<PAGE>

<PAGE>

                                                                  EXHIBIT 10.3

                                      LEASE

                              35 LANDSDOWNE STREET

                            CAMBRIDGE, MASSACHUSETTS





                                    LANDLORD
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                             FC 35 LANDSDOWNE, INC.





                                     TENANT
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                        MILLENNIUM PHARMACEUTICALS, INC.








                              DATED: AUGUST 4, 2000


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                                TABLE OF CONTENTS

<TABLE>
<CAPTION>

                                                                                                               PAGE
<S>                                                                                                            <C>
ARTICLE I

RECITALS AND DEFINITIONS..........................................................................................1
         Section 1.1       RECITALS...............................................................................1
         Section 1.2       DEFINITIONS............................................................................1

ARTICLE II

PREMISES AND TERM.................................................................................................3
         Section 2.1       PREMISES...............................................................................3
         Section 2.2       APPURTENANT RIGHTS.....................................................................4
         Section 2.3       LANDLORD'S RESERVATIONS................................................................4
         Section 2.4       PARKING PASSES.........................................................................5
         Section 2.5       COMMENCEMENT DATE; SCHEDULED RENT COMMENCEMENT DATE; RENT COMMENCEMENT DATE............6
         Section 2.6       EXTENSION OPTIONS......................................................................6

ARTICLE III

RENT AND OTHER PAYMENTS...........................................................................................9
         Section 3.1       ANNUAL FIXED RENT......................................................................9
         Section 3.2       REAL ESTATE TAXES......................................................................9
         Section 3.3       OPERATING EXPENSES....................................................................11
         Section 3.4       UTILITY CHARGES.......................................................................14
         Section 3.5       ABOVE-STANDARD SERVICES...............................................................14
         Section 3.6       NO OFFSETS............................................................................14
         Section 3.7       NET LEASE.............................................................................14

ARTICLE IV

ALTERATIONS......................................................................................................14
         Section 4.1       CONSENT REQUIRED FOR TENANT'S ALTERATIONS.............................................14
         Section 4.2       OWNERSHIP OF ALTERATIONS..............................................................15
         Section 4.3       CONSTRUCTION REQUIREMENTS FOR ALTERATIONS.............................................17
         Section 4.4       PAYMENT FOR TENANT ALTERATIONS........................................................18

ARTICLE V

RESPONSIBILITY FOR CONDITION OF BUILDING AND PREMISES............................................................18

                                      (ii)

<PAGE>

         Section 5.1       MAINTENANCE OF BUILDING AND COMMON AREAS BY LANDLORD..................................18
         Section 5.2       MAINTENANCE OF PREMISES BY TENANT.....................................................19
         Section 5.3       TENANT-PROVIDED SERVICES..............................................................19
         Section 5.4       DELAYS IN LANDLORD'S SERVICES.........................................................19
         Section 5.5       TENANT'S RESPONSIBILITIES REGARDING HAZARDOUS MATERIALS...............................20
         Section 5.6       LANDLORD'S RESPONSIBILITIES REGARDING HAZARDOUS MATERIALS.............................21
         Section 5.7       CROSS INDEMNIFICATION.................................................................21

ARTICLE VI

TENANT COVENANTS.................................................................................................21
         Section 6.1       PERMITTED USES........................................................................21
         Section 6.2       LAWS AND REGULATIONS..................................................................22
         Section 6.3       RULES AND REGULATIONS; SIGNS..........................................................22
         Section 6.4       SAFETY COMPLIANCE.....................................................................23
         Section 6.5       LANDLORD'S ENTRY......................................................................23
         Section 6.6       FLOOR LOAD............................................................................23
         Section 6.7       PERSONAL PROPERTY TAX.................................................................23
         Section 6.8       ASSIGNMENT AND SUBLEASES..............................................................23

ARTICLE VII

INDEMNITY AND INSURANCE..........................................................................................26
         Section 7.1       INDEMNITY.............................................................................26
         Section 7.2       LIABILITY INSURANCE...................................................................27
         Section 7.3       PERSONAL PROPERTY AT RISK.............................................................27
         Section 7.4       LANDLORD'S INSURANCE..................................................................28
         Section 7.5       WAIVER OF SUBROGATION.................................................................28
         Section 7.6       POLICY REQUIREMENTS...................................................................28

ARTICLE VIII

CASUALTY AND EMINENT DOMAIN......................................................................................29
         Section 8.1       RESTORATION FOLLOWING.................................................................29
         Section 8.2       LANDLORD'S TERMINATION ELECTION.......................................................29
         Section 8.3       TENANT'S TERMINATION ELECTION.........................................................29
         Section 8.4       CASUALTY AT EXPIRATION OF LEASE.......................................................30
         Section 8.5       EMINENT DOMAIN........................................................................30
         Section 8.6       RENT AFTER CASUALTY OR TAKING.........................................................30
         Section 8.7       TEMPORARY TAKING......................................................................31
         Section 8.8       TAKING AWARD..........................................................................31
         Section 8.9       CASUALTY OR EMINENT DOMAIN OR CONSTRUCTION DELAYS AFFECTING PARKING PRIVILEGES........31

                                     (iii)

<PAGE>

ARTICLE IX

DEFAULT..........................................................................................................33
         Section 9.1       TENANT'S DEFAULT......................................................................33
         Section 9.2       DAMAGES...............................................................................34
         Section 9.3       CUMULATIVE RIGHTS.....................................................................34
         Section 9.4       LANDLORD'S SELF-HELP..................................................................35
         Section 9.5       ENFORCEMENT EXPENSES; LITIGATION......................................................35
         Section 9.6       LATE CHARGES AND INTEREST ON OVERDUE PAYMENTS.........................................35
         Section 9.7       LANDLORD'S RIGHT TO NOTICE AND CURE; TENANT'S SELF-HELP RIGHTS........................36

ARTICLE X

MORTGAGEES'AND GROUND LESSORS'RIGHTS.............................................................................37
         Section 10.1      SUBORDINATION.........................................................................37
         Section 10.2      PREPAYMENT OF RENT NOT TO BIND MORTGAGEE..............................................37
         Section 10.3      TENANT'S DUTY TO NOTIFY MORTGAGEE: MORTGAGEE'S ABILITY TO CURE........................37
         Section 10.4      ESTOPPEL CERTIFICATES.................................................................38

ARTICLE XI

INTENTIONALLY OMITTED - SEE WORK LETTER..........................................................................39

ARTICLE XII

MISCELLANEOUS....................................................................................................39
         Section 12.1      NOTICE OF LEASE.......................................................................39
         Section 12.2      NOTICES...............................................................................39
         Section 12.3      SUCCESSORS AND LIMITATION ON LIABILITY................................................40
         Section 12.4      WAIVERS BY THE LANDLORD...............................................................40
         Section 12.5      ACCEPTANCE OF PARTIAL PAYMENTS OF RENT................................................40
         Section 12.6      INTERPRETATION AND PARTIAL INVALIDITY.................................................40
         Section 12.7      QUIET ENJOYMENT.......................................................................41
         Section 12.8      BROKERAGE.............................................................................41
         Section 12.9      SURRENDER OF PREMISES AND HOLDING OVER................................................41
         Section 12.10     GROUND LEASE..........................................................................42
         Section 12.11     FINANCIAL REPORTING...................................................................42
         Section 12.12     CAMBRIDGE EMPLOYMENT PLAN.............................................................42
         Section 12.13     TRUCK DELIVERY ROUTES; TRAFFIC MITIGATION MEASURES....................................42
         Section 12.14     PARKING AND TRANSPORTATION DEMAND MANAGEMENT..........................................43
         Section 12.15     LABORATORY ANIMALS....................................................................43
         Section 12.16     NO CONSEQUENTIAL DAMAGES..............................................................43
         Section 12.17     GOVERNING LAW.........................................................................43
         Section 12.18:    TERMINATION RIGHTS FOR FAILURE OF CONDITIONS..........................................43

                                      (iv)

<PAGE>

         Section 12.19     DECLARATION OF COVENANTS..............................................................45

EXHIBIT A           -      Basic Lease Terms
EXHIBIT B           -      Legal Description
EXHIBIT B-1         -      University Park Site Plan
EXHIBIT C           -      Work Letter
EXHIBIT D           -      Standard Services
EXHIBIT E           -      Rules and Regulations
EXHIBIT F           -      PTDM Traffic Mitigation Requirements
EXHIBIT G           -      Measurement Method
EXHIBIT H-1         -      Form of MIT Non-Disturbance Agreement
EXHIBIT H-2         -      Form of MIT Ground Lease
EXHIBIT I           -      Form of Permanent Lender's SNDA
EXHIBIT J           -      Dispute Resolution Process
EXHIBIT K           -      Construction Lender's SNDA
EXHIBIT L           -      Forest City Completion Guaranty
EXHIBIT M           -      Millennium Completion Guaranty

</TABLE>


                                      (v)

<PAGE>

                                      LEASE

                                    ARTICLE I

                            RECITALS AND DEFINITIONS

     Section 1.1 RECITALS. This Lease (this "Lease") is entered into this 4th
day of August, 2000 by and between FC 35 Lansdowne, Inc. (the "Landlord"), a
Massachusetts corporation, and Millennium Pharmaceuticals, Inc. (the "Tenant"),
a Delaware corporation.

     In consideration of the mutual covenants herein set forth, the Landlord and
the Tenant do hereby agree to the terms and conditions set forth in this Lease.

     Section 1.2 DEFINITIONS. The following terms shall have the meanings
indicated or referred to below:

     "Additional Rent" means all charges payable by the Tenant pursuant to this
Lease other than Annual Fixed Rent, including without implied limitation the
Tenant's parking charges as provided in Section 2.4; the Tax Expenses as
provided in Section 3.2; the Operating Expenses in accordance with Section 3.3;
amounts payable for special services pursuant to Section 3.5; costs for
alterations or additions to the Premises exceeding the Tenant's Allowance (as
described in the Work Letter); and the Landlord's share of any sublease or
assignment proceeds pursuant to Section 6.8.

     "Annual Fixed Rent" - See Exhibit A, and Section 3.1.

     "Base Building Improvements" - See the Work Letter.

     "Base Systems" - See the Work Letter.

     "Building" means the building to be constructed in accordance with the Work
Letter and located at 35 Landsdowne Street, Cambridge, Massachusetts, which
Building is also referred to as the Premises.

     "Commencement Date" - See Section 2.5.

     "Common Areas" - See Section 2.2.

     "Declaration of Covenants" means that certain Declaration of Covenants
dated as of December 15, 1997 made by Massachusetts Institute of Technology, as
declarant, recorded on March 12, 1998 in the Middlesex South Registry of Deeds
as Instrument No. 1065 and filed on March 12, 1998 in the Middlesex South
Registry District of the Land Court as Document No. 1058425.

                                       6

<PAGE>

     "Extension Term" - See Section 2.6.

     "Excusable Delay" - See the Work Letter.

     "External Causes" means, when referring to a party's responsibilities under
this Lease, collectively (i) Acts of God, war, civil commotion, fire, flood or
other casualty, strikes or other extraordinary labor difficulties, unusual
shortages of materials or equipment in the ordinary course of trade,
extraordinary weather conditions, government order or regulations or other cause
not reasonably within the control of such party, and not due to the fault or
neglect of such party, and (ii) any act, failure to act or neglect of the other
party or the other party's servants, agents, employees, licensees or, which in
the case of the Tenant shall include, without limitation, its sublessees,
licensees or other occupants deriving their rights under Tenant, which delays
such party in the performance of any act required to be performed by such party
under this Lease. External Causes shall not include financial inability nor the
inability of the responsible party therefor to obtain permits for work to be
performed under the Work Letter.

     "Initial Term" - See Exhibit A.

     "Land" means the parcel of land situated in Cambridge, Massachusetts,
described in Exhibit B and depicted in Exhibit B-1.

     "Landlord's Original Address" - See Exhibit A.

     "Lease Year" means each period of one year during the Term commencing on
the Rent Commencement Date or on any anniversary thereof.

     "Leasehold Improvements" - See the Work Letter.

     "Market Rate Parking Charge" means the monthly parking rate for structured
parking facilities charged from time to time by owners of parking facilities of
comparable quality at mixed use office/research parks, providing similar
convenience and proximity to the work environment, and similar services and
amenities in the geographical area of East Cambridge including University Park.

     "Permitted Uses" - See Exhibit A.

     "Premises" means the Building. See Exhibit A and Section 2.1.

     "Property" means the Land and the Building.

     "Removable Alterations" - See Section 4.2.

     "Rent Commencement Date" - See Section 2.5.

                                       7

<PAGE>

     "Scheduled Rent Commencement Date" means July 1, 2002, subject to Section
2.5.

     "Special Extension Term" - See Section 2.6.

     "Substantial Completion" - See the Work Letter.

     "Tenant's Original Address" - See Exhibit A.

     "Term" means the Initial Term together with the Special Extension Term
and/or the Extension Term, to the extent an extension option is timely exercised
by the Tenant under Section 2.6.

      "University Park" means the area in Cambridge, Massachusetts, bounded
on the North side by Massachusetts Avenue, Green and Blanche Streets, on the
East side by Landsdowne, Cross and Purrington Streets, on the South side by
Pacific Street and on the West side by Brookline Street.

     "Work Letter" means the letter agreement of even date herewith between the
Landlord and Tenant relating to the construction of the Building and the
leasehold improvements in the Premises attached hereto and made a part hereof as
Exhibit C.

                                   ARTICLE II

                                PREMISES AND TERM

     Section 2.1 PREMISES. The Landlord hereby leases to the Tenant, and the
Tenant hereby leases from the Landlord, for the Term, the Premises. The Premises
includes the entire rentable floor area of the Building. The Tenant acknowledges
that, except as expressly set forth in this Lease, there have been no
representations or warranties made by or on behalf of the Landlord with respect
to the Premises, the Building or the Property or with respect to the suitability
of any of them for the conduct of the Tenant's business. Prior to closing the
construction loan and commencing construction of the Building, the Landlord
shall deliver to the Tenant a statement from the Landlord's architect setting
forth the rentable floor area of the Premises and the total rentable floor area
of the Building as computed in accordance with Exhibit G (it being agreed that
the Building shall be designed to contain approximately 201,254 square feet of
rentable floor area as computed in accordance with Exhibit G) , provided that
rather than measuring the rooftop mechanical penthouse area, the parties agree
that it shall be deemed to contain 15,100 rentable square feet. Upon achievement
of the Tenant Construction Readiness Date, the rentable floor area of the
Premises shall be finally determined in accordance with Exhibit G. The Landlord
and the Tenant shall, after such determination at the request of either party,
confirm in writing the rentable square feet of floor area in the Premises,
initial Annual Fixed Rent and the total number of Parking Passes to be provided
to the Tenant hereunder.

                                       8

<PAGE>

     Section 2.2 APPURTENANT RIGHTS.

         (a) COMMON AREAS. The Tenant shall have as appurtenant to the Premises,
     the nonexclusive right to use in common with others, subject to reasonable
     rules of general applicability throughout University Park, common walkways
     and driveways necessary or reasonably convenient for access to the
     Building, and all other areas in and amenities of University Park outside
     of the Building as are made available generally to the occupants of
     University Park or the general public (collectively, "Common Areas").

         (b) ROOF RIGHTS. Subject to the provisions of Section 2.3(b) below, the
     Tenant shall have, as appurtenant to the Premises (and exclusively for use
     in connection with the occupancy of the Premises), the exclusive right of
     access to and use of the roof for the purpose of installing and maintaining
     mechanical equipment, antennae and dishes which, in each case, have been
     pre-approved by the Landlord as part of the initial construction of the
     Building or as otherwise approved by the Landlord under Article IV, subject
     however, to reasonable rules and regulations of general applicability
     within University Park of which the Tenant is given notice from time to
     time made by the Landlord.

         (c) PREFERENCE REGARDING DAY CARE CENTER. Landlord agrees to cause its
     Affiliate to enforce the existing provision(s) in the lease to Bright
     Horizons that tenants of University Park will be given preference regarding
     the day care center.

     Section 2.3 LANDLORD'S RESERVATIONS.

         (a) RESERVED COMMON AREA RIGHTS. The Landlord reserves the right from
     time to time, without unreasonable interference with the Tenant's use
     (including the specialized needs of Tenant's operations which Landlord
     hereby acknowledges) to alter or modify Common Areas, provided that (i) any
     such actions are effected in a good and workmanlike manner at no additional
     cost to Tenant (other than improvements thereto the costs of which are
     shared by tenants in University Park pursuant to the Declaration of
     Covenants) and (ii) such alterations or modifications do not impair
     Tenant's access to the Premises or its practical use and enjoyment thereof
     or of the Common Areas.

         (b) RESERVED ROOF RIGHTS. The Landlord reserves the right from time to
     time, without unreasonable interference with the Tenant's use, to access,
     install, use, maintain, repair, replace and relocate mechanical and
     communications equipment on the roof of the rooftop penthouse, and any
     related conduits, wires and appurtenant fixtures and equipment ("Landlord
     Reserved Roof Rights"), provided that the Landlord agrees that the exercise
     of Landlord's Reserved Roof Rights shall not be permitted to impair the
     Tenant's conduct of business operations in the Premises including, without
     limitation, the Tenant's equipment which is located on the roof and
     Tenant's communication with off-premises locations of the Tenant or its
     affiliates. Landlord agrees that it shall not exercise its reserved rights
     set forth in this Section 2.3(b) until the first anniversary of the

                                       9

<PAGE>

     Rent Commencement Date, unless it receives Tenant's prior written consent,
     which Tenant may withhold in its sole discretion. The Tenant acknowledges
     that the Tenant's right to make installations, improvements and alterations
     upon the roof of the Building, in addition to being limited by Article IV
     of this Lease, shall be restricted to mechanical equipment serving the
     Premises, and antennae, satellite dishes or other communications equipment
     that serves exclusively the business activities conducted in the Premises
     (exclusive of a communications business being conducted in the Premises)
     including, without limitation, the communication with off-premises
     locations of the Tenant or its affiliates.

         (c) GROUND FLOOR LOBBY. Tenant hereby acknowledges that the Landlord
     has an interest in the appearance and use of the ground floor lobby of the
     Building notwithstanding that the Building shall be occupied by a single
     tenant. The Tenant hereby agrees that visitors may enter the front door to
     the lobby during normal business hours, provided that they shall not have
     access to the elevators or through the lobby to the rear entrance/exit on
     Franklin Street. For purposes hereof, normal business hours shall mean
     Monday through Friday from 8:00 a.m. to 5:00 p.m. Accordingly, Tenant shall
     not alter the appearance of said lobby in a way contrary to the standards
     mutually agreed upon by Landlord and Tenant, without Landlord's prior
     written consent which shall not be unreasonably withheld, conditioned or
     delayed; provided that such consent shall be deemed given if not denied by
     Landlord within ten (10) business days of a request therefor and if still
     not denied within five (5) business days of a written reminder notice,
     which reminder notice shall state the "deemed approval" consequences of
     failure to respond.

     Section 2.4 PARKING PASSES. The Landlord shall provide and the Tenant shall
pay for Parking Passes (as defined in EXHIBIT A) for use by the Tenant's
employees in accordance with EXHIBIT A. Charges for the Tenant's Parking Passes
hereunder shall be determined in accordance with EXHIBIT A, shall constitute
Additional Rent and shall be payable monthly to the Landlord, during the Term,
from and after the Rent Commencement Date. The Tenant agrees that it and all
persons claiming by, through and under it, shall at all times abide by the Rules
and Regulations promulgated by the Landlord in accordance with Section 6.3, of
which the Tenant is given written notice, with respect to the use of the parking
facilities provided by the Landlord pursuant to this Lease. The Landlord
acknowledges that it is the Landlord's responsibility to assure that holders of
Parking Passes who fulfill the requirements of the Rules and Regulations are
able to find available parking spaces at 55 Franklin Street and/or 80 Landsdowne
Street in which to park their motor vehicle.

     At any time during the Term the Landlord shall have the right to assign the
Landlord's obligations to provide parking, as herein set forth, together with
the Landlord's right to receive Additional Rent for such parking spaces as
herein provided, to a separate entity created for the purpose of providing the
parking privileges set forth herein. In such event, the Landlord and the Tenant
agree to execute and deliver appropriate documentation, including documentation
with the new entity, reasonably necessary to provide for the new entity to
assume the Landlord's

                                       10

<PAGE>

obligations to provide the parking privileges to the Tenant as specified herein
and for the Tenant to pay the Additional Rent attributable to the parking
privileges directly to the new entity, provided, however, the Landlord shall at
all times remain obligated to the Tenant for the Tenant's parking privileges
hereunder as an obligation of the Landlord under this Lease, notwithstanding
that a new entity is providing the same to the Tenant pursuant to separate
documentation.

     Section 2.5 COMMENCEMENT DATE; SCHEDULED RENT COMMENCEMENT DATE; RENT
COMMENCEMENT DATE. "Commencement Date" means the date on which the Tenant is
first provided access to any portion of the Premises for the construction of the
Base Systems or the Leasehold Improvements. "Rent Commencement Date" for the
Building shall be the earlier of (i) September 1, 2002, which date shall be
extended as hereinafter set forth (the "Scheduled Rent Commencement Date") or
(ii) the date on which Tenant commences occupancy for business of any portion of
the Premises. Notwithstanding the foregoing, upon the following terms and
conditions, but not otherwise, if (i) the Landlord is delayed in achieving (a)
Tenant Construction Readiness beyond the Tenant Construction Readiness Date, or
(b) Substantial Completion of the Base Building Improvements beyond the date set
forth therefor on the Project Schedule (as such terms are defined in the Work
Letter), or (ii) the Tenant is delayed in Tenant's Construction Work by a
Landlord Delay (as such terms are defined in the Work Letter) and (iii) any such
delay is not due to Tenant Delay, then the Scheduled Rent Commencement Date
shall be postponed by the number of days that Tenant Construction Readiness
and/or Substantial Completion of the Base Building Improvements is so delayed,
up to 120, provided that Rent Commencement Date shall not be postponed by the
number of days of Landlord Delay if and to the extent Tenant is not actually
delayed because Landlord provides access to Tenant to commence its work prior to
Landlord's having fully achieved Tenant Construction Readiness. In addition to
the foregoing, in the event the aggregate number of days of Landlord Delay,
which cause Tenant to be actually delayed, exceeds one hundred twenty (120),
then Rent Commencement Date shall be postponed by the number of Landlord Delay
days in excess of 120, multiplied by 1.5.

     Without limitation of the foregoing, there shall be no postponement of the
Scheduled Rent Commencement Date to the extent that the Tenant Construction
Readiness Date, Substantial Completion of the Base Building Improvements or
Tenant's Construction Work is delayed on account of Tenant Delay. The
postponement of the Scheduled Rent Commencement Date, set forth above, shall
constitute the sole and liquidated damages with respect to damages associated
with any delay in the Tenant's ability to occupy the Premises by virtue of
Landlord Delay and, except as expressly otherwise provided in Article 9 of the
Work Letter, the Tenant shall have no right to terminate this Lease on account
thereof. Nothing in this paragraph is intended to limit or prevent Tenant from
occupying the Premises prior to the Rent Commencement Date.

     Section 2.6 EXTENSION OPTIONS. Provided that there has been no Event of
Default which is uncured and continuing on the part of the Tenant and the Tenant
or a Tenant Affiliate (as defined in Section 6.8) is, as of the date of

                                       11

<PAGE>

exercise and as of the commencement date of either the Special Extension Term or
the Extension Term (as such terms are defined below), actually occupying either
(i) at least seventy-five percent (75%) of the Premises, or (ii) at least
seventy-five percent (75%) of the rentable area leased by Tenant and any Tenant
Affiliates within University Park (not including subleases) for its own business
purposes, the Tenant shall have the following extension rights:

         (a) In the event that a lease is executed between Tenant or an
     affiliate of Tenant and an affiliate of Landlord wherein Tenant leases a
     building to be constructed at 40 Landsdowne Street, Cambridge,
     Massachusetts (the "40 Landsdowne Lease"), then Tenant shall have a
     one-time right (the "Special Extension Option") to extend the Term for a
     period of time (the "Special Extension Term") so as to be co-terminus with
     the term of the 40 Landsdowne Lease. In no event may Tenant shorten the
     term of this Lease to be co-terminus with the 40 Landsdowne Lease. The
     Special Extension Option may be exercised by Tenant's giving notice to
     Landlord on or prior to the tenth (10th) anniversary of the Rent
     Commencement date of this Lease. The Annual Fixed Rent during the Special
     Extension Term shall be the same as the Annual Base Fixed Rent in effect
     during the last year of the Initial Term.

         (b) In addition to or in lieu of, at the election of Tenant, the
     Special Extension Term described above, the Tenant shall have the right to
     extend the Term hereof for one (1) period of ten (10) years (such ten (10)
     year period the "Extension Term") on the terms and conditions set forth in
     this Section 2.6(b) and (c). Such right to extend the Term shall be
     exercised by the giving of notice by Tenant to Landlord at least eighteen
     (18) months prior to the expiration of the Initial Term or the Special
     Extension Term, as applicable. Upon the giving of such notice, this Lease
     and the Term hereof shall be extended for an additional term of ten (10)
     years without the necessity for the execution of any additional documents
     except a document memorializing the Annual Fixed Rent for the Extension
     Term to be determined as set forth below. Time shall be of the essence with
     respect to the Tenant's giving notice to extend the Term. In no event may
     the Tenant extend the Term under this Section 2.6 for more than ten (10)
     years after the expiration of the Initial Term or the Special Extension
     Term, as applicable.

         (c) The Extension Term shall be upon all the terms, conditions and
     provisions of this Lease except the Annual Fixed Rent during the ten (10)
     year Extension Term shall be the then Extension Fair Rental Value of the
     Premises for such Extension Term to be determined under Section 2.6(d)
     below, but in no case less than the Annual Fixed Rent that was applicable
     thereto immediately preceding the Extension Term with respect to which the
     Extension Fair Rental Value is to be established (the "Then Applicable
     Annual Fixed Rental Rate"). If the Tenant makes a written request to the
     Landlord for a proposal for the Extension Fair Rental Value for an
     Extension Term, the Landlord shall make such a written proposal to the
     Tenant within thirty (30) days after receipt of the Tenant's request
     therefor, but in no event shall the Landlord be required to deliver such a
     proposal sooner than twenty-one (21) months prior to the date as of which
     such proposal is to

                                       12

<PAGE>

     become effective. Alternatively, the Landlord may, at its election, propose
     an Extension Fair Rental Value to the Tenant without any request having
     been made.

         (d) For purposes of the Extension Term described in Section 2.6(b), the
     Extension Fair Rental Value of the Premises shall mean the then current
     fair market annual rent for leases of other space similarly improved,
     taking into account the condition to which such premises have been improved
     (excluding Removable Alterations) and the economic terms and conditions
     specified in this Lease that will be applicable thereto, including the
     savings, if any, due to the absence or reduction of brokerage commissions.
     The Landlord and the Tenant shall endeavor to agree upon the Extension Fair
     Rental Value of the Premises within forty five (45) days after the Tenant
     has exercised an option for an Extension Term. If the Extension Fair Rental
     Value of the Premises is not agreed upon by the Landlord and the Tenant
     within this time frame, each of the Landlord and the Tenant shall retain a
     real estate professional with at least ten (10) years continuous experience
     in the business of appraising or marketing similar commercial real estate
     in the Cambridge, Massachusetts area who shall, within thirty (30) days of
     his or her selection, prepare a written report summarizing his or her
     conclusion as to the Extension Fair Rental Value. The Landlord and the
     Tenant shall simultaneously exchange such reports; provided, however, if
     either party has not obtained such a report within ninety (90) days after
     the last day of the forty-five (45) day period referred to above in this
     Section 2.6(d), then the determination set forth in the other party's
     report shall be final and binding upon the parties. If both parties receive
     reports within such time and the lower determination is within ten percent
     (10%) of the higher determination, then the average of these determinations
     shall be deemed to be the Extension Fair Rental Value for the Premises. If
     these determinations differ by more than ten percent (10%), then the
     Landlord and the Tenant shall mutually select a person with the
     qualifications stated above (the "Final Professional") to resolve the
     dispute as to the Extension Fair Rental Value for the Premises. If the
     Landlord and the Tenant cannot agree upon the designation of the Final
     Professional within thirty (30) days of the exchange of the first valuation
     reports, either party may apply to the American Arbitration Association,
     the Greater Boston Real Estate Board, or any successor thereto, for the
     designation of a Final Professional. Within ten (10) days of the selection
     of the Final Professional, the Landlord and the Tenant shall each submit to
     the Final Professional a copy of their respective real estate
     professional's determination of the Extension Fair Rental Value for the
     Premises. The Final Professional shall not perform his or her own
     valuation, but rather shall, within thirty (30) days after such
     submissions, select the submission which is closest to the determination of
     the Extension Fair Rental Value for the Premises which the Final
     Professional would have made acting alone. The Final Professional shall
     give notice of his or her selection to the Landlord and the Tenant and such
     decision shall be final and binding upon the Landlord and the Tenant. Each
     party shall pay the fees and expenses of its real estate professional and
     counsel, if any, in connection with any proceeding under this paragraph,
     and one-half of the fees and expenses of the Final Professional. In the
     event that the commencement of the Extension Term occurs prior to a final
     determination of the Extension Fair Rental Value therefor (the "Extension
     Rent Determination Date"),

                                       13

<PAGE>

     then the Tenant shall pay the Annual Fixed Rent at the greater of (i) the
     rate specified by the Landlord in its proposed Extension Fair Rental Value
     or (ii) the Then Applicable Fixed Rental Rate. If the Annual Fixed Rent for
     the Extension Term is determined to be greater than the Annual Fixed Rent
     paid with respect to the Premises prior to the Extension Rent Determination
     Date, then the Tenant shall pay to the Landlord the amount of such
     underpayment within ten (10) days of the Extension Rent Determination Date,
     and if the Annual Fixed Rent for the Extension Term is determined to be
     less than the Annual Fixed Rent paid with respect to the Premises prior to
     the Extension Rent Determination Date, then the Landlord shall credit the
     amount of such overpayment against the monthly installments of Annual Fixed
     Rent thereafter coming due.

                                   ARTICLE III

                             RENT AND OTHER PAYMENTS

     Section 3.1 ANNUAL FIXED RENT. From and after the Rent Commencement Date
(as defined in Exhibit A), the Tenant shall pay, without notice or demand,
monthly installments of one-twelfth (1/12th) of the Annual Fixed Rent in effect
and applicable to the Premises in advance for each full calendar month of the
Term following the Rent Commencement Date and of the corresponding fraction of
said one-twelfth (1/12th) for any fraction of a calendar month at the Rent
Commencement Date. The Annual Fixed Rent applicable to the Premises during the
Initial Term shall be as set forth in Exhibit A.

     Section 3.2 REAL ESTATE TAXES. From and after the Rent Commencement Date,
during the Term, the Tenant shall pay to the Landlord, as Additional Rent, the
entirety of the Tax Expenses (as such term is hereinafter defined) for the
Premises in accordance with this Section 3.2. The terms used in this Section 3.2
are defined as follows:

         (a) "Fiscal Year" means the 12-month period beginning February 1 each
     year, or such other fiscal period of twelve (12) consecutive months
     hereinafter adopted by Landlord for lease administration purposes.

         (b) "The Tax Expenses" with respect to any Fiscal Year means the
     aggregate Real Estate Taxes on the Property with respect to that Fiscal
     Year, reduced by any abatement receipts with respect to that Fiscal Year.

         (c) "Real Estate Taxes" means (i) all taxes and special assessments of
     every kind and nature assessed by any governmental authority on the
     applicable property; and (ii) reasonable expenses of any proceedings for
     abatement or other reduction of such taxes or special assessments which
     expenses shall include fees paid to consultants in proactively negotiating
     assessments with the City of Cambridge prior to issuance of tax bills,
     provided that Tenant has been given prior notice of Landlord's intention to
     incur fees in excess of $5,000 per tax fiscal year which notice shall
     include a reference to any

                                       14

<PAGE>

     fee cap and has approved the same, such approval not to be unreasonably
     withheld or delayed. Any special assessments to be included within the
     definition of "Real Estate Taxes" shall be limited to the amount of the
     installment (plus any interest thereon) of such special tax or special
     assessment (which shall be payable over the longest period permitted by
     law) required to be paid during the Fiscal Year in respect of which such
     taxes are being determined. The Landlord hereby represents and warrants
     that, as of the date hereof, it has no knowledge of any special assessments
     affecting either the Premises or University Park. There shall be excluded
     from such taxes all income, estate, succession, inheritance, excess profit,
     franchise and transfer taxes, all so-called linkage payments and
     delinquency interest or penalties; provided, however, that if at any time
     during the Term the present system of AD VALOREM taxation of real property
     shall be changed so that in lieu of the whole or any part of the ad valorem
     tax on real property, there shall be assessed on the Landlord a capital
     levy or other tax on the gross rents received with respect to the Property,
     or a federal, state, county, municipal or other local income, franchise,
     excise or similar tax, assessment, levy or charge (distinct from any now in
     effect) based, in whole or in part, upon any such gross rents, then any and
     all of such taxes, assessments, levies or charges, to the extent so based,
     shall be deemed to be included within the term "Real Estate Taxes," based
     on the Building being the Landlord's only property. There shall also be
     excluded from the definition of "Real Estate Taxes" any taxes on the
     Building relating to capital improvements not approved by the Tenant,
     except for capital improvements required by law or required by the terms of
     this Lease.

     Payments by the Tenant on account of the Tax Expenses shall be made monthly
at the time and in the fashion herein provided for the payment of Annual Fixed
Rent and shall be in an amount of the greater of (i) one-twelfth (1/12th) of the
Tax Expenses for the current Fiscal Year as reasonably estimated by the
Landlord, or (ii) an amount reasonably estimated by any ground lessor of the
Land or holder of a first mortgage on the Property, to be sufficient, if paid
monthly, to pay the Landlord's Tax Expenses on the dates due to the taxing
authority.

     Not later than ninety (90) days after the Tax Expenses are determinable for
the first Fiscal Year of the Term or fraction thereof and for each succeeding
Fiscal Year or fraction thereof during the Term, the Landlord shall render the
Tenant a statement in reasonable detail showing for the preceding year or
fraction thereof, as the case may be, real estate taxes on the Property, and any
abatements or refunds of such taxes. Reasonable expenses incurred in obtaining
any tax abatement or refund not previously charged may be charged against such
tax abatement or refund before the adjustments are made for the Fiscal Year
provided that Tenant has been given prior notice of Landlord's intention to
incur fees in excess of $5,000 per tax fiscal year which notice shall include a
reference to any fee cap and has approved the same, such approval not to be
unreasonably withheld or delayed. If at the time such statement is rendered it
is determined with respect to any Fiscal Year, that the Tenant has paid (i) less
than the Tax Expenses or (ii) more than the Tax Expenses, then, in the case of
(i) the Tenant shall pay to the Landlord, as Additional Rent, within fifteen
(15) days of such statement the amount of such underpayment and, in the case of
(ii) the Landlord shall credit the amount of such overpayment against the
monthly installments of the Tax Expenses next thereafter coming due (or refund
such overpayment within

                                       15

<PAGE>

fifteen (15) days if the Term has expired and the Tenant has no further
obligation to the Landlord).

     To the extent that real estate taxes shall be payable to the taxing
authority in installments with respect to periods less than a Fiscal Year, the
statement to be furnished by the Landlord shall be rendered and payments made on
account of such installments. Landlord shall, upon Tenant's request therefor,
provide Tenant with copies of all applicable tax bills, statements, records and
the like, as well as copies of Landlord's calculations and all other relevant
information.

     At the reasonable request of the Tenant, the Landlord shall use reasonable
efforts to contest or seek abatement of any Real Estate Taxes affecting the
Premises. Should the Landlord contest or seek abatement of such taxes, then it
shall do so with diligence and shall keep the Tenant appropriately informed, in
the Landlord's reasonable discretion, as to such action.

     Section 3.3 OPERATING EXPENSES. From and after the Rent Commencement Date,
during the Term the Tenant shall pay to the Landlord, as Additional Rent, the
entirety of the Operating Expenses for the Property, as hereinafter defined, in
accordance with this Section 3.3.

         (a) "Operating Expenses" means the Landlord's actual and reasonable
     cost of operating, cleaning, maintaining and repairing the Property, the
     roads, driveways and walkways for providing access to the Building and
     shall include without limitation, the cost of fulfilling the maintenance
     and repair obligations required to be performed by Landlord under Section
     5.1 and the cost of services specified on Exhibit D; premiums for insurance
     carried pursuant to Section 7.4; the reasonable amount deductible from any
     insurance claim of the Landlord; reasonable compensation including, without
     limitation, reasonable fringe benefits, worker's compensation insurance
     premiums and payroll taxes paid to, for or with respect to all persons
     (University Park/Building general manager and below) directly engaged in
     the operating, maintaining or cleaning of the Property, to the extent
     allocable to the Property; interior landscaping and maintenance; steam,
     water, sewer, gas, oil electricity, telephone and other utility charges
     (excluding such utility charges either separately metered or separately
     chargeable to tenants for either measured or additional or special
     services); cost of building and cleaning supplies; the costs of routine
     environmental management programs operated by Landlord; market rental costs
     for equipment used in the operating, cleaning, maintaining or repairing of
     the Property, or the amortized cost of equipment owned by the Landlord;
     cost of cleaning; cost of maintenance, repairs and replacements (other than
     repairs and replacements reimbursed from contractors under guarantees or
     made by the Landlord pursuant to the Work Letter); cost of snow removal;
     cost of landscape maintenance; security services; payments under service
     contracts with independent contractors; management fees at reasonable rates
     consistent with comparable single-tenant buildings in campus settings in
     the Cambridge market with the type of occupancy and services rendered; the
     cost of any capital repair or improvement: (i) required by any law or
     regulation enacted or promulgated after the issuance of a building permit
     for the construction of the Building, (ii) which is required in

                                       16

<PAGE>

     order to maintain the Property in the condition it is required to be kept
     and maintained under Section 5.1 (excluding those repairs, however, which
     are necessitated by Landlord's negligence or willful misconduct or the
     errors or omissions by the Landlord's base building architect or
     contractor(s) or which are the responsibility of Landlord pursuant to the
     Work Letter), (iii) which reduces the Operating Expenses for the Property
     (but the same shall not be considered Operating Expenses to the extent the
     amortized cost of the capital improvement exceeds the amount of such
     reduction), (iv) which improves the management and operation of the
     Property in a manner reasonably acceptable to Tenant, or (v) which is
     required to maintain the Common Areas in the condition required by the
     Declaration of Covenants (but excluding any costs or expenses associated
     with the initial development of Common Areas and Facilities and Initial
     Open Space Improvements, as defined in the Declaration of Covenants) (all
     such capital costs to be amortized in accordance with generally accepted
     accounting principles, together with interest on the unamortized balance at
     the base lending rate announced by a major commercial bank designated by
     the Landlord, or such higher rate as may have been paid by the Landlord on
     funds borrowed for the purpose of constructing such capital improvements,
     with only the annual amortization amount being included in Operating
     Expenses with respect to any Fiscal Year); charges equitably and reasonably
     allocated to the Building for the operating, cleaning, maintaining,
     securing and repairing of University Park common areas and amenities; and
     all other reasonable and necessary (in the Landlord's reasonable judgment)
     expenses paid in connection with the operation, cleaning, maintenance and
     repair of the Property. Any costs related to the Common Areas shall, for
     the purposes of determining Operating Expenses for the Property, be
     allocated to the Building based upon the methodology set forth in the
     Declaration of Covenants, subject in all events to Section 12.20, hereof.

         Operating Expenses for the Property shall not include the following:
     the amortized cost of capital improvements expressly not allowed pursuant
     to the preceding paragraph unless otherwise approved by the Tenant; the Tax
     Expenses; cost of repairs or replacements (i) resulting from eminent domain
     takings, (ii) to the extent reimbursed by insurance or otherwise, (iii)
     resulting from correcting defects in the work for which the Landlord is
     obligated pursuant to the Work Letter, or (iv) resulting from Landlord's
     negligence or willful misconduct; replacement or contingency reserves;
     ground lease rents or payment of debt obligations; legal and other
     professional fees for matters not relating to the normal administration and
     operation of the Property, such as those fees incurred in connection with
     eviction proceedings and rent collection; promotional, advertising, public
     relations or brokerage fees and commissions paid in connection with
     services rendered for securing or renewing leases; interest or penalties
     for late payments; depreciation and other non-cash charges; and separately
     metered or submetered utilities.

         Notwithstanding anything to the contrary contained in this Section
     3.3(b), the reasonable costs of the Tenant-Provided Services (as defined in
     Section 5.3) shall be excluded from the calculation of the Operating
     Expenses for the Property.

                                       17

<PAGE>

         Payments by the Tenant for the Operating Expenses for the Property
     shall be made monthly at the time and in the fashion herein provided for
     the payment of Annual Fixed Rent. The amount so to be paid to the Landlord
     shall be an amount from time to time reasonably estimated by the Landlord
     to be sufficient to aggregate a sum equal to the Operating Expenses for the
     Property for each Fiscal Year.

     Not later than one hundred twenty (120) days after the end of each Fiscal
Year or fraction thereof during the Term or fraction thereof at the end of the
Term, the Landlord shall render the Tenant a statement in reasonable detail and
according to usual accounting practices certified by an officer of the Landlord,
showing for the preceding Fiscal Year or fraction thereof, as the case may be,
the Operating Expenses for the Property. Said statement to be rendered to the
Tenant also shall show for the preceding Fiscal Year or fraction thereof, as the
case may be, the amounts of Operating Expenses already paid by the Tenant. If at
the time such statement is rendered it is determined with respect to any Fiscal
Year, that the Tenant has paid (i) less than the Operating Expenses for the
Property or (ii) more than the Operating Expenses for the Property, then, in the
case of (i) the Tenant shall pay to the Landlord, as Additional Rent, within
thirty (30) days of such statement the amounts of such underpayment and, in the
case of (ii) the Landlord shall credit the amount of such overpayment against
the monthly installments of the Operating Expenses for the Property next
thereafter coming due (or refund such overpayment within thirty (30) days if the
Term has expired and the Tenant has no further obligation to the Landlord).

     The Tenant may examine or audit the accounts and original bills for
Operating Expenses upon ten (10) days' prior written notice to the Landlord, but
no more often than one (1) time in any Fiscal Year. The Landlord agrees that it
will make available to the Tenant in the Landlord's office in University Park,
during regular business hours, such information as the Landlord has available at
such office. In similar manner, the Tenant may examine such further records as
the Landlord (or its affiliates) may have, but such matters will be conducted
where the Landlord customarily keeps such records, which may be at the
headquarters of the Landlord's parent company. The Tenant shall bear the cost of
any such audit, unless the same discloses a discrepancy in excess of three
percent (3%) of the Tenant's Operating Expenses for the Fiscal Year in question,
in which event the Landlord shall reimburse the Tenant for such costs reasonably
incurred. For any given Fiscal Year of the Landlord, the Tenant must make any
such audit within two (2) years after the Tenant's receipt of itemized
statements (and any supporting documentation requested by the Tenant) referred
to in the preceding paragraph. The Tenant must further make any claim for
revision of Operating Expenses for such Fiscal Year by written notice to the
Landlord within said two (2) year period.

     Section 3.4 UTILITY CHARGES. During the Term, the Tenant shall pay directly
to the provider of the service all separately metered charges for steam, heat,
gas, electricity, fuel and other services and utilities furnished to the
Premises. The Tenant shall cause all electricity and gas service (specifically
serving the Premises and not the common areas) provided by a public utility to
be furnished on a separately metered basis.

                                       18

<PAGE>

     Section 3.5 ABOVE-STANDARD SERVICES. If the Tenant requests and the
Landlord elects to provide any services to the Tenant in addition to those
described in Exhibit D, the Tenant shall pay to the Landlord, as Additional
Rent, the amount billed by Landlord for such services at Landlord's standard
rates as from time to time in effect, so long as such rates are consistent with
comparable services in comparable buildings within the Cambridge market. If the
Tenant has requested that such services be provided on a regular basis, the
Tenant shall, if requested by the Landlord, pay for such services at their
actual cost to Landlord, including, without limitation, a reasonable overhead
component, at the time and in the fashion in which Annual Fixed Rent under this
Lease is payable. Otherwise, the Tenant shall pay for such additional services
within fifteen (15) days after receipt of an invoice from the Landlord.

     Section 3.6 NO OFFSETS. Annual Fixed Rent and Additional Rent shall be paid
by the Tenant without offset, abatement or deduction except as provided herein.

     Section 3.7 NET LEASE. It is understood and agreed that this Lease is a net
lease and that the Annual Fixed Rent is absolutely net to the Landlord excepting
only the Landlord's obligations to pay any debt service or ground rent on the
Property, to provide the Landlord's services, to repair or restore the Premises
after any casualty or condemnation as provided in Article VIII hereof, to pay
the real estate taxes and operating expenses which the Tenant is not required to
pay under this Lease and to otherwise comply with its obligations hereunder that
are specifically described herein as not subject to reimbursement by Tenant.

                                   ARTICLE IV

                                   ALTERATIONS

     Section 4.1 CONSENT REQUIRED FOR TENANT'S ALTERATIONS. The Tenant shall not
make alterations or additions to the Premises except in accordance with the
Construction Rules and Regulations, and with plans and specifications therefor
first approved by the Landlord, which approval shall not be unreasonably
withheld, conditioned or delayed, and shall be deemed granted if not denied
within ten (10) business days of request therefor and if still not denied within
five (5) business days of a written reminder notice, which reminder notice shall
state the "deemed approval" consequences of failure to respond. Notwithstanding
the foregoing, the Tenant may, from time to time without the Landlord's prior
consent and at the Tenant's own expense, make interior non-structural
alterations and changes in and to the Premises costing less than $100,000 in
each instance, provided that such alterations or changes (i) do not materially
diminish the value of the Building, (ii) are not incompatible with existing
mechanical or electrical, plumbing, HVAC or other systems in the Building, and
(iii) do not affect the exterior appearance of the Building, in each case, as
reasonably determined by the Landlord. Whether or not the Tenant's changes
and/or alterations require the Landlord's consent pursuant to this paragraph,
the Tenant shall give reasonable prior notice to the Landlord of any alterations
and changes in and to the Premises which the Tenant intends to undertake,
together with a reasonable description of the proposed work and such plans and
specifications, if any, as the Tenant has therefor. Tenant shall furnish
Landlord an "as-built" set of plans and specifications of the

                                       19

<PAGE>

Premises annually, on the first day of each Fiscal Year, in addition to any
other plans and specifications furnished by Tenant to Landlord from time to
time. The Landlord shall not be deemed unreasonable for withholding approval of
any alterations or additions which (i) involve or might affect any structural or
exterior element of the Building, any area or element outside of the Premises
(including, without limitation, interior alterations or additions that affect
the exterior appearance of the Building), or any facility serving any area of
the Building outside of the Premises or any publicly accessible major interior
features of the Building; (ii) will require unusual expense to readapt the
Premises to normal use as a biotechnology office and research and development
facility unless the Tenant first gives assurance acceptable to the Landlord that
such readaptation will be made prior to such termination without expense to the
Landlord; or (iii) would not be compatible with existing mechanical or
electrical, plumbing, HVAC or other systems in the Building; provided, however,
with respect to clause (iii), the Landlord shall not withhold its approval if
the Tenant agrees to, at its own cost and expense and within a reasonable period
of time as determined by the Landlord, make such alterations or additions
compatible with all Building systems and equipment. Neither the Landlord's
failure to object to any proposed alterations or additions, nor the Landlord's
approval of any plans and specifications furnished by Tenant to Landlord, shall
be construed as superseding in any respect, or as a waiver of Landlord's right
to enforce, the Tenant's obligation to fulfill all of the terms and conditions
of this Lease applicable to any work contemplated thereby.

     Notwithstanding anything to the contrary contained in this Section 4.1, if
any of the Tenant's proposed alterations and/or additions affect the roof or the
envelope of the Building, the following additional conditions shall apply: (i)
such alterations and changes will not in any way interfere with the proper
functioning of and Landlord's access to the roof of the Building or any
equipment thereon; (ii) adequate measures shall be taken to screen and otherwise
reduce the visibility and noise of such mechanical equipment, antennae and
dishes consistent with the appearance and design scheme required by the City of
Cambridge and other structures in University Park; and (iii) such alterations
and changes shall comply with the University Park Urban Design Guidelines which
have been delivered by Landlord to Tenant.

     Section 4.2 OWNERSHIP OF ALTERATIONS. All alterations and additions shall
be part of the Building and owned by the Landlord, except for certain
biotechnology laboratory equipment that is to be installed in the Premises,
which shall be deemed to be independent of the real property, and which have
been identified as Removable Alterations (as defined herein) in accordance with
the terms of this Section 4.2 and by mutual agreement between the Landlord and
Tenant either at the time of the Tenant's initial occupancy of the Premises or
from time to time thereafter; provided, however, that, other than with respect
to the original construction of the Millennium Project (as defined in the Work
Letter), the Landlord may require removal by Tenant of all or any portion of all
other alterations and additions so long as Landlord advised Tenant of such
requirement prior to the installation of the alteration or addition by Tenant.
If the Tenant fails to inform the Landlord, in writing, at least ten (10) days
prior to the installation of the alteration or addition, thereby preventing the
Landlord from making a determination as to whether it will want such addition or
alteration removed from the Premises prior to its installation, then the
Landlord may require such removal without exception. All specialized equipment,
trade fixtures and

                                       20

<PAGE>

furnishings not attached to the Premises (but excluding "Standard Installed
Laboratory Equipment", as defined in the Work Letter) shall remain the property
of the Tenant and shall be removed by the Tenant upon termination or expiration
of this Lease. Notwithstanding anything to the contrary contained in this
Section 4.2, all Tenant Construction Work funded by the Landlord Allowances and
installed as part of the Millennium Project (as defined in the Work Letter) (the
"Landlord Funded Alterations") shall be part of the Building and owned by the
Landlord, and all Tenant Construction Work, alterations and additions which are
necessary for the use of the Premises as an operational biotechnology laboratory
(the "Base Laboratory Alterations") as well as the Standard Installed Laboratory
Equipment, regardless of who funded their acquisition and installation, shall be
owned by Landlord, may not be removed by Tenant, and shall in no event
constitute Removable Alterations. All alterations and additions which: (i) are
not Landlord Funded Alterations; (ii) are not Base Laboratory Alterations; (iii)
are not Standard Installed Laboratory Equipment; and (iv) have been designated
as Removable Alterations by mutual agreement from time to time established
between the Landlord and the Tenant (such items being hereinafter collectively
referred to as the "Removable Alterations") shall remain the property of the
Tenant and shall be removed by the Tenant upon termination or expiration of this
Lease, unless the Landlord elects to purchase all or a portion of such Removable
Alterations from the Tenant for a price equal to the then depreciated book value
thereof established on the Tenant's books maintained in accordance with
generally accepted accounting principals consistently applied. If the Landlord
terminates this Lease on the basis of a default by the Tenant, and the Landlord
elects to purchase all or a portion of the Removable Alterations as provided
above, then the Landlord will credit such amounts due to the Tenant against any
sums owed by Tenant to Landlord, whether overdue Annual Fixed Rent, Additional
Rent, damages or otherwise and pay to Tenant the balance within sixty (60) days
thereafter. In connection with any removal by the Tenant of the Removable
Alterations, the Tenant shall cap off all utility connections behind the
adjacent interior finish and restore such interior finish to the extent
necessary so that the Premises are left with complete wall, ceiling and floor
finishes.

     The Tenant shall have the right to subject the Removable Alterations to a
prior security interest in connection with the financing of its Leasehold
Improvements (as defined in the Work Letter) so long as the security agreement
evidencing the secured party's interest in such Removable Alterations:

         (a) Requires the secured party to offer the Removable Alterations to
     the Landlord prior to offering them to third parties in the case of a
     default by the Tenant as part of the secured party's remedy under such
     security agreement;

         (b) Prohibits the sale of any Removable Alterations at the Premises
     (although secured lender(s) may conduct a viewing of the Removable
     Alterations at the Premises); and

                                       21

<PAGE>

         (c) Provides that the secured party agrees to fulfill the Tenant's
     restoration and repair obligations as a condition to recovering possession
     of the Removable Alterations.

     Section 4.3 CONSTRUCTION REQUIREMENTS FOR ALTERATIONS. All construction
work by the Tenant shall be done in a good and workmanlike manner employing only
first-class materials and in compliance with Construction Rules and Regulations
and with all applicable laws and all lawful ordinances, regulations and orders
of Governmental authority and insurers of the Building, all as more specifically
set forth in the Work Letter. The Landlord or Landlord's authorized agent may
(but without any implied obligation to do so) inspect the work of the Tenant at
reasonable times and shall give notice of observed defects. All of the Tenant's
alterations and additions and installation of furnishings shall be coordinated
with any work being performed by the Landlord and in such manner as to maintain
harmonious labor relations and not to damage the Building or interfere with
Building construction or operation and, except for installation of furnishings,
shall be performed by the Landlord's general contractor or by contractors or
workmen first approved by the Landlord, which approval the Landlord agrees not
to unreasonably withhold or delay. The Tenant, before starting any work, shall
comply with the Tenant Design and Construction Manual which has been delivered
by Landlord to Tenant and shall cause Tenant's contractors to comply therewith,
shall secure all licenses and permits necessary therefor and shall deliver to
the Landlord a statement of the names of all its contractors and subcontractors
and the estimated cost of all labor and material to be furnished by them and
security satisfactory to the Landlord in its reasonable discretion and
consistent with the security requirements for comparable work in comparable
buildings in the Cambridge market protecting the Landlord against liens arising
out of the furnishing of such labor and material; and cause each contractor to
carry worker's compensation insurance in statutory amounts covering all the
contractors' and subcontractors' employees and comprehensive general public
liability insurance with limits of $1,000,000 (individual ) and $5,000,000
(occurrence), or in such lesser amounts as Landlord may accept, covering
personal injury and death and property damage (all such insurance to be written
in companies approved reasonably by the Landlord and insuring the Landlord, such
individuals and entities affiliated with the Landlord as the Landlord may
designate, and the Tenant as well as the contractors and to contain a
requirement for at least thirty (30) days' notice to the Landlord prior to
cancellation, nonrenewal or material change), and to deliver to the Landlord
certificates of all such insurance.

     Section 4.4 PAYMENT FOR TENANT ALTERATIONS. The Tenant agrees to pay
promptly when due the entire cost of any work done on the Premises by the
Tenant, its agents, employees or independent contractors, and not to cause or
permit any liens for labor or materials performed or furnished in connection
therewith to attach to the Premises and promptly to discharge or bond over any
such liens which may so attach. If any such lien shall be filed against the
Premises and the Tenant shall fail to cause such lien to be discharged within
fifteen (15) days after receipt by the Tenant of notice of the filing thereof,
the Landlord after a further five (5) days' written notice may cause such lien
to be discharged by payment, bond or otherwise without investigation as to the
validity thereof or as to any offsets or defenses which the Tenant may have with
respect to the amount claimed. The Tenant shall reimburse the Landlord, as
Additional Rent, for any cost

                                       22

<PAGE>

so incurred and shall indemnify and hold harmless the Landlord from and against
any and all claims, costs, damages, liabilities and expenses (including
reasonable attorneys' fees) which may be incurred or suffered by the Landlord by
reason of any such lien or its discharge.

                                    ARTICLE V

              RESPONSIBILITY FOR CONDITION OF BUILDING AND PREMISES

     Section 5.1 MAINTENANCE OF BUILDING AND COMMON AREAS BY LANDLORD. Except as
otherwise provided in Section 5.3 and Article VIII, the Landlord shall make such
repairs to the major structural elements of the Building, including the roof,
exterior walls and floor slabs as may be necessary to keep and maintain the same
in good condition and repair. Landlord shall maintain and make repairs to the
mechanical systems, including HVAC systems, and equipment serving the Building
as are necessary to keep them in good order, condition and repair. If requested
by Tenant, Landlord shall maintain and repair additional building mechanical
equipment which are part of the building (such as exhaust fans) required as a
result of laboratory functions ("Additional Mechanical Equipment"), provided
that Landlord shall have no responsibility to maintain or repair Tenant's
laboratory equipment. The Landlord shall further perform the services designated
as Landlord's Services on Exhibit D. The Landlord shall in no event be
responsible to the Tenant for any condition in the Premises, the Building or the
Land caused by an act or neglect of the Tenant, or any invitee or contractor or
agent of the Tenant. Landlord's costs in performing such services shall be
reimbursed by the Tenant to the extent provided in Section 3.3. Without in any
way limiting the foregoing: (a) Tenant shall maintain any laboratories and other
specialized use areas within the Premises, and, except in the event of an
emergency, Landlord shall not enter the same without reasonable prior written
notice to the Tenant; (b) Tenant shall have the right to approve Landlord's
maintenance staff, such approval not to be unreasonably withheld, conditioned or
delayed; and (c) Landlord and Tenant shall work together to develop a mutually
satisfactory preventive maintenance plan.

     Section 5.2 MAINTENANCE OF PREMISES BY TENANT. The Tenant shall keep neat
and clean and maintain in good order, condition and repair the Premises and
every part thereof, reasonable wear and tear excepted and further excepting
those repairs for which the Landlord is responsible pursuant to Sections 5.1,
8.1 and 8.5. Tenant shall surrender the Premises and all alterations and
additions thereto, at the end of the Term, in good order, condition and repair,
first removing all goods and effects of the Tenant and, to the extent specified
by the Landlord by notice to the Tenant, all alterations and additions,
including the Removable Alterations, made by the Tenant, and repairing any
damage caused by such removal and restoring the Premises and leaving them clean
and neat. If the Tenant elects to provide the Tenant-Provided Services,
identified as such in Section 5.3, the Tenant shall perform the Tenant-Provided
Services promptly, as necessary and appropriate, with due diligence and in
accordance with the standards therefor established under Section 5.3. The Tenant
shall not permit or commit any damage (waste), and the Tenant shall be
responsible for the cost of repairs which may be made necessary by reason of
damage to the Building by the Tenant, or any of the contractors or invitees of
the Tenant. Unless Tenant

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requests Landlord to do so on its behalf pursuant to Section 5.1 hereof, Tenant
shall maintain and repair all Additional Mechanical Equipment and cause same to
be kept in good order, condition and repair consistent with prevailing standards
at comparable first-class biotechnology facilities.

     Section 5.3 TENANT-PROVIDED SERVICES. Notwithstanding anything to the
contrary contained in this Article V, the Tenant may choose to provide, at its
own cost and expense, in lieu of the Landlord providing the same under this
Lease, certain of the building services within the Premises, as specified in
Exhibit D (as modified from time to time by mutual consent of Landlord and
Tenant, such consent not to be unreasonably withheld, conditioned or delayed)
and such other services as may be hereafter approved by the Landlord in its
reasonable discretion. Such services which are paid for and provided by the
Tenant are hereinafter referred to as the "Tenant-Provided Services." The
provision by Tenant of Tenant-Provided Services shall be subject to reasonable
standards imposed by Landlord for the purpose of assuring the fulfillment of the
reasonable requirements of any ground lessee, mortgagee, tenant, governmental
authority or other third party pertaining to the maintenance and operation of
the Building in good order, condition and repair and in compliance with all
legal requirements.

     Section 5.4 DELAYS IN LANDLORD'S SERVICES. The Landlord shall not be liable
to the Tenant for any compensation or reduction of rent by reason or
inconvenience or annoyance or for loss of business arising from the necessity of
the Landlord or its agents entering the Premises for any purposes authorized in
this Lease, or for repairing the Premises or any portion of the Building. In
case the Landlord is prevented or delayed from making any repairs, alterations
or improvements, or furnishing any services or performing any other covenant or
duty to be performed on the Landlord's part, by reason of any External Cause,
the Landlord shall not be liable to the Tenant therefor, nor, except as
expressly otherwise provided in this Lease, shall the Tenant be entitled to any
abatement or reduction of rent by reason thereof, nor shall the same give rise
to a claim in the Tenant's favor that such failure constitutes actual or
constructive, total or partial, eviction from the Premises.

     The Landlord reserves the right to stop any service or utility system the
Landlord provides or causes to be provided under this Lease (i.e. exclusive of
any Tenant-Provided Services or other obligations of the Tenant under this
Lease) when necessary by reason of accident or emergency, until necessary
repairs have been completed; provided, however, that in each instance of
stoppage, the Landlord shall exercise reasonable diligence to eliminate the
cause thereof. Except in case of emergency repairs, the Landlord will schedule
contemplated stoppages at times reasonably approved by the Tenant and will use
reasonable efforts to avoid unnecessary inconvenience to the Tenant by reason
thereof. To the extent that the Landlord is providing or causing to be provided
heat, light or any utility or service, in no event shall the Landlord have any
liability to the Tenant for the unavailability of the same to the extent that
such unavailability is caused by External Causes, provided, however, that the
Landlord is obligated to exercise reasonable efforts to restore such services or
utility systems' operation. The Landlord agrees to carry rent interruption
insurance in commercially reasonable amounts which, to the extent commercially
reasonable, permits recovery within five (5) days after the insured peril. If
the unavailability of heat, light or any utility or service provided or caused
to be provided by

                                       24

<PAGE>

the Landlord other than the unavailability of the same due to the Tenant's acts
or omissions renders all or any portion of the Premises untenantable, and the
Tenant ceases to occupy the same for the conduct of its business, the Tenant
shall receive an abatement of rent as reasonably determined by the Landlord,
taking into account the extent of Tenant's loss of use of the Premises,
commencing with the day following the expiration of the deductible period
provided in Landlord's rent interruption insurance. For all purposes of this
Lease, if Tenant has responsibility for maintenance and repair of any aspect of
the Building or any equipment or system therein, the functioning and performance
of the same shall be the responsibility of the Tenant under this Lease, and
shall in no event constitute a service or utility system that the Landlord
provides or causes to be provided under this Lease.

     Section 5.5 TENANT'S RESPONSIBILITIES REGARDING HAZARDOUS MATERIALS. The
Tenant covenants and agrees that the Tenant shall not use, generate, store or
dispose, nor shall the Tenant suffer or permit the use, generation, storing or
disposal in the Premises or otherwise by any of Tenant's contractors, licensees,
invitees, agents or employees, of any oil, toxic substances, hazardous wastes or
hazardous materials (collectively, "Hazardous Materials") in, on or about the
Premises, the Building or the Land, except for Hazardous Materials that are
necessary or appropriate for Tenant's operation of an office and research
facility, and in all cases which Hazardous Materials must be used, generated,
stored and disposed of in compliance with all applicable law and regulations.
The Tenant covenants and agrees that Tenant shall comply with all applicable
laws and regulations in handling and disposing of materials used in its research
or other uses of the Premises, whether or not considered Hazardous Materials,
and no dumping, flushing or other introduction of Hazardous Materials into the
septic, sewage or other waste disposal systems serving the Premises shall occur,
except as specifically permitted by law and subject to the conditions and
qualifications imposed by any governmental license or permit. The Tenant shall
provide to the Landlord copies of all licenses and permits that the Tenant has
been required to obtain prior to the handling of any such Hazardous Materials,
and the Tenant must obtain all of such licenses and permits prior to the
commencement of operations in the Premises requiring the same. From time to time
during the Term of this Lease, and thereafter during which the Tenant occupies
any portion of the Premises, the Tenant shall provide the Landlord with such
reasonable substantiation of the Tenant's compliance with the requirements of
this Section 5.5 and any additional requirements set forth in Section 6.2 as the
Landlord may reasonably request. The Tenant covenants and agrees that the Tenant
shall, at its sole cost, promptly remove or remediate all Hazardous Materials
that are found upon the Premises, the Building or the Land by virtue of the
failure of the foregoing covenants and agreements to have been fulfilled, or
otherwise as the result of the act or omission of Tenant or its contractors,
licensees, invitees, agents or employees, in a manner complying with all
applicable laws and regulations and the provisions of this Lease. If the Tenant
should have any responsibility under this Section 5.5 to remove or remediate
Hazardous Materials, the Tenant shall keep the Landlord reasonably informed as
to the status of the environmental condition at issue, promptly furnish to the
Landlord copies of all regulatory filings with any governmental regulatory
agencies in connection therewith, and substantiate the performance of its
obligations under this Section 5.5.

                                       25

<PAGE>

     Section 5.6 LANDLORD'S RESPONSIBILITIES REGARDING HAZARDOUS MATERIALS. From
and after the date hereof, and during the Term of this Lease, if the removal or
remediation of Hazardous Materials from the Premises, Building or Land is
required to be undertaken, then except to the extent such obligation is the
responsibility of the Tenant under Section 5.5 hereof, the Landlord covenants
and agrees to undertake the same. Without limitation of the foregoing, if
necessary to comply with any applicable legal requirements, should the existing
environmental condition of the Land require the removal or remediation of
Hazardous Materials, such removal or remediation is expressly intended herein to
be the Landlord's responsibility under this Section 5.6. The Landlord shall keep
the Tenant reasonably informed as to the status of the environmental condition
at issue, promptly furnish to the Tenant copies of all regulatory filings with
any governmental regulatory agencies in connection therewith, and substantiate
the performance of its obligations under this Section 5.6.

     Section 5.7 CROSS INDEMNIFICATION. Each of the Landlord and the Tenant
shall defend and indemnify the other and hold the other harmless from and
against any damages, liability or expense associated with claims by governmental
or other third parties arising out of the presence, removal or remediation of
Hazardous Materials for which the indemnifying party is responsible for
compliance, removal or remediation under this Lease.

                                   ARTICLE VI

                                TENANT COVENANTS

     The Tenant covenants during the Term and for such further time as the
Tenant occupies any part of the Premises:

     Section 6.1 PERMITTED USES. The Tenant shall occupy the Premises only for
the Permitted Uses, and shall not injure or deface the Premises or the Property,
nor permit in the Premises any auction sale. The Tenant shall give written
notice to the Landlord, within twenty (20) days after the Rent Commencement Date
and thereafter once annually within twenty (20) days of each anniversary of the
Rent Commencement Date, of any materials on OSHA's right to know list or which
are subject to regulation by any other federal, state, municipal or other
governmental authority and which the Tenant intends to have present at the
Premises. The Tenant shall comply with all requirements of public authorities
and of the Board of Fire Underwriters in connection with methods of storage, use
and disposal thereof although nothing herein shall prevent the Tenant from
challenging the validity of such requirements. The Tenant shall not permit in
the Premises any nuisance, or the emission from the Premises of any
objectionable noise, odor or vibration, nor use or devote the Premises or any
part thereof for any purpose which is contrary to law or ordinance, or liable to
invalidate or increase premiums (above those normally incurred for Permitted
Uses) for any insurance on the Building or its contents (unless the Tenant pays
for any such increase in premiums or liable to render necessary any alteration
or addition to the Building, nor commit or permit any waste in or with respect
to the Premises.

                                       26

<PAGE>

     Section 6.2 LAWS AND REGULATIONS. The Tenant shall comply with all federal,
state and local laws, regulations, ordinances. executive orders, federal
guidelines, and similar requirements in effect from time to time, including,
without limitation, City of Cambridge ordinances numbered 1005 and 1086 and any
subsequently adopted ordinance for employment and animal experimentation with
respect to animal experiments and hazardous waste and any such requirements
pertaining to employment opportunity, anti-discrimination and affirmative
action. Tenant shall have the right to contest any notice of violation for any
of the foregoing by appropriate proceedings diligently conducted in good faith.

     Section 6.3 RULES AND REGULATIONS; SIGNS. The Tenant agrees to comply with
such reasonable and non-discriminatorily enforced rules and regulations of
general applicability ("Rules and Regulations") as (i) may from time to time be
made by the Landlord of which the Tenant is given written notice, so far as the
same relate to the use of the Building, the Land and the Tenant's appurtenant
parking privileges and (ii) may from time to time be promulgated under the
Declaration of Covenants (subject in all events to Section 12.20, hereof) with
respect to all or any portion of University Park. In the event of any conflict
between the provisions of this Lease and the provisions of any Rules and
Regulations made by the Landlord, the provisions of this Lease shall control.
The Tenant shall not obstruct in any manner any portion of the Property; and,
except as set forth in this Lease, shall not permit the placing of any signs,
curtains, blinds, shades, awnings or flagpoles, or the like, visible from
outside the Building.

     The Tenant shall have the right to install a single sign with its corporate
logo on the facade of the Building. Any such exterior sign, however, shall be
subject to prior approval by the City of Cambridge, and Landlord's reasonable
approval regarding the content, size, design and location on the Building facade
and all signs must be consistent with all applicable legal requirements.

     Section 6.4 SAFETY COMPLIANCE. The Tenant shall keep the Premises equipped
with all safety appliances required by law or ordinance or any other regulations
of any public authority because of the manner of use made by the Tenant and to
procure all licenses and permits so required because of such manner of use and,
if requested by the Landlord, do any work so required because of such use, it
being understood that the foregoing provisions shall not be construed to broaden
in any way the Tenant's Permitted Uses. Tenant shall conduct such periodic
tests, evaluations or certifications of safety appliances and laboratory
equipment as are required or recommended in accordance with generally accepted
standards for good laboratory practice to ensure that such safety appliances and
equipment remain in good working order, and shall, upon Landlord's reasonable
request but not more often than two (2) times in any Fiscal Year, provide to
Landlord copies of such reports, evaluations and certifications.

     Section 6.5 LANDLORD'S ENTRY. The Tenant shall permit the Landlord and it
agents, after reasonable notice except in the case of emergencies, to enter the
Premises at all reasonable hours for the purpose of inspecting or making repairs
to the same, monitoring Tenant's compliance with the requirements and
restrictions set forth in this Lease, and for the purpose of

                                       27

<PAGE>

showing the Premises to prospective purchasers and mortgagees at all reasonable
times and to prospective tenants within twelve (12) months of the end of the
Term provided that in connection with such entry, Tenant may provide procedures
reasonably designed so as not to jeopardize Tenant's trade secrets, proprietary
technology or critical business operations, including accompaniment of all such
persons by an employee of the Tenant. In case of an emergency, the Landlord
shall make good faith efforts to notify the Tenant in person or by telephone
prior to such entry, and in any event, the Landlord shall notify Tenant promptly
thereafter such entry.

     Section 6.6 FLOOR LOAD. The Tenant shall not place a load upon any floor in
the Premises exceeding the floor load per square foot of area which such floor
was designed to carry as set forth in Schedule C-1 of the Work Letter attached
hereto. The Tenant's machines and mechanical equipment shall be placed and
maintained by the Tenant at the Tenant's expense in settings sufficient to
absorb or prevent vibration or noise that may be transmitted to the Building
structure or to any other space in the Building.

     Section 6.7 PERSONAL PROPERTY TAX. The Tenant shall pay promptly when due
all taxes which may be imposed upon personal property (including, without
limitation, fixtures and equipment) in the Premises to whomever assessed. Tenant
shall have the right to contest the validity or amount of any such taxes by
appropriate proceedings diligently conducted in good faith.

     Section 6.8 ASSIGNMENT AND SUBLEASES. The Tenant shall not assign this
Lease or sublet (which term, without limitation, shall include granting of
concessions, licenses and the like) the whole or any part of the Premises (each
a "Transfer") without, in each instance, having first received the consent of
the Landlord which consent shall not be unreasonably withheld or delayed;
provided, however, that a Transfer shall include an assignment only to the
extent that it is an assignment of the Lease to a successor tenant and not a
collateral assignment. Tenant shall have no right to transfer its roof rights
granted under Section 2.2(b) other than in connection with a Transfer of the
whole or any part of the Premises for purposes of enabling the transferee to
occupy the same for the conduct of its business therein (provided that the
occupancy of part of the Premises in service of a business the substantial
orientation of which is roof communications shall not qualify as such an
occupancy). Except as specifically permitted herein, any Transfer made without
such consent shall be void, and in no event shall the Tenant have the right to
mortgage, pledge, hypothecate or otherwise transfer this Lease. The Landlord
shall not be deemed to be unreasonable in withholding its consent to any
proposed Transfer by the Tenant based on any of the following factors:

         (a) If the manner in which the proposed occupant conducts its business
     operations is not consistent, in Landlord's reasonable opinion, with the
     image and character of the University Park development as a first-class
     office/research and development park then the Landlord may reasonably
     withhold its consent.

         (b) If Landlord or any affiliate of the Landlord has already issued a
     written proposal to or received a written proposal from or on behalf of the
     proposed occupant

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<PAGE>

     regarding space within University Park that is or is to become available
     for lease, then the Landlord may reasonably withhold its consent.

If the Tenant desires to contest the Landlord's withholding of consent to any
proposed Transfer based on any of the foregoing factors, then Tenant shall,
within sixty (60) days after receipt of the Landlord's notice withholding
consent, send written notice of such disagreement to the Landlord. After the
Landlord's receipt of such notice, the Landlord and the Tenant shall have thirty
(30) days within which to resolve such controversy after which such controversy
shall be resolved by submittal to the binding Dispute Resolution Process
described in Exhibit J attached hereto and made a part hereof.

     Notwithstanding anything to the contrary contained in this Section, Tenant
shall have the right to assign or otherwise transfer this Lease or the Premises,
or part of the Premises, without obtaining the prior consent of Landlord, (a) to
an entity owning a majority of Tenant or to a majority owned subsidiary or to an
entity which is majority owned by the same entity which owns a majority of
Tenant (any of the foregoing being referred to herein as a "Tenant Affiliate"),
provided that (i) the transferee shall, subject to applicable law, regulation or
prior binding agreement, prior to the effective date of the transfer, deliver to
Landlord instruments evidencing such transfer and its agreement to assume and be
bound by all the terms, conditions and covenants of this Lease to be performed
by Tenant, all in form reasonably acceptable to Landlord, and (ii) at the time
of such transfer there shall not be an uncured Event of Default under this
Lease; or (b) to the purchaser of at least fifty percent (50%) of its assets or
stock, or to any entity into which the Tenant may be merged or consolidated
(along with all or substantially all of its assets) (the "Acquiring Company"),
provided that (i) the net worth of the Acquiring Company upon the consummation
of the transfer or merger shall not be less than the net worth of the Tenant at
the time immediately prior to such transfer or merger, (ii) the Acquiring
Company continues to operate the business conducted in the Premises consistent
with the Permitted Uses described in Exhibit A hereto, (iii) the Acquiring
Company shall assume in writing, in form reasonably acceptable to Landlord, all
of Tenant's obligations under this Lease, (iv) Tenant shall provide to Landlord
such additional information regarding the Acquiring Company as Landlord shall
reasonably request, and (v) Tenant shall pay Landlord's reasonable out-of-pocket
expenses incurred in connection therewith. Unless Landlord shall have objected
to such assignment or transfer by Tenant within ten (10) business days following
Landlord's receipt of the information or items described in (b)(i) and (iii)
above, then Landlord shall be deemed to have waived its right to object thereto.
Each of the transfers described in this paragraph is referred to hereinafter as
"Permitted Transfers." In no event shall any transaction consummated for the
purpose of evading Tenant's obligation to obtain Landlord's consent under this
Section 6.8 be construed as a Permitted Transfer, notwithstanding that such
transaction otherwise qualifies as a Permitted Transfer.

     Whether or not the Landlord consents, or is required to consent, to any
Transfer, the Tenant named herein shall remain fully and primarily liable for
the obligations of the tenant hereunder, including, without limitation, the
obligation to pay Annual Fixed Rent and Additional Rent provided under this
Lease.

                                       29

<PAGE>

     The Tenant shall give the Landlord notice of any proposed Transfer, whether
or not the Landlord's consent is required hereunder, specifying the provisions
thereof, including (i) the name and address of the proposed subtenant, assignee,
mortgagee or other transferee, (ii) a copy of the proposed subtenant's,
assignee's, mortgagee's or other transferee's most recent annual financial
statement, (iii) all of the terms and provisions upon which the proposed
Transfer is to be made including, without limitation, all of the documentation
effectuating such Transfer (which shall be subject to the Landlord's approval
not to be unreasonably withheld) and such other reasonable information
concerning the proposed Transfer or concerning the proposed subtenant, assignee,
mortgagee or other transferee as the Tenant has obtained in connection with the
proposed Transfer. The Tenant shall reimburse the Landlord promptly for
reasonable legal and other expenses incurred by the Landlord in connection with
any request by the Tenant for consent to any Transfer. If this Lease is
assigned, or if the Premises or any part thereof is sublet or occupied by anyone
other than the Tenant, or there is otherwise a Transfer after an Event of
Default the Landlord may, at any time and from time to time, collect rent and
other charges from the assignee, sublessee, occupant, mortgagee or transferee
and apply the net amount collected to the rent and other charges herein
reserved, but no such assignment, subletting, occupancy or collection shall be
deemed a waiver of the prohibitions contained in this Section 6.8 or the
acceptance of the assignee, sublessee or occupant as a tenant or a release of
the Tenant from the further performance by the Tenant of covenants on the part
of the Tenant herein contained. The Tenant shall pay to the Landlord fifty
percent (50%) of any amounts the Tenant receives from any subtenant, assignee,
mortgagee or other transferee as rent, additional rent or other forms of
compensation or reimbursement other than those which are less than or equal to
the aggregate of (i) the then due and payable (present valued in a manner
reasonably satisfactory to Landlord in the case of an assignment or similar
Transfer) proportionate monthly share of Annual Fixed Rent, Additional Rent and
all other monies due to Landlord pursuant to this Lease (allocable in the case
of a sublease to that portion of the Premises being subleased) and (ii) the
reasonable transaction costs associated with such a transaction, which shall
include brokerage commissions, fees for legal services and any expenses of
preparing the Premises or applicable portion thereof for occupancy by such
subtenant, assignee, mortgagee or other transferee (provided that only the
monthly amortization of the expenses incurred in preparing the space for
occupancy by such transferee, utilizing an interest component reasonably
satisfactory to Landlord, may be included in this calculation in case of a
sublease or similar Transfer). The preceding sentence shall not apply to any
Permitted Transfers. Neither the fact that the Landlord's consent may not be
required in order for the Tenant to effectuate a Permitted Transfer, nor the
consent by the Landlord to a Transfer for which the Landlord's consent is
required shall be construed to relieve the Tenant from obtaining the express
consent in writing of the Landlord to any further Transfer whether by the Tenant
or by anyone claiming by, through or under the Tenant including, without
limitation, any assignee, subtenant, mortgagee or other transferee, excluding
any Permitted Transfer.

     Landlord may elect, within thirty (30) days of receipt of written notice
from Tenant of any proposed assignment of Tenant's entire interest under this
Lease prior to approving or disapproving any such proposed assignment, to
repossess the Premises. Landlord may thereafter

                                       30

<PAGE>

lease the Premises in such a manner as the Landlord may in its sole discretion
determine. In the event Landlord elects to repossess the Premises as provided
above, then all of the Tenant's rights and obligations hereunder with respect to
the Premises shall cease and shall be of no further force and effect. The
provisions of this paragraph shall not apply to Permitted Transfers.

                                   ARTICLE VII

                             INDEMNITY AND INSURANCE

     Section 7.1 INDEMNITY. The Tenant shall indemnify and save harmless the
Landlord and the Landlord's ground lessees, mortgagees and managing agent for
the Building from and against all claims, loss, or damage of whatever nature
arising from (i) any breach by Tenant of any obligation of Tenant under this
Lease, or (ii) from any negligence or misconduct of the Tenant, or the Tenant's
contractors, licensees, invitees, agents, servants or employees, or (iii)
arising from any accident, injury or damage whatsoever caused to any person or
property in or about the Premises, occurring after the initial occupancy of all
or any portion of the Premises and until the end of the Term and thereafter, so
long as the Tenant is in occupancy of any part of the Premises, provided that
the foregoing indemnity shall not include any claims, loss or damage to the
extent arising from any negligence or misconduct of the Landlord, or the
Landlord's contractors, licensees, agents, servants or employees or the
Landlord's ground lessees, mortgagees or managing agent for the Building.

     The Landlord shall indemnify and save harmless the Tenant from and against
all claims, loss, or damage of whatever nature arising from (i) any breach by
Landlord of any obligation of Landlord under this Lease or (ii) from any
negligence or misconduct of the Landlord, or the Landlord's contractors,
licensees, agents, servants or employees, occurring after the initial occupancy
of the Premises provided that the foregoing indemnity shall not include any
claims, loss or damage to the extent arising from any act, omission or
negligence of the Tenant, or the Tenant's contractors, licensees, invitees,
agents, servants or employees.

     The foregoing indemnity and hold harmless agreements shall include
indemnity against reasonable attorneys' fees and all other costs, expenses and
liabilities incurred in connection with any such claim or proceeding brought
thereon, and the defense thereof, but shall be subject to the limitation
specified in Section 12.15.

     Section 7.2 LIABILITY INSURANCE. The Tenant agrees to maintain in full
force from the date upon which the Tenant first enters the Premises for any
reason, throughout the Term, and thereafter, so long as the Tenant is in
occupancy of any part of the Premises, a policy of comprehensive general
liability insurance under which the Landlord (and any individuals or entities
affiliated with the Landlord, any ground lessor and any holder of a mortgage on
the Property of whom the Tenant is notified by the Landlord) and the Tenant are
named as insureds, and under which the insurer provides a contractual liability
endorsement insuring against all cost, expense and liability arising out of or
based upon any and all claims, accidents, injuries and

                                       31

<PAGE>

damages described in Section 7.1, in the broadest form of such coverage from
time to time available. Each such policy shall be noncancellable and
nonamendable (to the extent that any proposed amendment reduces the limits or
the scope of the insurance required in this Lease) with respect to the Landlord
and such ground lessors and mortgagees without thirty (30) days' prior notice to
the Landlord and such ground lessors and mortgagees and at the election of the
Landlord, either a certificate of insurance or a duplicate original policy shall
be delivered to the Landlord. The minimum limits of liability of such insurance
as of the Commencement Date shall be Ten Million Dollars ($10,000,000.00) for
combined bodily injury (or death) and damage to property (per occurrence) with
an aggregate annual limit of liability of Ten Million Dollars ($10,000,000.00),
and from time to time during the Term such limits of liability shall be
increased to reflect such higher limits as are customarily required pursuant to
new leases of space in the Boston-Cambridge area with respect to similar
properties.

     Section 7.3 PERSONAL PROPERTY AT RISK. The Tenant agrees that all of the
furnishings, fixtures, equipment, effects and property of every kind, nature and
description of the Tenant and of all persons claiming by, through or under the
Tenant which, during the continuance of this Lease or any occupancy of the
Premises by the Tenant or anyone claiming under the Tenant which, during the
continuance of this Lease or any occupancy of the Premises by the Tenant or
anyone claiming under the Tenant, may be on the Premises or elsewhere in the
Building or on the Land or parking facilities provided hereby, shall be at the
sole risk and hazard of the Tenant, and if the whole or any part thereof shall
be destroyed or damaged by fire, water or otherwise, or by the leakage or
bursting of water pipes, steam pipes, or other pipes. by theft or from any other
cause, no part of said loss or damage is to be charged to or be borne by the
Landlord, except that the Landlord shall in no event be exonerated from any
liability to the Tenant or to any person, for any injury, loss, damage or
liability to the extent caused by Landlord's or its employees', agents' or
contractors' negligence or willful misconduct.

         Section 7.4 LANDLORD'S INSURANCE. The Landlord shall carry such all
risk casualty and liability insurance upon and with respect to operations at the
Building as may from time to time be deemed reasonably prudent by the Landlord
with deductibles in amounts carried at comparable buildings with similar uses
within the Cambridge market or required by any mortgagee holding a mortgage
thereon or any ground lessor of the Land, in an amount equal to the replacement
cost of the Building, including all leasehold improvements, exclusive of
foundations, site preparation and other nonrecurring construction costs. During
the period of construction of the Building, prior to the Rent Commencement Date,
Landlord shall carry Builder's Risk insurance with respect to the Building, and
the cost thereof shall be allocated between Landlord and Tenant based on the
proportionate costs of construction work to be done by Landlord (as described in
the Work Letter) and construction work to be done by Tenant (as described in the
Work Letter).

     Section 7.5 WAIVER OF SUBROGATION. Any insurance carried by either party
with respect to the Building, Land, Premises, parking facilities or any property
therein or occurrences thereon shall, without further request by either party,
if it can be so written without additional premium, or with an additional
premium which the other party elects to pay, include a clause or

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endorsement denying to the insurer rights of subrogation against the other party
to the extent rights have been waived by the insured prior to occurrence of
injury or loss. Each party, notwithstanding any provisions of this Lease to the
contrary, hereby waives any rights of recovery against the other for injury or
loss, including, without limitation, injury or loss caused by negligence of such
other party, due to hazards covered by insurance containing such clause or
endorsement to the extent of the indemnification received thereunder.

     Section 7.6 POLICY REQUIREMENTS. Any required insurance may be in the form
of blanket coverage, so long as the coverage required herein is maintained. Each
party shall cause a certificate, providing such information as reasonably
requested by the other party, evidencing the existence and limits of its
insurance coverage with respect to the Premises and the Building, as the case
may be, to be delivered to such other party upon the commencement of the Term.
Thereafter, each party shall cause similar certificates evidencing renewal
policies to be delivered to such other party at least thirty (30) days prior to
the expiration of the term of each policy and at such other times as reasonably
requested by the other party. The insurance policies and certificates required
by this Article VII shall contain a provision requiring the insurance company to
furnish Landlord and Tenant, as the case may be, thirty (30) days' prior written
notice of any cancellation or lapse, or the effective date of any reduction in
the amounts or scope of coverage.

                                  ARTICLE VIII

                           CASUALTY AND EMINENT DOMAIN

     Section 8.1 RESTORATION FOLLOWING. If, during the Term, the Building shall
be damaged by fire or casualty, subject to termination rights of the Landlord
and the Tenant provided below in this Article VIII, the Landlord shall proceed
promptly to exercise diligent efforts to restore, or cause to be restored, the
Building, to substantially the condition thereof just prior to time of such
damage, but the Landlord shall not be responsible for delay in such restoration
which may result from External Causes. The Landlord shall have no obligation to
expend in the reconstruction of the Building more than the sum of the amount of
any deductible and the actual amount of insurance proceeds made available to the
Landlord by its insurer. Any restoration of the Building shall be altered to the
extent necessary to comply with then current and applicable laws and codes.

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         Section 8.2 LANDLORD'S TERMINATION ELECTION. If the Landlord reasonably
determines, based upon certification by its architect or other design
professional, that (a) the amount of insurance proceeds available to the
Landlord is insufficient (by more than the amount of any deductible) to cover
the cost of restoring the Building, or (b) the Landlord will be unable to
restore the Building within twelve (12) months from the date of the casualty,
then the Landlord may terminate this Lease by giving notice to the Tenant. Any
such termination shall be effective on the date designated in such notice from
the Landlord, but in any event not later than sixty (60) days after such notice,
and if no date is specified, effective upon the delivery of such notice. Failure
by the Landlord to give the Tenant notice of termination within ninety (90) days
following the occurrence of the casualty shall constitute the Landlord's
agreement to restore the Building as contemplated in Section 8.1. A termination
of the Lease shall automatically terminate any separate parking lease that may
exist.

         Section 8.3 TENANT'S TERMINATION ELECTION. If the Landlord has not
terminated this Lease under Section 8.2, but the Landlord has failed to restore
the Premises, within twelve (12) months from the date of the casualty or taking,
such period to be subject, however, to extension (not exceeding ninety (90)
days) where the delay in completion of such work is due to External Causes or
the inability of the Landlord to obtain permits for the restoration work, the
Tenant shall have the right to terminate this Lease at any time after the
expiration of such 12-month period (as extended by such delay (not exceeding
ninety (90) days) due to External Causes or the inability of the Landlord to
obtain permits for the restoration work), until the restoration is substantially
completed, such termination to take effect as of the date of the Tenant's
notice. However, if the Landlord reasonably determines at any time, and from
time to time, during the restoration, based upon certification by its architect
or other design professional, that such restoration will not be able to be
completed before the deadline date after which the Tenant may terminate this
Lease under this Section 8.3, and the Landlord specifies in a notice to Tenant
to such effect a later date that the Landlord estimates will be the date upon
which such restoration will be completed, then the Tenant may terminate this
Lease within ten (10) days of the Landlord's notice as aforesaid, failing which
the deadline date shall be extended to the date set forth in Landlord's notice
(as extended by delay due to External Causes or the inability of the Landlord to
obtain permits for the restoration work). A termination of the Lease shall
automatically terminate any separate parking lease that may exist. The Landlord
shall exercise reasonable efforts to keep the Tenant advised of the status of
restoration work from time to time, and promptly following any request for
information during the course of the performance of the restoration work.

     Section 8.4 CASUALTY AT EXPIRATION OF LEASE. If the Premises shall be
damaged by fire or casualty in such a manner that the Premises cannot, in the
ordinary course, reasonably be expected to be repaired within one hundred twenty
(120) days from the commencement of repair work and such damage occurs within
the last eighteen (18) months of the Term (as the same may have been extended
prior to such fire or casualty), either party shall have the right, by giving
notice to the other not later than sixty (60) days after such damage, to
terminate this Lease, whereupon this Lease shall terminate as of the date of
such notice. Notwithstanding the foregoing, the Landlord shall not have the
right to terminate this Lease as aforesaid provided that

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<PAGE>

the Tenant shall have exercised its right to extend the Term of this Lease
pursuant to Section 2.6 hereof not later than forty-five (45) days after the
date of damage to the Premises.

     Section 8.5 EMINENT DOMAIN. Except as hereinafter provided, if the
Premises, or such portion thereof as to render the balance (if reconstructed to
the maximum extent practicable in the circumstances) unsuitable for the Tenant's
purposes as contemplated under this Lease, shall be taken by condemnation or
right of eminent domain, the Landlord or the Tenant shall have the right to
terminate this Lease and any separate parking lease by notice to the other of
its desire to do so, provided that such notice is given not later than thirty
(30) days after receipt by the Tenant of notice of the effective date of such
taking. If so much of the Building shall be so taken that the Landlord
reasonably determines that it would be reasonably necessary to raze or
substantially alter the Building, the Landlord shall have the right to terminate
this Lease by giving notice to the Tenant of the Landlord's desire to do so not
later than thirty (30) days after the effective date of such taking.

     Should any part of the Premises be so taken or condemned during the Term,
and should this Lease be not terminated in accordance with the foregoing
provisions, the Landlord agrees to use reasonable efforts to put what may remain
of the Premises into proper condition for use and occupation as nearly like the
condition of the Premises prior to such taking as shall be practicable, subject,
however, to applicable laws and codes then in existence.

     Section 8.6 RENT AFTER CASUALTY OR TAKING. If the Premises shall be damaged
by fire or other casualty, until the Lease is terminated or the Premises is
restored, the Annual Fixed Rent and Additional Rent shall be justly and
equitably abated and reduced according to the nature and extent of the loss of
use thereof suffered by the Tenant. In the event of a taking which permanently
reduces the area of the Premises, a just proportion of the Annual Fixed Rent
shall be abated for the remainder of the Term.

     Section 8.7 TEMPORARY TAKING. In the event of any taking of the Premises or
any part thereof for a temporary use not in excess of twelve (12) months, (i)
this Lease shall be and remain unaffected thereby and Annual Fixed Rent and
Additional Rent shall not abate, and (ii) the Tenant shall be entitled to
receive for itself such portion or portions of any award made for such use with
respect to the period of the taking which is within the Term.

     Section 8.8 TAKING AWARD. Except as otherwise provided in Section 8.7, the
Landlord shall have and hereby reserves and accepts, and the Tenant hereby
grants and assigns to the Landlord, all rights to recover for damages to the
Building and the Land, and the leasehold interest hereby created, and to
compensation accrued or hereafter to accrue by reason of such taking, damage or
destruction, as aforesaid, and by way of confirming the foregoing, the Tenant
hereby grants and assigns to the Landlord, all rights to such damages or
compensation. Nothing contained herein shall be construed to prevent the Tenant
from prosecuting in any condemnation proceedings a claim for relocation expenses
and improvements made by the Tenant in the Premises that constitute Tenant's
personal property, including the Removable Alterations.

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<PAGE>

     Section 8.9 CASUALTY OR EMINENT DOMAIN OR CONSTRUCTION DELAYS AFFECTING
PARKING PRIVILEGES. If, during the Term, there shall be damage by fire or
casualty, or a taking by condemnation or right of eminent domain (including a
change in law, ordinance, or regulation which amounts to a regulatory taking by
depriving Tenant of its parking privileges described herein), or if delays occur
in the construction of the 80 Landsdowne garage so that it is not completed upon
Tenant's taking occupancy of the Premises, in either event resulting in the
Landlord reducing the number of Parking Passes that permit the Tenant to park in
parking spaces at 55 Franklin Street and/or 80 Landsdowne Street (a "University
Park Parking Privileges Reduction"), then notwithstanding anything to the
contrary provided in Section 12.14 and EXHIBIT A, the Landlord may substitute
alternative parking arrangements ("Temporary Substitute Parking Arrangements")
within a one-mile radius of University Park that will provide parking areas with
respect to which the Tenant may (using the Parking Passes that may no longer be
used for parking within University Park) enjoy the parking privileges
contemplated hereby, provided that any such alternative parking areas shall be,
at Landlord's expense, reasonably well lit, and there shall be provided shuttle
bus transportation, also at Landlord's expense, to and from University Park from
such alternative parking areas, on terms reasonably satisfactory to the Tenant.
The Market Rate Parking Charge shall be abated during the duration of such
Temporary Substitute Parking Arrangements. The Landlord's furnishing of
Temporary Substitute Parking Arrangements shall constitute a satisfactory curing
of the adverse effects of a University Park Parking Privileges Reduction,
subject to the further provisions of this Section 8.9 below.

     The Landlord hereby covenants and agrees that any University Park Parking
Privileges Reduction shall be made on a fair and equitable basis with respect to
all of the tenants in University Park. The Landlord shall, within sixty (60)
days of any fire, casualty or taking affecting parking facilities at 55 Franklin
Street and/or 80 Landsdowne Street that results in a University Park Parking
Privileges Reduction, give the Tenant written notice ("Parking Notice") (i)
describing to what extent the Landlord will be able to provide Temporary
Substitute Parking Arrangements that fulfill the requirements of the immediately
prior paragraph of this Section 8.9 and (ii) providing an estimate of the likely
duration of such University Park Parking Privileges Reduction. The Landlord
shall, in the Parking Notice, designate the number of affected Parking Passes
and the location of the substitute parking areas, and cooperate with the Tenant
to establish reasonably satisfactory terms for shuttle bus transportation, as
soon as reasonably possible following the occurrence of an event resulting in a
University Park Parking Privileges Reduction.

     If the Landlord (i) does not, in the Parking Notice, timely make Temporary
Substitute Parking Arrangements that fulfill the requirements of the first
paragraph of this Section 8.9 with respect to at least seventy-five percent
(75%) of the Parking Passes that are made subject to the University Park Parking
Privileges Reduction, (ii) specifies in the Landlord's estimate of the likely
duration of the University Park Parking Privileges Reduction that the period
during which the Tenant must suffer a reduction of more than seventy-five
percent (75%) of its Parking Passes (I.E., not replaced through Temporary
Substitute Parking Arrangements) will exceed one (1) year, or (iii) specifies in
the Landlord's estimate of the likely duration of the University Park Parking
Privileges Reduction that the period during which a University Park Parking
Privileges Reduction will exist will likely exceed twenty four (24) months, then
the Tenant shall have the

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<PAGE>

right, exercisable upon written notice to the Landlord within thirty (30) days
after the Tenant receives the Parking Notice, to terminate this Lease.

     If the Tenant does not exercise its termination right, as aforesaid, then
the Landlord shall, to the extent practicable, proceed promptly after the
occurrence of a University Park Parking Privileges Reduction affecting the
Tenant to construct, restore, or cause to be restored, sufficient parking areas
in University Park to permit the Tenant to have the number of Parking Passes
contemplated in EXHIBIT A to be able to be used in parking areas within
University Park. However, if the Landlord is unable to so construct or restore
the Tenant's parking privileges within University Park within twenty four (24)
months from the date of the University Park Parking Privileges Reduction, then
the Tenant shall have the right to terminate this Lease by notice to the
Landlord of its desire to do so, provided such notice is given not later than
thirty (30) days after the Landlord notifies the Tenant as aforesaid.

     The Tenant's right to receive rent-free Temporary Substitute Parking
Arrangements and termination rights set forth in this Section 8.9 shall be the
Tenant's sole remedies for the Landlord's inability to provide the parking
privileges contemplated under this Lease in the event of construction delays
regarding the 80 Landsdowne garage or a casualty or taking affecting parking 55
Franklin garage or the 80 Landsdowne garage, provided that the Landlord has
complied with its obligation to act in a fair and equitable basis. Any such
termination of this Lease by the Tenant, as aforesaid, shall be effective on the
date specified by the Tenant in such termination notice, which specified
effective date may be up to twenty-four (24) months after the date of
termination notice. Such termination notice shall be binding on both Landlord
and Tenant, despite the fact that Landlord may subsequently complete the
construction or restoration of the affected garage. During such period after the
termination notice hereunder and until the effective date of such termination
notice, Landlord and Tenant shall continue to have all of their respective
rights and obligations specified in this Lease. Failure by the Tenant to timely
exercise any right to terminate this Lease under this Section 8.9 shall result
in the waiver by the Tenant of any such right to terminate this Lease.

                                   ARTICLE IX

                                     DEFAULT

     Section 9.1 TENANT'S DEFAULT. Each of the following shall constitute an
Event of Default:

         (a) Failure on the part of the Tenant to pay the Annual Fixed Rent,
     Additional Rent or other charges for which provision is made herein on or
     before the date on which the same become due and payable, if such condition
     continues for five (5) business days after written notice that the same are
     due; provided, however if Tenant shall fail to pay any of the foregoing
     when due two (2) times in any period of twelve (12) consecutive months,
     then Landlord shall not be required to give notice to Tenant of any future
     failure

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<PAGE>

     to pay during the remainder of the Term and any extension thereof, and such
     failure shall thereafter constitute an Event of Default if not cured within
     five (5) days after the same are due.

         (b) Failure on the part of the Tenant to perform or observe any other
     term or condition contained in this Lease if the Tenant shall not cure such
     failure within thirty (30) days after written notice from the Landlord to
     the Tenant thereof, provided that in the case of breaches that are not
     reasonably susceptible to cure within thirty (30) days through the exercise
     of due diligence, then so long as the Tenant commences such cure within
     thirty (30) days, and the Tenant diligently pursues such cure to
     completion, such breach shall not be deemed to create an Event of Default.

         (c) The taking of the estate hereby created on execution or by other
     process of law; or a judicial declaration that the Tenant is bankrupt or
     insolvent according to law; or any assignment of the property of the Tenant
     for the benefit of creditors; or the appointment of a receiver, guardian,
     conservator, trustee in bankruptcy or other similar officer to take charge
     of all or any substantial part of the Tenant's property by a court of
     competent jurisdiction, which officer is not dismissed or removed within
     ninety (90) days; or the filing of an involuntary petition against the
     Tenant under any provisions of the bankruptcy act now or hereafter enacted
     if the same is not dismissed within ninety (90) days; the filing by the
     Tenant of any voluntary petition for relief under provisions of any
     bankruptcy law now or hereafter enacted.

     If an Event of Default shall occur, then, in any such case, whether or not
the Term shall have begun, the Landlord lawfully may, immediately or at any time
thereafter, give notice to the Tenant specifying the Event of Default and this
Lease shall come to an end on the date specified therein as fully and completely
as if such date were the date herein originally fixed for the expiration of the
Lease Term, and the Tenant will then quit and surrender the Premises to the
Landlord, but the Tenant shall remain liable as hereinafter provided.

     Section 9.2 DAMAGES. In the event that this Lease is terminated, the Tenant
covenants to pay to the Landlord forthwith on the Landlord's demand, as
compensation, an amount (the "Lump Sum Payment") equal to the excess, if any, of
the discounted present value of the total rent reserved for the remainder of the
Term over the then discounted present fair rental value of the Premises for the
remainder of the Term. The discount rate for calculating such sum shall be the
then current rate of United States Treasury securities having a maturity date as
close as possible to the end of the Term (had the Lease not been terminated). In
calculating the rent reserved, there shall be included, in addition to the
Annual Fixed Rent and all Additional Rent, the value of all other considerations
agreed to be paid or performed by the Tenant over the remainder of the Term. In
addition, the Tenant shall pay punctually to the Landlord all the sums
("Periodic Payments") and perform all the obligations which the Tenant covenants
in this Lease to pay and to perform in the same manner and to the same extent
and at the same time as if this Lease had not been terminated until such time as
the entire Premises has been relet and the term under the new lease has
commenced. In calculating the amounts to be paid by the Tenant under

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<PAGE>

the foregoing covenant, the Tenant shall be credited with the net proceeds of
any rent obtained by reletting the Premises, after deducting all the Landlord's
expenses in connection with such reletting, including, without limitation, all
repossession costs, brokerage commissions, fees for legal services and expenses
of preparing the Premises for such reletting. The Tenant shall also be entitled
to credit against such payments which would otherwise become due the amount, if
any, paid to the Landlord as a Lump Sum Payment. The Landlord may (i) relet the
Premises, or any part or parts thereof, for a term or terms which may, at the
Landlord's option, exceed or be equal to or less than the period which would
otherwise have constituted the balance of the Term, and may grant such
concessions and free rent as the Landlord in its reasonable commercial judgment
considers advisable or necessary to relet the same and (ii) make such
alterations, repairs and improvements in the Premises as the Landlord in its
reasonable commercial judgment considers advisable or necessary to relet the
same. No action of the Landlord in accordance with foregoing or failure to relet
or to collect rent under-reletting shall operate to release or reduce the
Tenant's liability. The Landlord shall be entitled to seek to rent other
properties of the Landlord prior to reletting the Premises. Notwithstanding the
foregoing, the Landlord shall offer such Premises to lease in the same manner as
the Landlord offers other vacant space for lease in University Park.

     Section 9.3 CUMULATIVE RIGHTS. The specific remedies to which the Landlord
may resort under the terms of this Lease are cumulative and are not intended to
be exclusive of any other remedies or means of redress to which it may be
lawfully entitled in case of any breach or threatened breach by the Tenant of
any provisions of this Lease. In addition to the other remedies provided in this
Lease, the Landlord shall be entitled to seek the restraint by injunction of the
violation or attempted or threatened violation of any of the covenants,
conditions or provisions of this Lease or to a decree compelling specific
performance of any such covenants, conditions or provisions. Nothing contained
in this Lease shall limit or prejudice the right of the Landlord to prove for
and obtain in proceedings for bankruptcy, insolvency or like proceedings by
reason of the termination of this Lease, an amount equal to the maximum allowed
by any statute or rule of law in effect at the time when, and governing the
proceedings in which, the damages are to be proved, whether or not the amount be
greater, equal to, or less than the amount of the loss or damages referred to
above.

     Section 9.4 LANDLORD'S SELF-HELP. If there shall be an Event of Default or
circumstances which, upon the giving of notice or passage of time would
constitute an Event of Default by the Tenant in the performance of any
obligation under this Lease, then the Landlord shall have the right, but not the
obligation, after the giving by the Landlord of notice thereof to the Tenant and
the expiration of any applicable cure period (except in case of emergency in
which case no notice need be given nor must any applicable cure period expire)
and upon reasonable, but in no event more than ten (10) days' notice to the
Tenant (except in case of emergency in which case no such additional notice need
be given), to perform such obligation (including, without limitation, stopping
any service or utility system until necessary repairs have been completed). In
the event the Landlord exercises its rights under this Section 9.4 in case of
emergency, the Landlord shall notify the Tenant as soon as reasonably possible
after the taking of such action. The Landlord may exercise its rights under this
Section without waiving any other of its rights or releasing the Tenant from any
of its obligations under this Lease. The

                                       39

<PAGE>

Tenant shall be liable to the Landlord for all of the Landlord's reasonable
costs associated with effecting such cure.

     Section 9.5 ENFORCEMENT EXPENSES; LITIGATION. Each party hereto shall
promptly reimburse the other for all costs and expenses, including without
limitation legal fees, incurred by such party in exercising and enforcing its
rights under this Lease following the other party's failure to comply with its
obligations hereunder, whether or not such failure constitutes an Event of
Default pursuant to Sections 9.1 or 9.7 hereof.

     If either party hereto be made or becomes a party to any litigation
commenced by or against the other party by or against a third party, or incurs
costs or expenses related to such litigation, involving any part of the Property
and the enforcement of any of the rights, obligations or remedies of such party,
then the party becoming involved in any such litigation because of a claim
against such other party hereto shall receive from such other party hereto all
costs and reasonable attorneys' fees incurred by such party in such litigation.

     Section 9.6 LATE CHARGES AND INTEREST ON OVERDUE PAYMENTS. In the event
that any payment of Annual Fixed Rent or Additional Rent shall remain unpaid for
a period of five (5) days following notice by the Landlord to the Tenant that
such payment is overdue, there shall become due to the Landlord from the Tenant,
as Additional Rent and as compensation for the Landlord's extra administrative
costs in investigating the circumstances of late rent, a late charge of two
percent (2%) of the amount overdue. In addition, any Annual Fixed Rent and
Additional Rent not paid within five (5) days of the date when due shall bear
interest from the date due to the Landlord until paid at the variable rate (the
"Default Interest Rate") equal to the higher of (i) the rate at which interest
accrues on amounts not paid when due under the terms of the Landlord's financing
for the Building, as from time to time in effect, and (ii) one hundred and
twenty-five percent (125%) of the rate from time to time announced by FleetBank
N.A. as its base rate, or if such rate can no longer be determined, one hundred
and twenty-five percent (125%) of the rate from time to time announced by a
major commercial bank selected by the Landlord as the rate charged to
creditworthy commercial clients for short-term unsecured borrowings.

     Section 9.7 LANDLORD'S RIGHT TO NOTICE AND CURE; TENANT'S SELF-HELP RIGHTS.
The Landlord shall in no event be in default in the performance of any of the
Landlord's obligations hereunder unless and until the Landlord shall have failed
to perform such obligations within thirty (30) days, or such additional time as
is reasonably required to correct any such default, after notice by the Tenant
to the Landlord expressly specifying wherein the Landlord has failed to perform
any such obligation. If Landlord has failed to make any repair which results in
a material risk of damage or injury to persons or property within the Premises
within thirty (30) days or such additional time as is required to make such
repair, then the Tenant shall have the right, after providing an additional ten
(10) days' written notice to the Landlord, to perform such obligation so long as
the same may be done solely on the Property or within the common areas of the
Building. Notwithstanding the foregoing, in the case of an emergency, the Tenant
shall have the right to perform any such obligation without regard to the thirty
(30) day notice period, so

                                       40

<PAGE>

long as (a) the Tenant makes a good faith attempt to notify the Landlord prior
to taking such action and (b) notifies the Landlord as soon as possible
thereafter. The Landlord shall be liable to the Tenant for all of the Tenant's
reasonable costs associated with effecting such cure, provided that in no event
shall the Tenant be entitled to abate any Annual Fixed Rent or Additional Rent
or otherwise offset such costs against sums due the Landlord under this Lease.

                                    ARTICLE X

                     MORTGAGEES' AND GROUND LESSORS' RIGHTS

     Section 10.1 SUBORDINATION. This Lease shall, at the election of the holder
of any mortgage or ground lease on the Property, be subject and subordinate to
any and all mortgages or ground leases on the Property, so that the lien of any
such mortgage or ground lease shall be superior to all rights hereby or
hereafter vested in the Tenant, provided that such mortgagee or ground lessor
shall have entered into a subordination non-disturbance and attornment agreement
with Tenant, the form of which shall be furnished by the mortgagee or ground
lessor, as the case may be, with such reasonable modifications as Tenant shall
request within a reasonable time period. The form of non-disturbance and
attornment agreement attached hereto as Exhibit H is acceptable to Tenant in
connection with the Ground Lease held by MIT (as such terms are defined in
Section 12.10). The form of subordination, non-disturbance and attornment
agreement attached hereto as Exhibit I is acceptable to the Tenant with respect
to any such agreement to be entered into during the period following Substantial
Completion of the Premises, and the form of subordination, non-disturbance and
attornment agreement attached hereto as Exhibit K is acceptable to the Tenant
with respect to any such agreement to be entered into during the period prior
thereto, in connection with any mortgage to which this Lease shall be
subordinated.

     Section 10.2 PREPAYMENT OF RENT NOT TO BIND MORTGAGEE. No Annual Fixed
Rent, Additional Rent, or any other charge payable to the Landlord shall be paid
more than thirty (30) days prior to the due date thereof under the terms of this
Lease and payments made in violation of this provision shall (except to the
extent that such payments are actually received by a mortgagee or ground lessor)
be a nullity as against such mortgagee or ground lessor and the Tenant shall be
liable for the amount of such payments to such mortgagee or ground lessor.

         Section 10.3 TENANT'S DUTY TO NOTIFY MORTGAGEE: MORTGAGEE'S ABILITY TO
CURE. No act or failure to act on the part of the Landlord which would entitle
the Tenant under the terms of this Lease, or by law, to be relieved of the
Tenant's obligations to pay Annual Fixed Rent or Additional Rent hereunder or to
terminate this Lease, shall result in a release or termination of such
obligations of the Tenant or a termination of this Lease unless (i) the Tenant
shall have first given written notice of the Landlord's act or failure to act to
the Landlord's mortgagees or ground lessors of record, if any, of whose identity
and address the Tenant shall have been given notice, specifying the act or
failure to act on the part of the Landlord which would give basis to the
Tenant's rights; and (ii) such mortgagees or ground lessors, after receipt of
such notice, have

                                       41

<PAGE>

failed or refused to correct or cure the condition complained of within a
reasonable time thereafter; which shall include a reasonable time for such
mortgagee or ground lessors, but in no event more than thirty (30) days after
receipt of such notice, to obtain possession of the Property if possession is
necessary for the mortgagee or ground lessor to correct or cure the condition
and if the mortgagee or ground lessor notifies the Tenant of its intention to
take possession of the Property and correct or cure such condition; provided,
however, nothing contained in this Section 10.3 shall affect Tenant's right (a)
to terminate this Lease under Sections 8.3, 8.4, 8.5 or 8.9 or (b) to exercise
its self-help rights under Section 9.7.

     Section 10.4 ESTOPPEL CERTIFICATES. The Tenant shall from time to time,
upon not less than fifteen (15) days' prior written request by the Landlord,
execute, acknowledge and deliver to the Landlord a statement in writing
certifying to the Landlord or an independent third party, with a true and
correct copy of this Lease attached thereto, to the extent such statements are
true and accurate, (i) that this Lease is unmodified and in full force and
effect (or, if there have been any modifications, that the same is in full force
and effect as modified and stating the modifications); (ii) that the Tenant has
no knowledge of any defenses, offsets or counterclaims against its obligations
to pay the Annual Fixed Rent and Additional Rent and to perform its other
covenants under this Lease (or if there are any defenses, offsets, or
counterclaims, setting them forth in reasonable detail); (iii) that there are no
known uncured defaults of the Landlord or the Tenant under this Lease (or if
there are known defaults, setting them forth in reasonable detail); (iv) the
dates to which the Annual Fixed Rent, Additional Rent and other charges have
been paid; (v) that the Tenant has accepted, is satisfied with, and is in full
possession of the Premises, including all improvements, additions and
alterations thereto required to be made by Landlord under the Lease; (vi) that
the Landlord has satisfactorily complied with all of the requirements and
conditions precedent to the occurrence of the Rent Commencement Date with
respect to the entire Building; (vii) that the Tenant has been in occupancy
since the Rent Commencement Date and paying rent since the specified dates;
(viii) that no monetary or other considerations, including, but not limited to,
rental concessions for Landlord, special tenant improvements or Landlord's
assumption of prior lease obligations of Tenant have been granted to Tenant by
Landlord for entering into Lease, except as specified; (ix) that Tenant has no
notice of a prior assignment, hypothecation, or pledge of rents or of the Lease;
(x) that the Lease represents the entire agreement between Landlord and Tenant;
(xi) that no prepayment or reduction of rent and no modification, termination or
acceptance of Lease will be valid as to the party to whom such certificate is
addressed without the consent of such party; (xii) that any notice to Tenant may
be given it by certified or registered mail, return receipt requested, or
delivered, at the Premises, or at another address specified; and (xiii) such
other matters with respect to the Tenant and this Lease as the Landlord may
reasonably request. On or following the Rent Commencement Date hereunder, the
Tenant shall, within ten (10) days after receipt of Landlord's request therefor,
promptly execute, acknowledge and deliver to the Landlord a statement in writing
that the Rent Commencement Date has occurred with respect to the Premises, that
the Annual Fixed Rent has begun to accrue with respect thereto, and, to the
extent true, that the Tenant has taken occupancy of the Premises. Any statement
delivered pursuant to this Section may be relied upon by any prospective
purchaser, mortgagee or ground lessor of the Premises and shall be binding on
the Tenant.

                                       42

<PAGE>

     Landlord shall from time to time, upon not less than fifteen (15) days'
prior written request by the Tenant, execute, acknowledge and deliver to the
Tenant a statement in writing certifying to the Tenant or an independent third
party, with a true and correct copy of this Lease attached thereto, to the
extent such statements are true and accurate (i) that this Lease is unmodified
and in full force and effect (or, if there have been any modifications, that the
same is in full force and effect as modified and stating the modifications);
(ii) that the Landlord has no knowledge of any defenses, offsets or
counterclaims against its obligations to perform its covenants under this Lease
(or if there are any defenses, offsets, or counterclaims, setting them forth in
reasonable detail); (iii) that there are no known uncured defaults of the Tenant
or the Landlord under this Lease (or if there are known defaults, setting them
forth in reasonable detail); (iv) the dates to which the Annual Fixed Rent,
Additional Rent and other charges have been paid; (v) that the Tenant is in full
possession of the Premises; (vi) that Landlord has no notice of a prior
assignment of the Lease or sublease of space therein; (vii) that the Lease
represents the entire agreement between Landlord and Tenant; (viii) that any
notice to Landlord may be given if by certified or registered mail, return
receipt requested, or delivered to the Landlord's address listed on Exhibit A,
or at another address specified; and (xii) such other matters with respect to
the Tenant and this Lease as the Tenant may reasonably request. Any statement
delivered pursuant to this Section may be relied upon by any prospective
assignee or sublessee of Tenant and shall be binding on the Landlord.

                                   ARTICLE XI

                    INTENTIONALLY OMITTED - SEE WORK LETTER.

                                   ARTICLE XII

                                  MISCELLANEOUS

     Section 12.1 NOTICE OF LEASE. The Tenant agrees not to record this Lease,
but upon request of either party, both parties shall execute and deliver a
memorandum of this Lease in form appropriate for recording or registration, an
instrument acknowledging the Commencement Date of the Term, and if this Lease is
terminated before the Term expires, an instrument in such form acknowledging the
date of termination.

     Section 12.2 NOTICES. Whenever any notice, approval, consent, request,
election, offer or acceptance is given or made pursuant to this Lease, it shall
be in writing. Communications and payments shall be addressed, if to the
Landlord, at the Landlord's Address for Notices as set forth in Exhibit A or at
such other address as may have been specified by prior notice to the Tenant; and
if to the Tenant, at the Tenant's Original Address or at such other place as may
have been specified by prior notice to the Landlord. Any communication so
addressed shall be deemed duly given on the earlier of (i) the date received,
(ii) on the third business day following

                                       43

<PAGE>

the day of mailing if mailed by registered or certified mail, return receipt
requested, or (iii) on the next business day if sent by a nationally recognized
overnight courier service. If the Landlord by notice to the Tenant at any time
designates some other person to receive payments or notices, all payments or
notices thereafter by the Tenant shall be paid or given to the agent designated
until notice to the contrary is received by the Tenant from the Landlord.

     Section 12.3 SUCCESSORS AND LIMITATION ON LIABILITY. The obligations of
this Lease shall run with the land, and this Lease shall be binding upon and
inure to the benefit of the parties hereto and their respective successors and
assigns, except that the original Landlord named herein and each successor
Landlord shall be liable only for obligations accruing during the period of its
ownership. The obligations of the Landlord under this Lease shall be binding
upon the assets of the Landlord consisting of an equity ownership of the
Property, including its interest under the Ground Lease (and including any
proceeds realized from the sale of such Property), but not upon other assets of
the Landlord and neither the Tenant, nor anyone claiming by, under or through
the Tenant, shall be entitled to obtain any judgment in enforcing the terms and
conditions of this Lease creating personal liability on the part of the Landlord
or enforcing any obligations of the Landlord against any assets of the Landlord
other than an equity ownership of the Property. The obligations of the Tenant
under this Lease shall be binding upon the assets of the Tenant and neither the
Landlord, nor anyone claiming by, under or through the Landlord, shall be
entitled to obtain any judgment in enforcing the terms and conditions of this
Lease creating personal liability on the part of any of Tenant's, officers,
employees, directors or shareholders.

     Section 12.4 WAIVERS BY THE LANDLORD. The failure of the Landlord or the
Tenant to seek redress for violation of, or to insist upon strict performance
of, any covenant or condition of this Lease, shall not be deemed a waiver of
such violation nor prevent a subsequent act, which would have originally
constituted a violation, from having all the force and effect of an original
violation. The receipt by the Landlord of Annual Fixed Rent or Additional Rent
with knowledge of the breach of any covenant of this Lease shall not be deemed a
waiver of such breach. No provision of this Lease shall be deemed to have been
waived by the Landlord or the Tenant, as the case may be, unless such waiver be
in writing signed by the Landlord or the Tenant, as the case may be. No consent
or waiver, express or implied, by the Landlord or Tenant to or of any breach of
any agreement or duty shall be construed as a waiver or consent to or of any
other breach of the same or any other agreement or duty.

     Section 12.5 ACCEPTANCE OF PARTIAL PAYMENTS OF RENT. No acceptance by the
Landlord of a lesser sum than the Annual Fixed Rent and Additional Rent then due
shall be deemed to be other than a partial installment of such rent due, nor
shall any endorsement or statement on any check or any letter accompanying any
check or payment as rent be deemed an accord and satisfaction, and the Landlord
may accept such check or payment without prejudice to the Landlord's right to
recover the balance of such installment or pursue any other remedy in this Lease
provided. The delivery of keys to any employee of the Landlord or to the
Landlord's agent or any employee thereof shall not operate as a termination of
this Lease or a surrender of the Premises.

                                       44

<PAGE>

     Section 12.6 INTERPRETATION AND PARTIAL INVALIDITY. If any term of this
Lease, or the application thereof to any person or circumstances, shall to any
extent be invalid or unenforceable, the remainder of this Lease, or the
application of such term to persons or circumstances other than those as to
which it is invalid or unenforceable, shall not be affected thereby, and each
term of this Lease shall be valid and enforceable to the fullest extent
permitted by law. The titles of the Articles are for convenience only and not to
be considered in construing this Lease. This Lease contains all of the
agreements of the parties with respect to the subject matter thereof and
supersedes all prior dealings between them with respect to such subject matter.

     Section 12.7 QUIET ENJOYMENT. So long as the Tenant pays Annual Fixed Rent
and Additional Rent, performs all other Tenant covenants of this Lease and
observes all conditions hereof, the Tenant shall peaceably and quietly have,
hold and enjoy the Premises free of any claims by, through or under the
Landlord.

     Section 12.8 BROKERAGE. Each party represents and warrants to the other
that it has had no dealings with any broker or agent other than Meredith & Grew,
Incorporated and Insignia/ESG (collectively, the "Broker") in connection with
this Lease and shall indemnify and hold harmless the other from claims for any
brokerage commission (other than by the Broker) arising out a breach of the
foregoing representations. Landlord shall be responsible for any commission due
to the Broker pursuant to the terms of a separate agreement.

     Section 12.9 SURRENDER OF PREMISES AND HOLDING OVER. The Tenant shall
surrender possession of the Premises on the last day of the Term and the Tenant
waives the right to any notice of termination or notice to quit. The Tenant
covenants that upon the expiration or sooner termination of this Lease, it
shall, without notice, deliver up and surrender possession of the Premises in
the same condition in which the Tenant has agreed to keep the same during the
continuance of this Lease and in accordance with the terms hereof, normal wear
and tear and damage by fire or other casualty excepted, first removing therefrom
all goods and effects of the Tenant and any leasehold improvements Landlord
specified for removal pursuant to Section 4.2, and repairing all damage caused
by such removal. Upon the expiration of this Lease or if the Premises should be
abandoned by the Tenant, or this Lease should terminate for any cause, and at
the time of such expiration, abandonment or termination, the Tenant or Tenant's
agents, subtenants or any other person should leave any property of any kind or
character on or in the Premises, the fact of such leaving of property on or in
the Premises shall be conclusive evidence of intent by the Tenant, and
individuals and entities deriving their rights through the Tenant, to abandon
such property so left in or upon the Premises, and such leaving shall constitute
abandonment of the property. Landlord shall have the right and authority without
notice to the Tenant or anyone else, to remove and destroy, or to sell or
authorize disposal of such property, or any part thereof, without being in any
way liable to the Tenant therefor and the proceeds thereof shall belong to the
Landlord as compensation for the removal and disposition of such property.

                                       45

<PAGE>

     If the Tenant fails to surrender possession of the Premises upon the
expiration or sooner termination of this Lease, the Tenant shall pay to
Landlord, as rent for any period after the expiration or sooner termination of
this Lease an amount equal to one hundred fifty percent (150%) of the Annual
Fixed Rent and the Additional Rent required to be paid under this Lease as
applied to any period in which the Tenant shall remain in possession. Acceptance
by the Landlord of such payments shall not constitute a consent to a holdover
hereunder or result in a renewal or extension of the Tenant's rights of
occupancy. Such payments shall be in addition to and shall not affect or limit
the Landlord's right of re-entry, Landlord's right to collect such damages as
may be available at law, or any other rights of the Landlord under this Lease or
as provided by law.

     Section 12.10 GROUND LEASE. The Land is owned by the Massachusetts
Institute of Technology ("MIT"). MIT as lessor and the Landlord as lessee shall
enter into a ground lease (the "Ground Lease") of the Land which Ground Lease
shall be in substantially the form attached hereto as EXHIBIT H-2 and which
shall specify a term and permitted uses which are consistent with the terms and
provisions of this Lease and this Lease shall in all respects be subject to such
Ground Lease. If the Ground Lease shall terminate during the Term for any reason
whatsoever, except as may otherwise be agreed between MIT and the Tenant, this
Lease shall terminate with the same force and effect as if such termination date
had been named herein as the date of expiration hereof. However, this Lease is
subject to the execution by MIT, the Tenant and the Landlord of a
non-disturbance agreement substantially in the form attached hereto as Exhibit
H-1. Each party shall pay its own expenses related to such non-disturbance
agreement. The Landlord represents and warrants to the Tenant that, upon
execution of the Ground Lease by the Landlord, or upon assignment of this Lease
to the Limited Partnership and the execution of the Ground Lease by the Limited
Partnership, the Landlord or the Limited Partnership, as the case may be, shall
have the full right and authority to grant the estate demised herein and the
appurtenant rights granted herein.

     Section 12.11 FINANCIAL REPORTING. Tenant shall from time to time (but not
less frequently than quarterly) provide Landlord with financial statements of
Tenant, together with related statements of Tenant's operations for Tenant's
most recent fiscal year then ended, certified by an independent certified public
accounting firm. Notwithstanding the foregoing, so long as the Tenant is a
public company, it shall be in compliance with its financial reporting
obligations provided that it submits all 10-Q and 10-K reports to the Landlord
within ten (10) business days of filing the same with the Securities and
Exchange Commission.

     Section 12.12 CAMBRIDGE EMPLOYMENT PLAN. The Tenant agrees to sign an
agreement with the Employment and Training Agency designated by the City Manager
of the City of Cambridge as provided in subsections (a)-(g) of Section 24-4 of
Ordinance Number 1005 of the City of Cambridge, adopted April 23, 1984.

     Section 12.13 TRUCK DELIVERY ROUTES; TRAFFIC MITIGATION MEASURES. Tenant
agrees to exercise good faith efforts to cooperate with any efforts by the City
of Cambridge to direct truck

                                       46

<PAGE>

traffic to certain streets and away from certain other streets, in connection
with the making of deliveries to the Premises.

     Section 12.14 PARKING AND TRANSPORTATION DEMAND MANAGEMENT. Tenant
covenants and agrees to work cooperatively with Landlord to develop a parking
and transportation demand management ("PTDM") program that comprises part of a
comprehensive PTDM for University Park. In connection therewith, the use of
single occupant vehicle commuting will be discouraged and the use of alternative
modes of transportation and/or alternative work hours will be promoted. Without
limitation of the foregoing, Tenant agrees that its PTDM program (and Tenant
will require in any sublease or occupancy agreement permitting occupancy in the
Premises that such occupant's PTDM program) will include offering a subsidized
MBTA transit pass, either constituting a full subsidy or a subsidy in an amount
equal to the maximum deductible amount therefor allowed under the federal tax
code, to any employee working in the Premises requesting one. Tenant agrees to
comply with the traffic mitigation measures required by the City of Cambridge
described as of the date hereof on Exhibit F hereto, and Tenant shall otherwise
comply with all legal requirements of the City of Cambridge pertaining thereto,
provided that if such compliance causes Tenant to be deprived of parking rights
described herein, then Landlord shall substitute Temporary Substitute Parking
Arrangements as provided in Section 8.9. Landlord represents and warrants that
neither Landlord nor any affiliate of Landlord has entered into or shall enter
into any agreement with respect to parking or transportation that imposes or
would impose on Tenant any obligations in excess of those set forth in this
Section 12.14 or which would impair Tenant's parking rights under this Lease.
Nothing in this Section 12.14 shall in any way limit Tenant's express rights set
forth in this Lease, including without limitation, its rights with respect to
parking.

     Section 12.15 LABORATORY ANIMALS. The Landlord acknowledges that the Tenant
will be conducting biotechnology research and development at the Premises and as
such may require the use of certain laboratory animals at the Premises in order
to carry out such research and development.

     Section 12.16 NO CONSEQUENTIAL DAMAGES. In no event shall either Landlord
or Tenant be liable to the other for consequential damages, provided that
damages incurred by the Landlord in connection with any holding over by Tenant
in the Premises, including without limitation those associated with loss, cost,
liability or expense arising by virtue of the existence of aggrieved third
parties (e.g. lenders and prospective tenants), shall not constitute
consequential damages.

     Section 12.17 GOVERNING LAW. This Lease shall be governed by and construed
in accordance with the laws of the Commonwealth of Massachusetts.

     Section 12.18: TERMINATION RIGHTS FOR FAILURE OF CONDITIONS. The
effectiveness of this Lease shall be subject to the timely satisfaction of each
of the conditions specified below, unless the satisfaction of a condition is
waived or deemed waived, on or before the deadline date specified below for the
satisfaction thereof:

                                       47

<PAGE>

         (a) TENANT'S TITLE DUE DILIGENCE: That the Tenant is reasonably
     satisfied, on or before fourteen (14) days following the date hereof, that
     any matters specified in that certain ALTA leasehold title insurance
     commitment with respect to this Lease and a boundary survey of the Land
     specifying the location of any easements or restrictions, which have been
     heretofore furnished by the Landlord to the Tenant, will not materially and
     adversely interfere with Tenant's use of the Premises, and the timely
     occupancy thereof as contemplated under this Lease, for the Permitted Uses.
     The Tenant's failure to terminate this Lease under this clause (a) on the
     basis of the condition herein described, on or before fourteen (14) days
     following the date hereof, shall constitute a waiver by the Tenant of such
     condition, except that Tenant shall continue to have a termination right
     for any new material, adverse title matters which may arise after the date
     of such title commitment and survey, respectively, but prior to the
     recordation of a Notice of Lease for this Lease.

         (b) CLOSING OF CONSTRUCTION LOAN: that a loan financing construction of
     the Building shall have closed on or before January 1, 2001, which
     condition shall be for the benefit of both the Landlord and the Tenant, and
     cannot be waived as a condition unless waived by both the Landlord and the
     Tenant. If the construction loan is not closed on or before January 1,
     2001, this Lease may be terminated by either the Landlord or the Tenant on
     or before February 1, 2001 upon notice to such effect to the other.
     Landlord agrees to use good faith efforts to obtain financing which is
     non-recourse (other than including completion guaranties from Forest City
     Enterprises and Tenant) on commercially reasonable terms, from an
     institutional lender with loan provisions consistent with the terms of this
     Lease and with loan provisions substantially similar to the financing of
     the construction of the building at 45/75 Sidney Street, Cambridge,
     Massachusetts. Tenant agrees to cooperate and deliver estoppel certificates
     reasonably requested by Lender. Landlord shall periodically keep Tenant
     apprised of its discussions with potential lenders. If Landlord has not
     obtained a financing commitment by September 1, 2000 for such financing
     described above, then Landlord and Tenant agree to explore alternative
     means to achieving funding for the Millennium Project which alternative
     solutions might include some financing by Tenant or a partial guaranty by
     Tenant. If Landlord elects to terminate the Lease, Tenant shall have the
     right to supersede such Landlord election, in a notice to Landlord, in
     which notice Tenant shall specify a time period (which may not exceed sixty
     (60) days), during which Tenant and Landlord shall continue to be bound to
     the terms of this Lease, and during which Landlord shall continue to use
     good faith efforts to obtain financing from an institutional lender with
     loan provisions consistent with the terms of this Lease.

         (c) SUBORDINATION, NON-DISTURBANCE AND ATTORNMENT AGREEMENT: that the
     Tenant has been furnished with a subordination, non-disturbance and

                                       48

<PAGE>

     attornment agreement, substantially in the form attached hereto as Exhibit
     K on or before the closing of the construction loan to which reference is
     made in clause (c) above.

         (d) INTENTIONALLY OMITTED.

         (e) FOREST CITY ENTERPRISES GUARANTY OF COMPLETION: that guaranties of
     completion for the benefit of the Tenant, Construction Lender and Ground
     Lessor, in substantially the forms attached hereto as Exhibit L (the
     "Forest City Completion Guaranties") are furnished to the Tenant,
     Construction Lender and Ground Lessor on or before the closing of the
     construction loan to which reference is made in clause (c) above, provided
     that certain provisions of the completion guaranty to Construction Lender
     may be further modified so long as such modification does not materially
     adversely affect the rights of Tenant under the Forest City Completion
     Guaranty to be delivered to Tenant.

         (f) MILLENNIUM GUARANTY OF COMPLETION: that guaranties of completion
     for the benefit of the Landlord, Construction Lender and Ground Lessor in
     substantially the forms attached hereto as Exhibit M (the "Millennium
     Completion Guaranties") are furnished to the Landlord, Construction Lender
     and Ground Lessor on or before the closing of the construction loan to
     which reference is made in clause (c) above, provided that paragraph
     2(c)(ii) of Exhibit A thereto may be further modified so long as such
     modification does not materially adversely affect the rights of Landlord,
     Construction Lender and Ground Lessor under the Millennium Completion
     Guaranty.

         (g) LENDER'S PERMITTING OPINION: that the Tenant has been furnished
     with a copy of any zoning or permitting opinion given by the Landlord's
     counsel to the lender making the construction loan to which reference is
     made in clause (b) above on or before the closing of such construction
     loan, which opinion does not reveal, in the Tenant's reasonable judgment,
     any issues that will materially and adversely interfere with Tenant's use
     of the Premises and the timely occupancy thereof as contemplated in this
     Lease for the Permitted Uses.

         (h) MIT GROUND LEASE AND SNDA: that MIT as lessor and Landlord as
     lessee shall have entered into the Ground Lease, and MIT, Tenant and
     Landlord shall have entered into the non-disturbance agreement contemplated
     by Section 12.10 hereof no later than the date of closing of the
     construction loan referenced in Section 12.18(b) above.

     Section 12.19 DECLARATION OF COVENANTS. Notwithstanding any provision in
this Lease to the contrary, the following shall apply with respect to the
Declaration of Covenants:

                                       49

<PAGE>

         (a) In no event shall Operating Expenses under the Declaration of
     Covenants include any late charges or penalties assessed or incurred by
     Landlord or any entity controlled by, controlling or under common control
     with Landlord.

         (b) Landlord shall direct the Manager under the Declaration of
     Covenants to deliver to Tenant copies of all default notices sent to
     Landlord with respect to the Declaration of Covenants and shall request of
     the Manager under the Declaration of Covenants estoppel certificates, from
     time to time, on request of the Tenant.

         (c) If at any time assessments due under the Declaration of Covenants
     are not paid when due, then at its election Tenant may make such assessment
     payments directly, and may deduct amounts so paid from Annual Fixed Rent or
     Additional Rent otherwise due hereunder.

         (d) Landlord agrees that it shall not, without obtaining Tenant's prior
     written consent, vote for any amendment to the Declaration of Covenants
     that would materially increase Operating Expenses thereunder.

                                       50

<PAGE>

     IN WITNESS WHEREOF, this Lease has been executed and delivered as of the
date first above written as a sealed instrument.

                                         LANDLORD:

                                         FC 35 LANDSDOWNE, INC.

                                         By:  /s/ GAYLE W. FRIEDLAND
                                              -------------------------
                                              Gayle W. Friedland
                                              Vice President

                                         TENANT:

                                         MILLENNIUM PHARMACEUTICALS, INC.

                                         By:  /s/ KEVIN STARR
                                              -------------------------
                                              Kevin Starr
                                              Chief Financial Officer

                                       51




<PAGE>

                                    EXHIBIT A

                                BASIC LEASE TERMS

Annual Fixed Rent
for the Initial Term:               Lease Years One (1) through Five (5):
                                    $38.70 per rentable square foot.
                                    Lease Years Six (6) through Eleven (11):
                                    $41.20 per rentable square foot.
                                    Lease Years Twelve (12) through Seventeen
                                    (17): $43.70 per rentable square foot.

TI Security Deposit:                $5,000,000;  See Work Letter.

Landlord's TI Allowance:            $45 per rentable square foot;
                                    See Work Letter.

Landlord's Base Systems
Allowance:                          $10,000,000;  See Work Letter.

MEP Design
Allowance:                          $175,000

Tenant Work L/C:                    $10,000,000.  See Work Letter.

Initial Term:                       Approximately seventeen (17) years,
                                    commencing on the Rent Commencement Date,
                                    and expiring on the last day of the month
                                    during which the seventeenth (17th)
                                    anniversary of the Rent Commencement Date
                                    occurs.

Extension
Options:                            Tenant shall have (a) one Special Extension
                                    Option so as to make this Lease co-terminus
                                    with a lease which may be executed for
                                    40 Landsdowne Street, Cambridge and (b)
                                    one (1) option to extend the term of this
                                    Lease for an additional ten (10) years, all
                                    as described in Section 2.6 of the Lease.

Landlord's Original
Address:                            FC 35 Landsdowne, Inc.
                                    1100 Terminal Tower
                                    50 Public Square
                                    Cleveland, Ohio  44130
                                    Attention:  James Ratner

                                       52

<PAGE>

Landlord's Address for
Notices:                            FC 35 Landsdowne, Inc.
                                    38 Sidney Street
                                    Cambridge, Massachusetts  02139-4234
                                    Attention:  Gayle Friedland

                                    With a copy to:

                                    Forest City Commercial Management
                                    38 Sidney Street
                                    Cambridge, Massachusetts  02139-4234
                                    Attention:  General Manager

Premises:                           Approximately 197,000 total rentable square
                                    feet ("rsf") of space as depicted on
                                    Exhibit B-1, as such calculation may be
                                    adjusted in accordance with Section 2.1 of
                                    the Lease.

Parking Privileges:                 During the Term, the Tenant shall be
                                    entitled to parking passes (each a
                                    "Parking Pass"), each of which shall entitle
                                    the parking of a single motor vehicle in an
                                    unreserved parking space in University Park.
                                    Tenant's Parking Passes shall entitle
                                    Tenant's employees to park in the garage at
                                    55 Franklin Street, provided that Landlord
                                    may, upon (30) days prior written notice to
                                    Tenant, relocate some or all Parking Passes
                                    to the garage, once constructed, at 80
                                    Landsdowne Street. Tenant shall receive a
                                    number of parking passes equal to the
                                    product of (a) 1.5 multiplied by (b) each
                                    1,000 rentable square feet of floor area in
                                    the Premises. The parking spaces relating to
                                    the Parking Passes shall be accessible
                                    twenty four (24) hours per day, seven (7)
                                    days per week. In no event are Parking
                                    Passes transferable other than to the
                                    holder, from time to time, of the tenant's
                                    interest under this Lease or a subtenant
                                    that has been demised all or a portion of
                                    the Premises in conformity with the
                                    requirements of this Lease, and use of the
                                    Parking Pass is limited to use by employees,
                                    business invitees and visitors of either of
                                    the foregoing. The charge for each Parking
                                    Pass shall be equal to the Market Rate
                                    Parking Charge, established by the parking
                                    garage operator from time to time. "Market
                                    Rate Parking Charge" means the monthly
                                    parking rate for parking facilities charged
                                    from time to time by owners of parking
                                    facilities of comparable quality at mixed
                                    use office/research parks in East Cambridge.
                                    If the Tenant desires additional Parking
                                    Passes, and at the time in question there
                                    are sufficient Parking Passes in the
                                    reasonable judgment of the parking garage
                                    operator, that are not committed to

                                       53

<PAGE>

                                    others, then Tenant may request from the
                                    parking garage operator additional Parking
                                    Passes which may be made available, on a
                                    month-to-month basis, subject to
                                    availability (availability to be determined
                                    in a fair and reasonable manner vis-a-vis
                                    the other tenants in University Park that
                                    have outstanding requests for surplus
                                    Parking Passes) and subject to any
                                    limitations imposed by any applicable
                                    Parking and Transportation Demand Management
                                    Plan or other applicable legal requirements.

Permitted Uses:                     General business and administrative
                                    offices, pharmaceutical research and
                                    manufacturing, and customary accessory uses
                                    supporting the foregoing.

Commencement Date:                  See Section 2.5.

Rent Commencement Date:             July 1, 2002, subject to adjustment per
                                    Section 2.5.

Tenant's Address for
Notices:                            Millennium Pharmaceuticals, Inc.
                                    75 Sidney Street
                                    Cambridge, Massachusetts  02139-4211
                                    Attention: General Counsel

                                    With a copy to:

                                    Katharine E. Bachman, Esq.
                                    Hale and Dorr LLP
                                    60 State Street
                                    Boston, Massachusetts  02109

                                       54

<PAGE>

                                    EXHIBIT B

                              35 LANDSDOWNE STREET

A parcel of land situated in the City of Cambridge, Middlesex County
Commonwealth of Massachusetts, being more particularly bounded and described as
follows:

         Beginning at the intersection of the northwesterly line of
         Landsdowne Street and the southwesterly line of Franklin Street;

         Thence running S 59 degrees 10' 19" W, along said northwesterly line
         of Landsdowne Street, a distance of 180.57 feet to a point;

         Thence running by land, now or formerly of Massachusetts
         Institute of Technology the following four (4) courses;

         N 30 degrees 49' 41" W, a distance of 4.12 feet, to a point of
         curvature;

         Northwesterly on a curve to the left being non-tangent and having a
         radius of 273.56 feet, an arc distance of 158.49 feet and a chord
         bearing of N 46 degrees 54' 57" W, to a point;

         N 51 degrees 34' 47" W, a distance of 76.91 feet, to a point;

         and N 37 degrees 18' 54" E, a distance of 154.66 feet, to a point
         at the aforesaid southwesterly line of Franklin Street;

         Thence running S 51 degrees 35' 25" E, along said southwesterly line
         of Franklin Street, a distance of 303.49 feet, to the point of
         beginning.

         The above described parcel containing 42,061 square feet more or less

<PAGE>

                                    EXHIBIT C

                                   WORK LETTER
                         35 Landsdowne Street, Cambridge



                                TABLE OF CONTENTS

<TABLE>
<CAPTION>

<S>                                                                                                               <C>
ARTICLE 1

         DEFINITIONS..............................................................................................1
         1.1      DEFINITIONS.....................................................................................1
         1.2      SCHEDULES.......................................................................................4

ARTICLE 2

ENGAGEMENT OF ARCHITECT; MEP ENGINEER AND TENANT'S REPRESENTATIVE.................................................5
         2.1      ENGAGEMENT OF ARCHITECT; MEP ENGINEER...........................................................5
         2.2      BASE SYSTEM DESIGN..............................................................................6
         2.3      REPRESENTATIVES.................................................................................6
         2.4      PARTICIPATION IN DESIGN PROCESS.................................................................7

ARTICLE 3

PLANS AND SPECIFICATIONS..........................................................................................7
         3.1      PLANS AND SPECIFICATIONS........................................................................7
         3.2      DEVELOPMENT OF PLANS AND SPECS..................................................................8
         3.3      LANDLORD'S APPROVAL.............................................................................8
         3.4      TENANT'S APPROVAL...............................................................................9
         3.5      MAJOR CHANGES TO BASE BUILDING AND C/L/F PLANS AND SPECS........................................9

ARTICLE 4

CONSTRUCTION OF THE PROJECT......................................................................................10
         4.1      CONTRACTOR.....................................................................................10
         4.2      CONSTRUCTION OF BASE BUILDING..................................................................11
         4.3      TENANT'S CONSTRUCTION WORK.....................................................................11
         4.4      PERMITS........................................................................................12
         4.5      COORDINATION...................................................................................12
         4.6      TENANT CONSTRUCTION READINESS..................................................................12
         4.7      INTERIM PROJECT SUBSTANTIAL COMPLETION.........................................................13

<PAGE>

ARTICLE 5

CHANGES IN THE WORK..............................................................................................15
         5.1      TENANT REQUESTED CHANGE ORDERS TO BASE BUILDING AND C/L/F......................................15
         5.2      BASE SYSTEMS CHANGE ORDERS.....................................................................15
         5.3      TI CHANGE ORDERS...............................................................................15
         5.4      LANDLORD WORK CHANGE ORDERS....................................................................16
         5.5      CONSTRUCTION LENDER APPROVAL OF CHANGE ORDERS..................................................16

ARTICLE 6

PAYMENT OF COSTS; CONSTRUCTION ALLOWANCES AND TENANT'S CASH ESCROW; MEP DESIGN ALLOWANCE.........................17
         6.1      PAYMENT OF COSTS; MEP DESIGN ALLOWANCE.........................................................17
         6.2      ELIGIBLE TENANT EXPENSES.......................................................................17
         6.3      EXCESS TENANT COSTS............................................................................18
         6.4      DRAW REQUESTS..................................................................................18
         6.5      APPLICATION OF FUNDS...........................................................................18
         6.6      MEASUREMENT....................................................................................19
         6.7      FUNDING OF MEP DESIGN ALLOWANCE................................................................19

ARTICLE 7

SUBSTANTIAL COMPLETION; DELAYS; MATERIAL TENANT CONSTRUCTION DEFAULTS............................................20
         7.1      SUBSTANTIAL COMPLETION.........................................................................20
         7.2      COMPLIANCE WITH PROJECT SCHEDULE...............................................................20
         7.3      TENANT DELAY...................................................................................21
         7.4      LANDLORD DELAY.................................................................................21
         7.6      EXTENSIONS DUE TO PRIOR DELAYS.................................................................22
         7.7      LANDLORD COSTS; RENT COMMENCEMENT DATE DELAY...................................................22
         7.8      MATERIAL TENANT CONSTRUCTION DEFAULTS..........................................................22

ARTICLE 8

COMPLETION OF BASE BUILDING IMPROVEMENTS; PUNCH LIST...............................................................
         24
         8.1      COMPLETION OF BASE BUILDING IMPROVEMENTS.......................................................24
         8.2      SUBSTANTIAL COMPLETION INSPECTION AND PUNCH LIST...............................................24
         8.3      ACCEPTANCE OF WORK.............................................................................25
         8.4      AS BUILT PLANS.................................................................................25

ARTICLE 9

WARRANTIES.......................................................................................................25

         9.1      LANDLORD WARRANTY..............................................................................25
         9.2      TENANT WARRANTY................................................................................25

<PAGE>

ARTICLE 10


         CONSTRUCTION SECURITY...................................................................................26
         10.1     SECURITY FOR LANDLORD'S CONSTRUCTION OBLIGATIONS...............................................26
         10.2     SECURITY FOR TENANT'S CONSTRUCTION OBLIGATIONS.................................................26
         10.3     PLEDGE TO LENDER AND RETURN OF LETTERS OF CREDIT...............................................27

</TABLE>

<PAGE>

                                    EXHIBIT C

                                   WORK LETTER

                         35 Landsdowne Street, Cambridge

     All capitalized terms used herein and not otherwise defined shall have the
meaning ascribed to said terms in the Lease to which this Work Letter is
attached as EXHIBIT C. This Work Letter is expressly subject to the provisions
of the Lease and supplements the Lease. The provisions herein should be read
consistently with the Lease, provided, however, in the event of any
inconsistency between this Work Letter and the Lease, the terms and conditions
of this Work Letter shall, in all instances, and for all purposes, control.

     Attached to this Work Letter as SCHEDULE C-1 is the Project Work
Allocation. In the event of any inconsistency between this Work Letter and the
Project Work Allocation, the terms and conditions of the Project Work Allocation
shall, in all instances, and for all purposes, control. The Millennium Project
is intended by both Tenant and Landlord to be a fully integrated corporate
headquarters and research facility, to be owned by a real estate company that
takes a long term view to ownership and management of its real estate and to be
occupied by a company that regards flexibility as a highly critical component of
both the initial design and construction process and its long-term operations.
As such, the parties have agreed upon a unique allocation of responsibilities
for design and construction, placing greater responsibility with Tenant but also
requiring an integrated team approach with very close cooperation among all team
members.

                                    ARTICLE 1

                                   DEFINITIONS

     1.1 DEFINITIONS. The following terms shall have the meanings indicated or
referred to below:

         "Architects" mean, collectively, the Base Building Architect and the
Tenant's Architect.

         "Base Building Architect" means Elkus/Manfredi Architects Ltd. (or
replacement therefor in accordance with Section 2.1).

         "Base Building Improvements" means the Building structural elements,
facade, roof and other components of the Building envelope, egress stairs,
loading areas and site work around the Building, including landscaping of the
Property and within the University Park Quadrangle, as shown on the plans
prepared by the Halvorson Company, Inc. dated May 1, 2000, and any other work
described in Column 1 of the Project Work Allocation for which responsibility
for design and construction has been allocated to Landlord in the Project Work
Allocation. No Core/Lobby/Finish Work is included within the definition of Base
Building Improvements.

                                       1

<PAGE>

         "Base Building Plans and Specs" -- See Section 3.1.

         "Base Building C/L/F Design Development Plans and Specs" means the
design development plans and outline specifications prepared by the Base
Building Architect for the Base Building Improvements and the Core/Lobby/Finish
Work identified on SCHEDULE C-2 attached hereto.

         "Base Building Plans and Specs" - See Section 3.1.

         "Base Systems" means the base building mechanical systems, electrical
systems, plumbing systems and life safety systems, which shall be designed by
Tenant's MEP Engineer and approved by Landlord's MEP Engineer, as set forth in
the Project Work Allocation.

         "Base Systems Allowance" - See Section 6.1.

         "Base Systems Change Order" -- See Section 5.2.

         "Base Systems Description and Outline Specs" -- See Section 3.2 and
SCHEDULE C-3 attached hereto.

         "Base Systems Design" - See Section 2.2.

         "Building Standards" means the standards associated with the Millennium
Project specified in the Project Work Allocation, the Base Building C/L/F Design
Development Plans and Specs, the Base Systems Description and Outline Specs
Exhibit, the TI Schematic Plans and Specs and the project design and
construction manual to be mutually agreed upon by Landlord and Tenant.

         "C/L/F Plans and Specs" - See Section 3.1.

         "Civil Engineer" means SEA Consultants, Inc.

         "Construction Allowances" means, collectively, the Base Systems
Allowance and the Leasehold Improvements Allowance.

         "Construction Lender" means Landlord's construction lender who shall
finance a portion of the Construction Allowances and the MEP Design Allowance.

         "Contractor" means the Construction Manager mutually selected and
engaged by each of Landlord and Tenant to construct the entire Millennium
Project.

         "Core/Lobby/Finish Work" means construction of Building core elements,
restrooms, lobbies, elevators, and any other work which is designated in Column
2 of the Project Work Allocation as "Landlord Design Tenant Construct".

                                       2

<PAGE>

         "Deferred Area L/C" -- See Section 10.2.

         "Developer's Administrative Fee" means a fee of Seventy Five Thousand
Dollars ($75,000.00), to be charged against the Leasehold Improvement Allowance
by Forest City Development for construction administration services, in fifteen
(15) equal monthly installments of Five Thousand Dollars ($5,000.00) each,
commencing on the first day of the month following the date upon which Landlord
commences construction of the Building.

         "Eligible Tenant Expenses" -- See Section 6.2.

         "Excusable Delay" means any delay in the satisfaction of the conditions
in question to the extent the same is a consequence of External Causes
including, without limitation, any governmental embargo restrictions, or actions
of local, state or federal governments but excluding any failure due to
financial incapacity and any delays in issuing building permits, certificates of
occupancy or other similar permits or certificates.

         "Interim Project Substantial Completion" shall mean that the Base
Building, Base Systems, Core/Lobby/Finish Work and the Leasehold Improvements
for all but the Deferred Area(s) (as defined in Section 4.7) shall be
substantially complete and Landlord and Tenant shall have obtained a temporary
certificate of occupancy for the Millennium Project.

         "Landlord Construction Documents" mean the plans, specifications and
drawings for the Base Building Improvements prepared and approved in accordance
with Section 3.1, which shall be constructed by Landlord.

         "Landlord Delay" -- See Section 7.4.

         "Landlord's Lighting Consultant" means Joe Kaplan Architectural
Lighting.

         "Landlord's MEP Engineer" means AHA Consulting.

         "Landlord's Structural Engineer" means McNamara Salvia.

         "Leasehold Improvement Allowance" -- See Section 6.1.

         "Leasehold Improvements" means the build-out of the Premises into a
first class biotechnology research and development laboratory and office
facility as contemplated by the Project Work Allocation and the TI Plans and
Specs, including without limitation office and laboratory improvements,
casework, fume hoods and other laboratory components that are integrated into
the Building, and any other work described in Column 3 of the Project Work
Allocation for which responsibility for design and construction has been
allocated to Tenant in the Project Work Allocation.

                                       3

<PAGE>

         "MEP Design Allowance" -- See Section 6.1.

         "Material Tenant Construction Default" -- See Section 7.8.

         "Millennium Project" means, collectively, the Base Building
Improvements, the Base Systems, the Core/Lobby/Finish Work and the Leasehold
Improvements.

         "Project Schedule" -- See Section 3.1 and SCHEDULE C-4 attached hereto.

         "Project Work Allocation" means the schedule denoting aspects of the
Millennium Project and the allocation of design and construction
responsibilities therefor to either Landlord or Tenant (or partially to Landlord
and the balance to Tenant) attached hereto as SCHEDULE C-1.

         "Substantial Completion" -- See Section 7.1.

         "Tenant's Architect" means Elkus /Manfredi Architects Ltd. (or
replacement therefor in accordance with Section 2.1).

         "Tenant Construction Documents" mean the plans, specifications and
drawings for the Tenant Construction Work prepared and approved in accordance
with Section 3.1.

         "Tenant Construction Readiness Date"-- See Section 4.6.

         "Tenant's Construction Work" means, collectively, the construction of
the Base Systems, the Core/Lobby/Finish Work and the Leasehold Improvements.

         "Tenant Delay" -- See Section 7.3.

         "Tenant's MEP Engineer" means Shooshanian Engineering, Inc. who is
engaged by Tenant's Architect.

         "Tenant Requested Change Order" -- See Section 5.1.

         "TI Plans and Specs" -- See Section 3.1.

         "TI Schematic Plans and Specs" means the schematic plans and outline
specifications to be prepared by the Tenant's Architect for the Leasehold
Improvements, based on the TI Schematic Plans, if any, attached hereto as
SCHEDULE C-7.

         "Tenant's Representative" -- See Section 2.3.

     1.2 SCHEDULES. The following Schedules, which are attached to this Work
Letter, are incorporated herein:

                                       4

<PAGE>

      Schedule C-1:   Project Work Allocation
      Schedule C-2:   Base Building C/L/F Design Development Plans and Specs
      Schedule C-3:   Base Systems Description and Outline Specifications
      Schedule C-4:   Project Schedule (Including Times for Performance)
      Schedule C-5:   Description of Standard Installed Laboratory Equipment
      Schedule C-6:   Construction Rules and Regulations
      Schedule C-7:   TI Schematic Plans
      Schedule C-8:   Noise Standards
      Schedule C-9:   Intentionally Omitted
      Schedule C-10:  Form of Pledge Agreement
      Schedule C-11:  Form of Letter of Credit

                                    ARTICLE 2

        ENGAGEMENT OF ARCHITECT; MEP ENGINEER AND TENANT'S REPRESENTATIVE

     2.1 ENGAGEMENT OF ARCHITECT; MEP ENGINEER . Landlord has engaged the Base
Building Architect to develop schematic plans, drawings and specifications for
the Base Building Improvements and for the Core/Lobby/Finish Work. Tenant has
engaged the Tenant's Architect to develop schematic plans, drawings and
specifications for the Base Systems and the Leasehold Improvements. Landlord and
Tenant shall hereafter enter into separate formal architect's contracts with the
Base Building Architect and the Tenant's Architect, respectively. The Base
Building Architect will be engaged by Landlord to prepare more fully developed
plans, drawings and specifications, work with Landlord through the bid process
and supervise the construction of the Base Building Improvements. The Tenant's
Architect will be engaged by Tenant to prepare the Base Systems Design (working
through Tenant's MEP Engineer), prepare more fully developed TI Plans and Specs,
work with Tenant through the bid process and supervise the construction of the
Tenant's Construction Work. Landlord and Tenant agree to work together to engage
the services of the Base Building Architect and the Tenant's Architect,
respectively, under contracts that properly allocate the design responsibilities
to each of Landlord and Tenant, and to assure that the construction of the
Millennium Project is well coordinated. In order to assure such coordination,
Landlord shall furnish a copy of its architect's contract to Tenant, and it is
intended by the parties that the Base Building Architect's contract shall serve
as a guide for the preparation of Tenant's Architect's contract. Tenant shall
furnish a copy of its proposed architect's contract with the Tenant's Architect
to Landlord prior to its execution for Landlord's review and approval, which
shall not be unreasonably withheld or delayed. With respect to either such
contract, neither Landlord nor Tenant shall permit any provision to be included
therein which would influence the Architect to prioritize work for one party
hereto over the other's or otherwise interfere with the Architect's obligations
to the other party hereto (including, by way of example but not limitation,
penalties for delayed completion of work, retainage or bonuses for early
completion of work other than to the extent which may be commercially
customary). In either case, purely economic terms that a party may reasonably
believe are appropriately confidential (excluding retainage provisions,
penalties and other provisions which could be contrary to the preceding
sentence) may be redacted. Tenant's agreement with the

                                       5

<PAGE>

Tenant's Architect shall require the Tenant's Architect to incorporate the
Structural Engineer, the Civil Engineer and Landlord's Lighting Consultant into
its team and to work closely with such parties so as to ensure coordination of
the complete design package. If either Landlord or Tenant should elect to
replace (i) the Base Building Architect, Landlord's MEP Engineer or Landlord's
Structural Engineer, or (ii) the Tenant's Architect or Tenant's MEP Engineer,
respectively, and engage a replacement architect or engineer to fulfill the
responsibilities contemplated to be undertaken by the respective Architect or
engineer on behalf of such party, (the parties hereby agreeing that such a
replacement of an Architect or engineer will not be implemented without good
cause but agreeing, further that non-competitive prices shall be deemed good
cause) the identity of the replacement shall be subject to the other party's
approval, which shall not be unreasonably withheld or delayed.

     2.2 BASE SYSTEM DESIGN. The Tenant's Architect shall engage Tenant's MEP
Engineer to provide comprehensive engineering services related to Base Systems
("Base System Design") and to the entire Millennium Project. Tenant hereby
acknowledges that Landlord and Tenant retain joint rights of approval over the
basic design approach, system capacities, equipment selection and other matters
associated with Base Systems components throughout the Building, but that Tenant
is responsible for the overall efficacy of the Base System Design; provided,
however that Landlord may not require Base Systems in excess of those set forth
on SCHEDULE C-3. All systems shall comply with noise control standards
established within zoning and related ordinances by the City of Cambridge,
summarized in their current form as set forth on SCHEDULE C-8, and requirements
established by Landlord's insurance underwriters (Factory Mutual). In addition,
Tenant's MEP Engineer will include Landlord's objectives for long-term
durability and flexibility together with Tenant's specific operational
objectives in proposing systems, controls, and other components of the Building
Base Systems. The Base Systems design shall be consistent with the Base Systems
Description and Outline Specs, which is attached hereto as SCHEDULE C-3.
Tenant's MEP Engineer shall provide to the Base Building Architect all Base
Systems plans, details, specifications and other information necessary to obtain
a building permit for the Base Building Improvements (notwithstanding that Base
Systems are not a part of Base Building Improvements) in accordance with the
Project Schedule. In accordance with the Project Schedule, Tenant shall provide
the necessary planning information specified on the Project Schedule to Tenant's
MEP Engineer so that the Base System Design can be completed so as to maintain
the Project Schedule.

     2.3 REPRESENTATIVES. Tenant shall enter into an agreement (which may be one
or more employment relationships or consulting agreements) with a project
manager or project management team ("Tenant's Representative"), which Project
Manager will perform certain services for and on behalf of Tenant during the
design and construction phases of Tenant's Construction Work. Tenant shall, at
its sole cost and expense, and in accordance with the terms and conditions of
its agreement with Tenant's Representative, compensate Tenant's Representative
for providing such services. Landlord hereby agrees that Tenant's Representative
shall receive copies of all notices to which reference is made in this Work
Letter given by Landlord to Tenant. Tenant hereby agrees that Tenant's
Representative shall have authority to act as Tenant's representative in
connection with its participation in meetings and otherwise, and that except to
the extent Landlord has been given contrary instructions in writing from Tenant

                                       6

<PAGE>

with respect to any matter with which Tenant's Representative has been involved,
Landlord is entitled to rely on Tenant's Representative as the party having
authority to make decisions and establish schedules for the performance of work.
Tenant agrees further to continue to engage Tenant's Representative to render
the services contemplated hereunder, or a qualified successor subject to
Landlord approval, which shall not be unreasonably withheld, until such time as
final completion of the Tenant's Construction Work and occupancy by Tenant in
the Premises has been achieved. Landlord shall appoint a project manager
("Landlord's Representative"), which project manager will perform certain
services on behalf of Landlord during the design and construction phase of the
Base Building Improvements. Tenant hereby agrees that all notices to which
reference is made in this Work Letter given by Tenant to Landlord shall be
delivered to the attention of Landlord's Representative. Landlord hereby agrees
that Landlord's Representative shall have authority to act as Landlord's
representative in connection with its participation in meetings and otherwise,
and that except to the extent Tenant has been given contrary instructions in
writing from Landlord with respect to any matter with which Landlord's
Representative has been involved, Tenant is entitled to rely on Landlord's
Representative as the party having authority to make decisions and establish
schedules for the performance of work.

     2.4 PARTICIPATION IN DESIGN PROCESS. Notwithstanding the specific
responsibilities allocated to each of Tenant and Landlord, it is understood and
agreed that each party has a direct interest in and specific goals for certain
aspects of the other party's responsibilities. For example, Landlord has a
direct interest in the design approach to the centralized Base Systems as well
as the overall quality level of Leasehold Improvements, particularly as viewed
from the exterior within the context of the University Park campus context and
design guidelines. In the same manner, Tenant has a direct interest in the
functionality, design and finishes of lobby and core elements that will be
designed by Landlord. Therefore, it is jointly acknowledged that a basic premise
of this approach will be open participation by all parties in all phases of
design and construction, together with reasoned consideration by one party of
legitimate comments or concerns raised by the other. Notwithstanding the
foregoing, Landlord and Tenant agree that the lobby of the Building shall be
substantially as shown on the Base Building C/L/F Design Development Plans and
Specs and with a level of finish mutually agreeable to both parties.

                                       7

<PAGE>

                                    ARTICLE 3

                            PLANS AND SPECIFICATIONS

     3.1 PLANS AND SPECIFICATIONS . Landlord shall, at its sole cost and expense
(except as provided in Section 3.5 and Article 5), as soon as reasonably
possible, but in any event in accordance with the Project Schedule attached
hereto as SCHEDULE C-4, cause the completion of the Base Building Plans and
Specs and the C/L/F Plans and Specs as necessary to obtain a building permit and
commence construction of the Base Building Improvements on or before the date
contemplated therefor in the Project Schedule. For all purposes hereof, the
"Base Building Plans and Specs" and the "C/L/F Plans and Specs" mean
respectively, the plans, specifications and working drawings for the Base
Building Improvements and the Core/Lobby/Finish Work approved by Tenant as
herein provided, as the same may be modified consistently with the terms and
conditions hereof. Landlord shall also cause the Base Building Architect to
prepare the Landlord's Construction Documents so that Landlord may bid the work
described therein in accordance with the Project Schedule. Tenant shall at its
sole cost and expense (except as provided in Section 3.5 and Article 5), as soon
as reasonably possible, but in any event in accordance with the Project
Schedule, cause the completion of Base System Design and Landlord shall
incorporate such design into the Base Building Plans and Specs and the C/L/F
Plans and Specs and submit a set of plans and specifications so as to obtain a
building permit in accordance with the Project Schedule. Tenant shall also, at
its own cost and expense, cause the TI Plans and Specs to be completed in
accordance with the Project Schedule to permit construction of Tenant's
Construction Work to proceed in accordance with the Project Schedule. For all
purposes hereof, the "TI Plans and Specs" means the plans, specifications and
working drawings for the completion of the Leasehold Improvements approved by
Landlord as herein provided, as the same may be modified consistently with the
terms and conditions hereof. Landlord shall share with Tenant in accordance with
the Project Schedule the Base Building Plans and Specs and the C/L/F Plans and
Specs so that Tenant may proceed with the design of the Leasehold Improvements
in accordance with the Project Schedule.

     3.2 DEVELOPMENT OF PLANS AND SPECS.. Landlord, Tenant, the Engineers and
Architects shall work together in accordance with the Project Schedule to create
design concepts, plans and specifications for the Millennium Project that will
be mutually satisfactory to Landlord and Tenant. The results of such efforts as
of the date of this Lease are the Project Work Allocation, the preliminary
agreed upon standards and capacities for the Base Systems which are specified in
SCHEDULE C-3 attached hereto (the "Base Systems Description and Outline Specs"),
the designs set forth in the Base Building C/L/F Design Development Plans and
Specs, the designs for the Leasehold Improvements set forth in the TI Schematic
Plans attached hereto as SCHEDULE C-7 and the description of standard installed
laboratory equipment identified on SCHEDULE C-5 attached hereto. The parties
agree to work together with the Architects, Engineers and any other retained
design professionals to finalize the Base Building Plans and Specs, the C/L/F
Plans and Specs, the Base Systems Design, and the TI Plans and Specs in
accordance with the Project Schedule. Landlord and Tenant agree to furnish to
each other in accordance with the Project Schedule such information as may be
requested by the other party, to promptly "sign off" or specify objections or
concerns as to matters where a "sign off" is requested by one party or the

                                       8

<PAGE>

other and to otherwise undertake all such actions in accordance with the Project
Schedule as are reasonably necessary in order to assure the timely commencement
of the construction of the Base Building Improvements and, upon the Tenant
Construction Readiness Date, the Tenant Construction Work.

     3.3 LANDLORD'S APPROVAL. The TI Plans and Specs and the Base System
Design, including without limitation each iteration thereof (at schematic,
design development and construction document stages), and each change proposal
with respect thereto, through and including the "as-built" version thereof, and
the work contemplated to be performed in accordance therewith, shall be subject
to Landlord's prior approval, which approval shall not be unreasonably withheld
or delayed and shall, in any event, be given or denied within the applicable
time frame set forth in the Project Schedule. Landlord shall not be considered
unreasonable if it withholds approval to changes which are visible from the
exterior, are inconsistent with the Building Standards or are inconsistent with
the use for standard, multi-purpose, first-class biotechnology space, including
a mix of office, laboratory and manufacturing. Without limitation of the
foregoing, Landlord shall not be deemed to be unreasonable for requiring that
the standards provided in Article IV of this Work Letter are being fulfilled.
The Base System design shall be subject to peer review on behalf of Landlord by
Landlord's MEP Engineer (at schematic, design development and construction
document stages) to ensure consistency with Landlord's objectives; provided,
however, that Landlord may not require Base Systems in excess of those set forth
on SCHEDULE C-3. Neither the requirement that Base System Design and TI Plans
and Specifications be submitted to Landlord nor Landlord's agents, nor their
actual or implied review thereof or changes thereto shall in any way be deemed
to be an agreement by Landlord that (i) the work contemplated thereby or any
other aspect thereof complies with legal or other requirements, (ii) that any
plans or specifications will be approved by any governmental agency having
jurisdiction thereover, or (iii) that the Base System Design or the TI Plans and
Specs are free from errors, omissions or inconsistencies or are coordinated
within themselves, with each other or with the then existing Base Building Plans
and Specs and without limitation, any delay associated with any of the foregoing
(i), (ii) and (iii) shall constitute Tenant Delay.

     3.4 TENANT'S APPROVAL. The Base Building Plans and Specs and the C/L/F
Plans and Specs, including without limitation each iteration thereof (at
schematic, design development and construction document stages), and each change
proposal with respect thereto, through and including the "as-built" version
thereof, and the work contemplated to be performed in accordance therewith,
shall be subject to Tenant's prior approval, which approval shall not be
unreasonably withheld or delayed and shall, in any event, be given or denied
within the applicable time frame set forth in the Project Schedule. Without
limitation of the foregoing, Tenant shall not be deemed to be unreasonable for
requiring that the standards provided in Article IV of this Work Letter are
being fulfilled. Neither the requirement that the Base Building Plans and Specs
and the C/L/F Plans and Specs be submitted to Tenant nor Tenant's agents, nor
their actual or implied review thereof or changes thereto shall in any way be
deemed to be an agreement by Tenant that (i) the work contemplated thereby or
any other aspect thereof complies with legal or other requirements, (ii) that
any plans or specifications will be approved by any governmental agency having
jurisdiction thereover, or (iii) that the Base Building Plans and

                                       9

<PAGE>

Specs and the C/L/F Plans and Specs are free from errors, omissions or
inconsistencies or are coordinated within themselves, with each other or with
the then existing Base System Design or the TI Plans and Specs and without
limitation, any delay associated with any of the foregoing (i), (ii) and (iii)
shall constitute Landlord Delay.

     3.5 MAJOR CHANGES TO BASE BUILDING AND C/L/F PLANS AND SPECS. The Base
Building Plans and Specs and the C/L/F Plans and Specs to be developed by
Landlord under Section 3.1 shall always remain consistent with the Building
Standards and the Base Building C/L/F Design Development Plans and Specs and, if
during the development and completion of the Base Building Plans and Specs and
the C/L/F Plans and Specs, Landlord desires to make any change that would not be
so consistent (a "Major Change"), such Major Change shall be subject to Tenant's
consent, which shall not be unreasonably withheld or delayed. Tenant may, in any
event, condition its approval to a Major Change proposed by Landlord upon
Landlord expressly acknowledging its responsibility for any increase in the
costs associated with the Tenant Construction Work resulting from such Major
Change and any delay under the Project Schedule resulting therefrom as a
Landlord Delay, based on input from the Contractor. If, during the development
and completion of the Base Building Plans and Specs and the C/L/F Plans and
Specs, Tenant desires to propose a Major Change, Tenant shall be responsible for
reimbursing the Landlord for the reasonable actual cost of having Landlord's
Architect prepare any necessary modifications to the Base Building Plans and
Specs and/or the C/L/F Plans and Specs, as the case may be, and any such Major
Change shall be subject to Landlord's approval, which Landlord shall not
unreasonably withhold or delay. Landlord may, in any event, condition its
approval to a Major Change proposed by Tenant upon Tenant's expressly
acknowledging its responsibility for any increase in the costs associated with
the Base Building Improvements resulting from such Major Change and any delay
under the Project Schedule resulting therefrom as a Tenant Delay, based on input
from the Contractor. Either party may withhold consent to a Major Change if such
Major Change is reasonably anticipated to cause a delay and such delay, when
added to other then existing delays (but excluding delays caused by the party
with the consent right and Excusable Delays), would, in the aggregate, be
reasonably anticipated to cause the Base Systems or C/L/F Work to be delayed
beyond the Base Systems/C/L/F Outside Date. If Landlord should approve Tenant's
proposed Major Change, Tenant shall cause all plans and specifications
associated therewith to be prepared and furnished to Landlord and Tenant for
approval. Without in any way limiting the provisions of Article 9, hereof,
Tenant acknowledges that Landlord has made no representation or warranty with
respect to the Base Building Improvements or Core/Lobby/Finish Work being
suitable for any particular use or purpose, Tenant having the responsibility to
make such determination.

                                    ARTICLE 4

                           CONSTRUCTION OF THE PROJECT

     4.1 CONTRACTOR. Landlord and Tenant shall, under separate contracts,
engage the Contractor, and enter into such other arrangements as are
appropriate, to cause the Base Building Improvements (Landlord's contract) and
the Tenant's Construction Work (Tenant's contract),

                                       10

<PAGE>

respectively, to be timely constructed, installed and completed. The
Contractor's duties shall include preparing and updating from time to time a
construction progress schedule setting forth key dates for construction of the
Millennium Project. Tenant hereby acknowledges that all major construction
projects in University Park are undertaken on a union basis, and the Contractor
must provide a payment and performance bond on all work. Tenant shall furnish a
copy of its proposed construction contract with the Contractor, prior to its
execution for Landlord's review and approval, which shall not be unreasonably
withheld or delayed. It is contemplated that Tenant's construction contract with
the Contractor be consistent with the terms and conditions set forth in
Landlord's contract with the Contractor, and due to the fact that the
construction of the Base Building Improvements and of the Tenant's Construction
Work are to be performed contemporaneously in some respects, the coordination of
the construction process and the proper undertaking of responsibility by the
Contractor on behalf of Landlord and Tenant is critically important. In
furtherance of such coordination, Landlord shall furnish Tenant a copy of
Landlord's construction contract, and it is intended by the parties that
Landlord's construction contract shall serve as a guide for the preparation of
Tenant's construction contract. With respect to either such contract, neither
Landlord nor Tenant shall permit any provision to be included therein which
would influence the Contractor to prioritize work for one party hereto over the
other's or otherwise interfere with the Contractor's obligations to the other
party hereto (including, by way of example but not limitation, penalties for
delayed completion of work, retainage or bonuses for early completion of work
other than to the extent which may be commercially customary). Any decision to
replace the Contractor (the parties hereby agreeing that such a replacement of
the Contractor will only be implemented for good cause, if the Contractor is not
performing and, the success of the Millennium Project is thereby imperiled)
prior to Substantial Completion of the Base Building Improvements may only be
made by mutual agreement of Landlord and Tenant. Following Substantial
Completion of the Base Building Improvements, upon a mutual agreement to
terminate the Contractor, the selection of the replacement shall be made by
Tenant, subject to Landlord's approval, which shall not be unreasonably withheld
or delayed. Each of Landlord and Tenant agree to notify the other party in the
event such party desires, or the Contractor elects, to terminate a construction
contract entered into by such party and the Contractor.

     4.2 CONSTRUCTION OF BASE BUILDING. Landlord shall cause to be installed,
furnished and performed, as the case may be, with reasonable diligence and, in
any event, in accordance with the Project Schedule, and in a good and
workmanlike manner and at its sole cost and expense (except as otherwise
expressly provided herein to the contrary), the facilities, materials, labor,
supplies and work required for the construction of the Base Building
Improvements in accordance with the Base Building Plans and Specs.

     4.3 TENANT'S CONSTRUCTION WORK. Tenant shall cause to be installed,
furnished, and performed, as the case may be, with reasonable diligence and, in
any event, in accordance with the Project Schedule (but subject to Section 4.7
below), and in a good and workmanlike manner and at its own cost and expense
(except as otherwise expressly provided to the contrary herein) the facilities,
materials, labor, supplies and work required for the construction of the
Tenant's Construction Work, in accordance with the Tenant Construction Documents
or otherwise as approved by Landlord. Tenant shall have the right to use that
one-third portion of the Quadrangle

                                       11

<PAGE>

closest to the Building to locate cranes for the installation of rooftop
equipment, provided such installation is completed and such cranes removed on or
before March 1, 2002. Subject to the provisions hereof, the Tenant's
Construction Work shall be commenced promptly following the Tenant Construction
Readiness Date, and shall be constructed, installed and performed in accordance
with the Rules and Regulations for Tenant Construction which are set forth as
SCHEDULE C-6 and with any and all reasonable and customary requirements of
Construction Lender which relate to construction of the Tenant Work or
disbursement of the Construction Allowances such as title policy endorsements
(the obtaining of which shall be Landlord's responsibility), requirements
regarding materials stored off-site, retainage, collateral assignment of
construction contracts, lien waivers, inspections by Construction Lender's
representative, payment and performance bonds and required certifications that
the Millennium Project is "on schedule" (with reference to the Outside Dates
described in SCHEDULE C-4) and, with each draw request, a Tenant certification
as to the current Tenant Cost Budget, with Tenant reaffirming its obligation to
pay for Excess Tenant Costs (hereinafter defined). (The foregoing provisions are
not intended to make Tenant liable for Landlord obligations under its loan
documents which are Landlord's obligations under this Work Letter.) As
construction of the Base Building Improvements and the Tenant Construction Work
will, to a certain extent, be conducted contemporaneously, each of Landlord and
Tenant expressly acknowledge that in conducting their respective work, due care
must be exercised to avert interference in the conduct by the other party of its
work, and each party covenants and agrees to exercise reasonable efforts to
avert such interference. Landlord and Tenant acknowledge that, prior to the
Tenant Construction Readiness Date, Tenant shall be permitted to undertake
certain elements of the Tenant Construction Work. This work includes, but is not
necessarily limited to, equipment deliveries and rough-in work to the interior
building systems. Prior to the Tenant Construction Readiness Date, Landlord
shall not be required to delay any aspect of Landlord construction of Base
Building Improvements on account of any aspect of Tenant Construction Work
unless Tenant acknowledges in writing its responsibility for the scheduling and
cost effects of the delay and Tenant agrees to waive the affected component of
Landlord's work as a requirement for Tenant Construction Readiness. From and
after the Tenant Construction Readiness Date, Tenant shall not be required to
delay any aspect of Tenant Construction Work on account of any aspect of
Landlord's Base Building Improvements unless Landlord acknowledges in writing
its responsibility for the scheduling and cost effects of the delay.

     4.4 PERMITS. All building permits, certificates of occupancy and other
governmental approvals required to construct the Base Building Improvements
shall be obtained by Landlord at Landlord's sole cost and expense.
Notwithstanding the foregoing, to the extent building permit fees paid by
Landlord are based on work which constitutes Base Systems or
Core/Lobby/Finish Work, then such portion of the permit fees shall be
reimbursed by Tenant, or paid to Landlord out of the Construction Allowances.
All building permits, certificates of occupancy, and other governmental
approvals required to construct the Tenant Construction Work, other than work
which is the subject of permits which Landlord is required to obtain, and to
occupy and operate Tenant's business in the Premises shall be obtained by
Tenant at Tenant's sole cost and expense.

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<PAGE>

     4.5 COORDINATION. Landlord and Tenant agree to work together
cooperatively so as to coordinate the management, administration, and
scheduling of the Base Building Improvements and the Tenant Construction
Work. Such cooperation shall include without limitation, coordination of the
contractors' schedules, regular meetings, generally to be held weekly, during
the construction period with the Contractor, the attendance at such meetings
to include Landlord's Representative and Tenant's Representative. Landlord
and Tenant each agree that they shall respectively assure the availability of
such representatives at reasonable times after reasonable notice and that
each party shall provide the other reasonable advance notice of all
construction meetings.

     4.6 TENANT CONSTRUCTION READINESS. The Base Building Improvements shall
for all purposes hereof be deemed at the stage of completion sufficient to
permit Landlord to establish that the "Tenant Construction Readiness Date" has
occurred on such date as the Base Building Improvements are in such condition,
to permit efficient progress of the Tenant Construction Work within the Building
without adverse effects from the on-going work on Base Building Improvements nor
weather conditions, and all Base Building Improvements are complete with the
following items as acceptable deficiencies:

-        Caulking and trim to facade

-        1st floor slab, and associated underslab services. Note: the following
         shall be completed as a condition of Tenant Construction Readiness:
         loading bay walls; all elevator pit slabs and walls.

-        Interior wall construction at 1st floor.

-        Equipment related to the redundant primary electrical service entrance.

-        Sections of the main building and/or penthouse facade left open for
         construction access, equipment installation and/or ventilation as
         required by the contractor to perform the Tenant's Construction Work.

-        5th floor roofing ballast

-        Lightning protection system

-        Security system

-        Signage

-        Landscaping and final grading.

-        Any other deficiencies that Landlord and Tenant may agree are
         appropriate to accommodate the efficient scheduling and progress of
         Tenant Work.

Landlord shall diligently proceed with the construction of the Base Building
Improvements so as to cause the Tenant Construction Readiness Date to occur on
or before the date set forth in the Project Schedule, and Substantial Completion
of the Base Building Improvements to occur on or before the date specified
therefor on the Project Schedule.

     Tenant Construction Readiness shall be determined by a joint inspection of
the Landlord, Base Building Architect, Tenant and the Tenant's Architect within
five (5) working days of notification by the Landlord that Tenant Construction
Readiness has been achieved. The Base

                                       13

<PAGE>

Building Architect and Tenant's Architect shall issue a certificate certifying
the date Tenant Construction Readiness has been achieved, or a list of
deficiencies in the case of the determination the Base Building Improvements are
not complete to achieve Tenant Construction Readiness.

     4.7 INTERIM PROJECT SUBSTANTIAL COMPLETION. The parties acknowledge
Tenant's need for some flexibility in planning and occupying its space, but also
Landlord's needs for closure, for a permanent certificate of occupancy for the
Millennium Project and the constraints of construction lenders and permanent
lenders. The Project Schedule calls for Tenant Improvements Substantial
Completion, by the applicable Outside Date set forth on the Project Schedule.
Notwithstanding, the parties agree that Tenant may elect, by written notice to
Landlord ("Tenant's Deferral Election"), to delay the completion of Leasehold
Improvements for up to 50,000 square feet of building area (the "Deferred
Area(s)"), which notice shall describe the size and location of such Deferred
Area(s). Such election by Tenant shall not relieve Tenant of its obligations to
complete all other Tenant Work by the applicable Outside Dates specified in the
Project Schedule. If Tenant exercises Tenant's Deferral Election, Tenant shall
complete the Leasehold Improvements for all but 25,000 square feet of such
Deferred Area(s) by the 1st Deferred Space Substantial Completion Outside Date,
and shall complete the balance by the 2nd Deferred Space Substantial Completion
Outside Date. As a condition precedent to Tenant's exercising Tenant's Deferral
Election, Tenant shall deliver the Deferred Area L/C described hereafter in
Section 10.2 to secure the build-out of the Deferred Area(s).

     If, at Interim Project Substantial Completion, Leasehold Improvement
Allowance funds have not yet been drawn down due to Tenant's Deferral Election
hereunder, then, provided that Tenant has delivered the Deferred Area L/C
described in Section 10.2 to secure future build-out of the Deferred Areas as
provided above, either (a) the remaining Leasehold Allowance funds shall be
drawn down to reimburse Tenant for funds it has previously advanced to pay for
then existing Leasehold Improvements ("Option 1"), or (b) the balance shall be
drawn down and placed in escrow with Construction Lender, with interest to
accrue to Tenant, for payment of costs to be incurred by Tenant later in its
build-out of Leasehold Improvements for the Deferred Area(s) ("Option 2").
Within thirty (30) days of receipt of notice of Tenant's Deferral Election,
Landlord will confer with the Construction Lender and shall provide notice to
Tenant of its election of Option 1 or Option 2. In the event that Option 2 is
used, such escrowed amount of Leasehold Improvement Allowance funds (the
"Deferred Area Escrow") shall reduce, on a dollar-for-dollar basis, the required
amount of the Deferred Area L/C described in Section 10.2. For example, if
Tenant were to elect a Deferred Area of 30,000 square feet, and Landlord were to
elect Option 2, then Leasehold Improvement Allowance funds equal to the product
of $45 multiplied by 30,000 (or $1,350,000) would be drawn down and placed in
escrow with Construction Lender. The amount of the Deferred Area L/C would be
[the product of $100 multiplied times 30,000] minus $1,350,000, or $1,650,000.

     Prior to commencement of construction of Deferred Areas Tenant shall
establish, and Landlord shall review, a budget for the anticipated cost and
expense of construction of the build-out of the Deferred Areas and a timetable
for completion, which timetable shall be consistent with the applicable Outside
Dates set forth in the Project Schedule. As construction of

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<PAGE>

the Deferred Area(s) progresses, the Deferred Area Escrow may be drawn down to
reimburse Tenant for a portion of its construction costs incurred in such
build-out. The portion of any monthly draw requests to be paid out of the
Deferred Area Escrow would be based on a ratio, the numerator of which is the
Deferred Area Escrow and the denominator of which is the amount of the Deferred
Area build-out budget (including only Eligible Tenant Expenses). The Tenant
shall be solely responsible for amounts due on each Contractor requisition
associated with non-Eligible Expenses or any Tenant Change Orders that have
resulted in actual increases to the build-out budget and for any budgeted
build-out expenses in excess of the Deferred Area Escrow. The amount of the
Deferred Area L/C described in Section 10.2 may be reduced, at Tenant's request,
no more frequently than once per quarter, provided that the amount remaining
shall at no time, when added to the amount of the then remaining Deferred Area
Escrow, be less than the estimated budget to build-out the Deferred Areas.

                                    ARTICLE 5

                               CHANGES IN THE WORK

     5.1 TENANT REQUESTED CHANGE ORDERS TO BASE BUILDING AND C/L/F. Tenant may
request changes, subject to the Landlord's approval as hereinafter provided, in
the Base Building Plans and Specs and the C/L/F Plans and Specs. Any such change
in the Base Building Plans and Specs and the C/L/F Plans and Specs which is
approved by Landlord may be authorized by Tenant as a written change order to
the contract with the Contractor who performed the work affected by the change
(herein, a "Tenant Requested Change Order") and any costs associated therewith
shall be paid for by Tenant. Landlord's rights with respect to approving or
withholding approval to a proposed Tenant Requested Change Order shall be
governed by the same criteria as are applicable to a requested change to the
Base Building Plans and Specs and the C/L/F Plans and Specs under Section 3.5,
with the additional criteria that Landlord may withhold consent to a Tenant
Requested Change Order if the Tenant Requested Change Order is reasonably
anticipated to cause a delay and such delay, when added to other then existing
delays (but excluding Landlord delays and Excusable Delays) would, in the
aggregate, cause a delay in completion of Base Building Improvements beyond the
Base Building Substantial Completion Outside Date. If Tenant determines that a
Tenant Requested Change Order will result in an increase in the cost of the Base
Building Improvements, as calculated below, Tenant shall, in turn, notify
Landlord as to whether Tenant wishes to proceed with the Tenant Requested Change
Order in question. Any delay in the occurrence of the Tenant Construction
Readiness Date, Substantial Completion of the Base Building Improvements, the
Interim Project Substantial Completion Date and/or Substantial Completion of the
Millennium Project caused by a Tenant Requested Change Order shall be deemed
Tenant Delay for all purposes of this Work Letter.

     5.2 BASE SYSTEMS CHANGE ORDERS. Tenant may, subject to obtaining the
Landlord's approval as hereinafter provided, make changes to the Base Systems
consisting of additions or deletions to, or other revisions in the Base Systems
Design. Any such change in the Base System Design which is approved by Landlord
may be authorized by Tenant as a written change order to Tenant's contract with
the Contractor, with Tenant being responsible for any cost increases

                                       15

<PAGE>

associated with such changes, and Tenant being responsible for any delays
resulting therefrom, as a Tenant Delay ("Base Systems Change Order"). Landlord's
rights with respect to approving or withholding approval to a proposed Base
Systems Change Order shall be governed by the same criteria as are applicable to
the approval of Base Systems Design and TI Plans and Specs under Section 3.3.,
with the additional criteria that Landlord may withhold consent to a Base
Systems Change Order if the proposed Base Systems Change Order is reasonably
anticipated to cause a delay and such delay, when added to other then existing
delays (but excluding Landlord Delays and Excusable Delays), would, in the
aggregate, cause a delay in substantial completion of Base Systems beyond the
Base Systems/CLF Outside Date.

     5.3 TI CHANGE ORDERS. Subject to Section 5.5, Tenant may, subject to
notifying Landlord at least five (5) days prior to the effective date of a
change order, which notice shall describe the proposed change order, make
changes to the Leasehold Improvements ("TI Change Orders"), and appropriate
adjustment, if any, to the Tenant Cost Budget and to the Project Schedule, with
Tenant being responsible for any cost increases associated with such changes,
and Tenant being responsible for any delays resulting therefrom, as a Tenant
Delay, provided that Landlord shall have the right to disapprove within said
five (5) day period only those proposed TI Change Orders which are visible from
the exterior, are inconsistent with the Building Standards or are inconsistent
with use for standard multi-purpose, first-class biotechnology space including a
mix of office, laboratory and manufacturing. Notwithstanding the foregoing,
Landlord may withhold approval of a TI Change Order if the proposed TI Change
Order is reasonably anticipated to cause a delay and such delay, when added to
other then existing delays (but excluding Landlord Delays and Excusable Delays),
would, in the aggregate, cause a delay in substantial completion of Leasehold
Improvements beyond the Tenant Improvement Substantial Completion Outside Date.

     5.4 LANDLORD WORK CHANGE ORDERS. Landlord may, subject to obtaining the
Tenant's approval as hereinafter provided, make changes to the Base Building
Plans and Specs or the C/L/F Plans and Specs, with an appropriate adjustment,
if any, to the Project Schedule ("Landlord Work Change Order"). Such Tenant's
rights with respect to approving or withholding approval to a Landlord Work
Change Order shall be governed by the same criteria as applicable to the
approval of the Base Building Plans and Specs and the C/L/F Plans and Specs
under Section 3.4. If Landlord elects to make a change to the Base Building
Plans and Specs or the C/L/F Plans and Specs that would have the effect of
requiring modifications to the Tenant Construction Documents, and/or increase
the cost to Tenant of constructing the Tenant Construction Work and Tenant
approves the same, then Landlord agrees that the Construction Allowances
shall be increased by the amount of such increase. Any delay in the Project
Schedule caused by a Landlord Work Change Order shall be deemed a Landlord
Delay for all purposes of this Work Letter. Tenant may withhold consent to a
Landlord Work Change Order if the Landlord Work Change Order is reasonably
anticipated to cause a delay and such delay, when added to other then
existing delays (but excluding delays caused by Tenant and Excusable Delays),
would, in the aggregate, cause a delay in completion of the Base Building
Improvements beyond the Base Building Substantial Completion Outside Date.
Nothing in this Section is intended to impose the burden upon Tenant of any
violation by Landlord in its preparation of the Base Building Plans and Specs
or the Base Building C/L/F Plans and Specs of

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<PAGE>

any applicable legal requirements in effect at the time Landlord obtained the
building permit therefor, or any breach by Landlord of any existing agreements
with municipal authorities.

     5.5 CONSTRUCTION LENDER APPROVAL OF CHANGE ORDERS. Notwithstanding any
other provision herein to the contrary, Landlord shall have the right to review
and approve any change order regarding any component of the Millennium Project
if (i) Construction Lender has the right to approve such change order subject to
the provisions of this Section 5.5 and (ii) Construction Lender withholds or
conditions such approval under the construction loan agreement. The construction
loan agreement shall provide that Construction Lender shall not have approval
rights regarding Leasehold Improvements with an associated cost of Seventy-Five
Thousand Dollars ($75,000) or less (herein "Minor TI Change Orders") unless the
Minor TI Change Orders, in the aggregate, including prior Minor TI Change
Orders, have an aggregate associated cost greater than Two Hundred Fifty
Thousand Dollars ($250,000). The construction loan agreement shall also provide
that the Construction Lender shall not have approval rights regarding a proposed
TI Change Order unless such proposed TI Change Order is visible from the
exterior, is inconsistent with the Building Standards or is inconsistent with
use for standard multi-purpose biotechnology space including a mix of office,
laboratory and manufacturing.

                                    ARTICLE 6

             PAYMENT OF COSTS; CONSTRUCTION ALLOWANCES AND TENANT'S
                       CASH ESCROW; MEP DESIGN ALLOWANCE

     6.1 PAYMENT OF COSTS; MEP DESIGN ALLOWANCE. Landlord shall establish an
allowance (the "Leasehold Improvements Allowance") equal to the product of
Forty-Five and 00/100 Dollars ($45.00) times the rentable square footage of
the Premises for purposes of being applied to certain costs and expenses,
more particularly set forth below, incurred by or on behalf of Tenant in
connection with the construction of the Leasehold Improvements. Landlord
shall establish an additional allowance (the "Base Systems Allowance") equal
to the amount of Ten Million Dollars ($10,000,000.00), for purposes of being
applied to certain costs and expenses, more particularly set forth below,
incurred by or on behalf of Tenant in connection with the construction of the
Core/Lobby/Finish work and the Base Systems. Landlord shall establish a third
allowance (the "MEP Design Allowance") equal to the amount of One Hundred
Seventy-Five Thousand Dollars ($175,000.00) for purposes of being applied to
the cost to design the Base Systems. The Leasehold Improvement Allowance and
the Base Systems Allowance are collectively referred to as the "Construction
Allowances". As described in Article 10 below, Tenant shall deposit with
Landlord or the Construction Lender Letters of Credit to secure Tenant's
construction of Tenant Construction Work. To the extent Tenant incurs costs
in connection with the construction of the Tenant Construction Work in excess
of the Construction Allowances, or in connection with the Base Systems Design
in excess of the MEP Design Allowance, then except as otherwise expressly
provided in Section 5.3 and Section 7.7, all costs shall be borne by Tenant,
in accordance with the terms and conditions specified in Section 6.3 below.

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<PAGE>

     6.2 ELIGIBLE TENANT EXPENSES. The application of the Construction
Allowances shall be limited to payment of the costs incurred by Tenant in
connection with the following costs and expenses incurred by or on behalf of
Tenant in connection with the construction of the Tenant Construction Work
(collectively "Eligible Tenant Expenses"): the actual documented and verified
cost of the labor and materials, together with the associated contractor's
overhead and profit and general conditions and the Developer's Administrative
Fee, incurred in the construction of the Tenant Construction Work, except for
the making of Removable Alterations or other improvements, installation of
fixtures or incorporation of other items which (x) by virtue of their quantity
or quality (whether greater or less) would not be of general utility to other
laboratory tenants that might later occupy the Premises, whether at the
expiration of the Term or by virtue of the earlier termination of this Lease, or
(y) are moveable rather than permanent improvements, examples of which are
furniture, telephone and security systems, communication systems, research
systems and bench-top laboratory equipment items such as microscopes. Any other
expenses incurred by Tenant in the Tenant Construction Work, unless approved by
Landlord as an Eligible Tenant Expense, shall constitute a "non-Eligible
Expense" hereunder.

     6.3 EXCESS TENANT COSTS. No later than thirty (30) days prior to the
Tenant Construction Readiness Date, Tenant shall establish, and Landlord shall
review, the anticipated cost and expense of construction of the Tenant
Construction Work (the "Tenant Cost Budget", or "TCB" and a budget, based upon
the TCB, that includes only Eligible Tenant Expenses (the Tenant Eligible Cost
Budget" or "TECB".) The parties intend that the TCB and The TECB shall reflect
the actual Contractor's budget as accurately as possible. Tenant shall be
responsible for all non-Eligible Tenant Expenses. If for any reason, other than
Landlord's failure to comply with its obligations hereunder, the anticipated
cost and expense of construction of Tenant Construction Work should increase,
including any increase in construction period interest due to Tenant Delays,
Tenant shall pay such Excess Tenant Costs.

     6.4 DRAW REQUESTS. The Contractor shall deliver separate monthly
requisitions to each of Landlord and Tenant, for work performed pursuant to
their respective construction contracts, which requisitions shall comply with
the Construction Rules and Regulations and with customary, commercially
reasonable requirements imposed by the Construction Lender which relate to
disbursement procedures regarding the Construction Allowances such as title
policy endorsements (the obtaining of which shall be Landlord's responsibility),
lien waivers, inspections by Construction Lender's representative and Tenant
certifications as to the current Tenant Cost Budget, with Tenant reaffirming its
obligation to pay for Excess Tenant Costs. During the construction of the Tenant
Construction Work, Tenant shall, on a monthly basis (as the Contractor submits
to Tenant its application for payment less retainage of not less than five
percent (5%) together with lien waivers), deliver to Landlord a draw request for
payment from the Construction Allowances, which draw request shall show the cost
of the Tenant Construction Work and the amount of the current payment requested
from Landlord. Tenant's draw request shall break out and show separately any
material non-Eligible Expenses. Each of the draw requests shall also show the
amounts paid to date and the percent complete of the total scope of the Tenant
Construction Work. Landlord will deliver such draw requests to the Lender
together with its own requisition for payment of costs associated with Base
Building Improvements, and shall promptly, but in any event within thirty (30)
days of receipt of such draw request, pay to the

                                       18

<PAGE>

Tenant, for further payment to the Contractor, the portion received from the
Construction Allowances. Following the completion of the Tenant Construction
Work, Tenant shall deliver to the Landlord, within ninety (90) days of
completion, a statement showing the final costs of the Tenant Construction Work,
the total of Tenant's Excess Tenant Costs, the amounts paid to date to, or on
behalf of the Tenant, and any amounts available for release of retainage.
Landlord shall ensure that the Construction Lender advances monies properly and
duly owed to Contractor for the Tenant Construction Work. Notwithstanding
anything herein to the contrary, except to the extent that Construction Lender
has failed to make a timely advance because of Tenant's failure to comply with
its obligations hereunder, including any Tenant Delay, Landlord shall fund,
within thirty (30) days of any failure by Construction Lender to fund payment to
the Tenant the portion due from the Construction Allowances, if the Construction
Lender has failed to do so, with respect to a regular draw or with respect to
final payment.

     6.5 APPLICATION OF FUNDS.

              (i) The first Five Million Dollars ($5,000,000) worth of Tenant's
              draw requests on account of Contractor's requisitions, to the
              extent they include Eligible Tenant Expenses, shall be paid from
              the Base Systems Allowance. Thereafter, payments to the Tenant for
              Tenant's draw requests on account of Contractor requisitions, to
              the extent they include Eligible Tenant Expenses (excluding
              additional costs, if any, associated with Tenant Change Orders),
              shall be made from the then remaining Landlord Construction
              Allowances in accordance with a ratio, the numerator of which is
              the amount of the then remaining Landlord Construction Allowances
              and the denominator of which is the amount of the Tenant Eligible
              Cost Budget less Five Million Dollars ($5,000,000). The Tenant
              shall be solely responsible for amounts due on each Contractor
              requisition associated with non-Eligible Tenant Expenses and
              Tenant Change Orders that have resulted in actual increases to the
              TECB.

              Example: Assume Landlord Construction Allowances total $19,000,000
              and the TECB is $32,000,000. Landlord would pay the first
              $5,000,000 of Contractor requisitions representing eligible
              construction costs. The remaining TECB would be $27,000,000. The
              Landlord payment ratio would be ($14,000,000 divided by
              27,000,000), i.e. {(19,000,000 less 5,000,000) divided by
              (32,000,000 less 5,000,000)}, or .518. Assume further that a
              $1,100,000 requisition includes $50,000 of non-Eligible Tenant
              Expenses and $50,000 associated with a Tenant Change Order.
              Landlord would pay an amount equal to .518 times ($1,100,000 less
              $100,000), or $518,000.

              (ii) Tenant shall maintain the Tenant Work LC as provided in
              Section 10.2. The Tenant Work LC may be reduced, at Tenant's
              request, no more frequently than once per quarter, such that the
              amount remaining shall at no time be less than an amount equal to
              the product of the estimated unpaid balance of the Tenant Eligible
              Cost budget multiplied by a ratio, the numerator of which is Ten
              Million Dollars ($10,000,000) and the denominator of which is the
              TECB less Five

                                       19

<PAGE>

              Million Dollars ($5,000,000). For example, if the TECB less
              $5,000,000 is $27,000,000, and the estimated unpaid balance of
              Eligible Tenant Expenses is $15,000,000, the Tenant Construction
              LC shall be no less than ($10,000,000 divided by $27,000,000)
              times $15,000,000, or $5,550,000.

     6.6 MEASUREMENT. All of the payments to be made by Landlord and Tenant
described in this Article 6, shall be made based upon the rentable floor area of
the Premises calculated by the Base Building Architect in accordance with
Section 2.1 of the Lease and EXHIBIT G to the Lease. After a final determination
of such rentable floor area pursuant to Section 2.1 of the Lease, the aforesaid
payments shall be adjusted between the parties within thirty (30) days of such
final determination.

     6.7 FUNDING OF MEP DESIGN ALLOWANCE. The MEP Design Allowance shall be
funded by Landlord in connection with Tenant's costs incurred with respect to
the Base Systems Design within thirty (30) days of Tenant's forwarding to
Landlord of invoices with respect to such validly incurred costs; provided that
Landlord shall not be required to advance the MEP Design Allowance at rates
greater than (a) fifteen (15%) percent of the MEP Design Allowance at completion
of schematic design, (b) twenty (20%) percent of the MEP Design Allowance at
completion of design development, (c) forty (40%) percent of the MEP Design
Allowance at completion of construction documents, and (d) twenty-five (25%)
percent of the MEP Design Allowance at the issuance of a certificate of
occupancy.

                                    ARTICLE 7

      SUBSTANTIAL COMPLETION; DELAYS; MATERIAL TENANT CONSTRUCTION DEFAULTS

     7.1 SUBSTANTIAL COMPLETION. For all purposes hereof, "Substantial
Completion" of the Base Building Improvements shall be deemed to have taken
place once the Base Building Improvements have been completed in substantial
accordance with the Base Building Plans and Specs as certified by the Base
Building Architect, notwithstanding that minor or insubstantial details of
construction remain to be performed, provided that the same do not materially
adversely affect Tenant's use or enjoyment of the Building or the performance of
Tenant's Construction Work hereunder. Substantial Completion of the Base
Building Improvements can be achieved with the following items as acceptable
deficiencies:

-        Sections of the facade left open for construction access, equipment
         installation and/or ventilation as required by the contractor to
         perform the Tenant's Construction work.

-        Security system

-        Signage

-        Exterior site improvements and landscaping

-        Other deficiencies of a "punchlist" nature.

                                       20

<PAGE>

The acceptable deficiencies noted above shall be completed contemporaneously
with the Tenant Construction Work as mutually agreed by the Tenant, Landlord,
and Contractor which completion dates shall be made specific by the construction
progress schedule. For all purposes hereof, "Substantial Completion" of the
Tenant Construction Work shall be deemed to have taken place once the Tenant
Construction Work has been completed in substantial accordance with the Tenant
Construction Documents as certified by Tenant's Architect, notwithstanding that
minor or insubstantial details of construction, mechanical adjustment, balancing
or decorating remain to be performed. When Substantial Completion of the Base
Building Improvements has occurred, and Substantial Completion of all Tenant
Construction Work has occurred, such that a permanent certificate of occupancy
has been obtained from the City of Cambridge for the entire Millennium Project,
then the "Substantial Completion" of the Millennium Project will be deemed to
have been achieved.

     7.2 COMPLIANCE WITH PROJECT SCHEDULE. Landlord and Tenant shall
respectively complete final iterations of Base Building Plans and Specs and
C/L/F Plans and Specs (in the case of Landlord) and Tenant Construction
Documents (in the case of Tenant), and respectively execute construction
contracts with the Contractor on or before the dates contemplated therefor in
the Project Schedule. Landlord shall diligently proceed with construction so as
to meet the Tenant Construction Readiness Date for the Building on or before the
date set forth therefor on the Project Schedule, and Landlord shall cause
Substantial Completion of the Base Building Improvements on or before the Base
Building Substantial Completion Date set forth therefor on the Project Schedule.
Without in any way limiting the provisions of Section 4.3, from and after the
Tenant Construction Readiness Date, Tenant shall diligently proceed with Tenant
Construction Work so as to achieve Substantial Completion of the Tenant
Construction Work by the Tenant Work Target Date, as set forth in the Project
Schedule.

     7.3 TENANT DELAY. As used herein, "Tenant Delay" shall mean any delay
(measured in calendar days) in (a) the Landlord's satisfaction of the condition
in question (e.g. the Tenant Construction Readiness Date or the Base Building
Improvements Substantial Completion Date) or (b) the Tenant's satisfaction of
the condition in question (e.g., Substantial Completion of all Tenant's
Construction Work), beyond the Tenant Work Target Date as set forth in the
Project Schedule, in any event to the extent the same are a consequence of any
act, omission or neglect of Tenant, Tenant's MEP Engineer or the Tenant's
Architect (in connection with Base System Design, TI Plans and Specs or Tenant
Requested Change Orders), or of any other employee, agent, Contractor (in its
performance of the Tenant Construction Work), subcontractor, sub-subcontractor,
agent or representative of Tenant, including without limitation, any failure by
Tenant, Tenant's Architect or Tenant's MEP Engineer to comply with the Project
Schedule, (including, without limitation the failure to obtain permits as
specified therein) any errors, omissions or inconsistencies in Tenant
Construction Documents or the lack of coordination caused by Tenant, Tenant's
Architect or Tenant's MEP Engineer of Base System Design or TI Plans and Specs
with the Base Building Plans and Specs or the C/L/F Plans and Specs, delays
attributable to any Major Changes requested by Tenant, Tenant Requested Change
Orders, Base Systems Change Orders, TI Change Orders or otherwise from
interference that could reasonably have been averted in the manner by which
Tenant's construction of the Tenant Construction Work is conducted.

                                      21

<PAGE>

     7.4 LANDLORD DELAY. As used in this Work Letter, "Landlord Delay" shall
mean any delay (measured in calendar days) in (a) in the Landlord's satisfaction
of the condition in question (e.g. the Tenant Construction Readiness Date or the
Base Building Substantial Completion Date) or (b) the Tenant's satisfaction of
the condition in question (e.g., Substantial Completion of Tenant's Construction
Work) in any event to the extent the same are a consequence of any act, omission
or neglect of Landlord, of the Base Building Architect (in connection with the
Base Building Plans and Specs or the C/L/F Plans and Specs or Landlord Work
Change Orders), or of any other employee, agent, Contractor (in its performance
of the Base Building Improvements), subcontractor, sub-subcontractor, agent or
representative of Landlord, including without limitation any failure by
Landlord, the Base Building Architect, Landlord's Lighting Consultant,
Landlord's MEP Engineer or Landlord's Structural Engineer to comply with the
Project Schedule (including, without limitation the failure to obtain permits as
specified therein), any errors, omissions or inconsistencies in Base Building
Plans and Specs or C/L/F Plans and Specs or the lack of coordination caused by
Landlord, the Base Building Architect, Landlord's Lighting Consultant,
Landlord's MEP Engineer or Landlord's Structural Engineer of the Base Building
Plans and Specs or the C/L/F Plans and Specs, with the Base System Design or TI
Plans and Specs, or due to delays by Landlord in funding amounts due from the
Construction Allowances for reasons which do not include Tenant Delay, or
otherwise from interference that could reasonably have been averted in the
manner in which Landlord's construction of the Base Building Improvements is
conducted.

     7.5 NOTICE OF DELAY. In the event that either Landlord or Tenant anticipate
or learn that it will not be able to meet a milestone set forth in the Project
Schedule, for whatever reason, such party shall notify the other, which notice
shall indicate the cause of delay and the probable duration thereof. Each of
Landlord and Tenant agree to work together to mitigate the duration of any
delays.

     7.6 EXTENSIONS DUE TO PRIOR DELAYS. To the extent that the performance of
any obligation set forth on the Project Schedule is actually delayed as the
result of a Landlord Delay (with respect to obligations of the Tenant, Tenant's
Architect or Tenant's MEP Engineer) or is actually delayed as the result of a
Tenant Delay (with respect to obligations of the Landlord, the Base Building
Architect, Landlord's Lighting Consultant, Landlord's MEP Engineer or Landlord's
Structural Engineer) the date for performing the same shall be extended by the
same time period as the delay in question and, to the extent that such extension
actually delays the performance of any other item set forth on the Project
Schedule, the date for performing such other item shall be similarly extended
and such further actual delay shall be deemed a Landlord Delay (to the extent
the original delay in question was a Landlord Delay) or a Tenant Delay (to the
extent the original delay in question was a Tenant Delay). However, to the
extent a Landlord Delay or a Tenant Delay causes no actual delay in the other
party's ability to perform its obligations, then there shall be no extension as
to subsequent obligations.

     7.7 LANDLORD COSTS; RENT COMMENCEMENT DATE DELAY. If Landlord must incur
change order costs under its construction contract with the Contractor with
respect to the Base Building Improvements by virtue of Tenant Delay, such costs
shall be payable by Tenant as

                                       22

<PAGE>

incurred. If Tenant must incur change order costs under its construction
contract with the Contractor with respect to the Tenant Construction Work by
virtue of Landlord Delay, such costs shall be payable by Landlord as incurred.
As set forth in Section 2.5 of the Lease and subject to the conditions therein,
the failure by Landlord to meet the Tenant Construction Readiness Date or Base
Building Substantial Completion Date as set forth on the Project Schedule and
any other Landlord Delay which affects the Project Schedule, except for delays
which the Landlord and Tenant may mutually agree are in their mutual best
interests, shall delay the Rent Commencement Date.

     7.8 MATERIAL TENANT CONSTRUCTION DEFAULTS. The occurrence of any of the
following shall constitute a Material Tenant Construction Default:

              (a) Tenant's failure to complete Base System Design so as to
         enable Landlord to submit plans for a Base Building building permit by
         the "Submit Building Permit Set" Outside Date set forth on the Project
         Schedule (subject to extension for Landlord Delays and Excusable
         Delays);

              (b) Tenant's failure to achieve Base Systems and C/L/F Substantial
         Completion by the applicable Outside Date set forth in the Project
         Schedule (subject to extension for Landlord Delays and Excusable
         Delays);

              (c) Tenant's failure to achieve Tenant Improvement Substantial
         Completion (without regard to Deferred Areas if Tenant has made
         Tenant's Deferral Election under Section 4.7) by the applicable Outside
         Date set forth in the Project Schedule (subject to extension for
         Landlord Delays and Excusable Delays);

              (d) Tenant's failure to achieve 1st Deferred Space Substantial
         Completion by the applicable Outside Date set forth on the Project
         Schedule (subject to extension for Excusable Delays);

              (e) Tenant's failure to achieve 2nd Deferred Space Substantial
         Completion by the applicable Outside Date set forth on the Project
         Schedule (subject to extension for Excusable Delays);

              (f) Tenant's failure to pay for the costs of Tenant's Work (other
         than amounts to be funded from the Construction Allowances) which
         failure results in a lien against the Property which is not released of
         record with fifteen (15) days' of notice to Tenant; and

              (g) Tenant's failure to construct Tenant's Work in accordance with
         applicable laws and with plans and specifications approved by Landlord
         as provided in this Work Letter, and such failure is not cured within
         fifteen (15) days of Landlord's notice to Tenant, provided that it
         shall not be a Material Tenant Construction Default if such failure is
         not capable of being cured within fifteen

                                       23

<PAGE>

         (15) days but Tenant commences cure during such fifteen (15) days and
         thereafter diligently pursues such cure and ultimately completes such
         cure prior to the applicable Outside Date specified in the Project
         Schedule.

Upon the occurrence of a Material Tenant Construction Default, Landlord shall
have the sole and exclusive rights and remedies described below:

         (i) Landlord shall be entitled, on ten (10) days prior written notice
         to Tenant, to cure any Material Tenant Construction Default and be
         reimbursed by Tenant in full, and may draw upon the Letters of Credit
         to fund costs incurred by Landlord in curing such Material Tenant
         Construction Default. In the case of Tenant's failure to complete
         build-out of Tenant Improvements, including the Deferred Area(s), this
         shall include such build-out as may be necessary to obtain a permanent
         certificate of occupancy and/or to satisfy reasonable and customary
         requirements of Construction Lender and/or a permanent lender related
         to Tenant's Work.

         (ii) In the event a lien is filed against the Premises as a result of
         any component of the Tenant Work, and such lien is not removed or
         bonded within fifteen (15) days of notice to Tenant, Landlord or
         Construction Lender may remove such lien and be reimbursed by Tenant in
         full, and may draw on the Letters of Credit to fund costs incurred by
         Landlord in connection therewith.

         (iii) If Landlord incurs direct costs as a result of a Material Tenant
         Construction Default, including without limitation legal fees, lender's
         legal fees, default rate interest or other default-related charges
         under the construction loan, increased construction period interest
         under the construction loan, or loss of permanent loan commitment fees,
         where Landlord can demonstrate that Tenant Delays, and not Landlord
         Delays, are the cause of a permanent lender's refusal to fund, or
         Landlord's enforcement costs (but excluding consequential damages of
         Landlord), then Tenant shall be responsible for such direct costs, and
         Landlord may draw upon the Letters of Credit in the amount of such
         actual direct damages.

The amounts of the Letters of Credit are not intended to be a cap on the
Tenant's liability in the event of a Material Tenant Construction Default.

                                    ARTICLE 8

              COMPLETION OF BASE BUILDING IMPROVEMENTS; PUNCH LIST

     8.1 COMPLETION OF BASE BUILDING IMPROVEMENTS. Notwithstanding that
Substantial Completion of the Base Building Improvements shall be deemed to have
occurred with respect to the Building, subject to the provisions of this Section
8, Landlord shall cause the remaining Base

                                     24

<PAGE>

Building Improvements to be completed by the date set forth therefor on the
Project Schedule, subject to Tenant Delays.

     8.2 SUBSTANTIAL COMPLETION INSPECTION AND PUNCH LIST. Within fifteen (15)
days after Landlord notifies Tenant that it has achieved Substantial Completion
of the Base Building, Tenant shall conduct an inspection of the Base Building
Improvements with respect thereto, giving Landlord prior notice and an
opportunity to attend the same, and furnish to Landlord a notice ("Punch List
Notice") specifying any aspect of Base Building Improvements which remains to be
completed or which, if completed, is not substantially in accordance with the
Base Building Plans and Specs (other than defects which could not reasonably
have been discovered by a reasonable investigation at that time). Landlord shall
cause any incomplete work to be completed and/or remedy any such work not
substantially in accordance with the Base Building Plans and Specs promptly
following the establishment of the punchlist. In no event shall Landlord be
obligated for any costs associated with the repair of any damage to the Base
Building Improvements caused by Tenant, its employees, agents or contractors.
Nothing in this Section 8.2 shall limit any of Tenant's rights or Landlord's
obligations under the Lease with respect to the maintenance or operation of the
Building.

     8.3 ACCEPTANCE OF WORK. If Tenant shall fail to conduct an inspection of
Base Building Improvements and timely give to Landlord a Punch List Notice as
provided in Section 8.1 above, then it shall be deemed that, except for latent
defects, and work which cannot be inspected or tested due to seasonal factors,
the Base Building Improvements with respect to the building in question has been
fully completed in accordance with the Base Building Plans and Specs. If Tenant
shall conduct an inspection and give Landlord a Punch List Notice as provided in
Section 8.1, then except for the items specified in the Punch List Notice,
latent defects and work which cannot be inspected due to seasonal factors, the
Base Building Improvements with respect to the building in question has been
fully completed in accordance with the Base Building Plans and Specs. Nothing in
this Section 8.3 shall limit any of Tenant's rights or Landlord's obligations
under the Lease with respect to the maintenance or operation of the Building.

     8.4 AS BUILT PLANS. Tenant shall furnish Landlord, promptly after the
Substantial Completion of the Millennium Project, an "as built" iteration of the
Tenant Construction Documents.

                                    ARTICLE 9

                                   WARRANTIES

     9.1 LANDLORD WARRANTY. Landlord warrants to the Tenant that the materials
and equipment furnished in connection with the Base Building Improvements will
be of good quality and new unless otherwise required or permitted by the Base
Building Plans and

                                       25

<PAGE>

Specs and that the work with respect thereto will be free from defects not
inherent to the quality required or permitted and that such work will conform
with the requirements of the Base Building Plans and Specs and such warranty
shall remain in full force and effect for one (1) year after the Rent
Commencement Date. In furtherance of the same, Landlord shall require the
Contractor to provide a warranty on the Base Building Improvements on the same
terms and conditions for at least such period. Landlord shall share with Tenant
the benefits and rights under all extended warranties for Base Building
Improvements components.

     9.2 TENANT WARRANTY. Tenant warrants to the Landlord that the materials
and equipment furnished in connection with Base Systems and the
Core/Lobby/Finish Work will be of good quality and new unless otherwise required
or permitted by the Tenant Construction Documents and that the work with respect
thereto will be free from defects not inherent to the quality required or
permitted and that such work will conform with the requirements of the Tenant
Construction Documents and such warranty shall remain in full force and effect
for one (1) year after the date of Substantial Completion of the Millennium
Project. In furtherance of the same, Tenant shall require the Contractor to
provide a warranty on the Base Systems and the Core/Lobby/Finish Work on the
same terms and conditions for at least such period. Tenant shall share with
Landlord the benefits and rights under all extended warranties for Base Systems
or C/L/F Work components.

                                   ARTICLE 10

                              CONSTRUCTION SECURITY

     10.1 SECURITY FOR LANDLORD'S CONSTRUCTION OBLIGATIONS. In order to secure
Landlord's obligations to construct and pay for the costs of the Base Building
Improvements, in accordance with the Project Schedule, Forest City Enterprises
agrees to execute on or before the closing of Landlord's construction loan with
Construction Lender, for the benefit of Tenant, Construction Lender, and the
Ground Lessee, a completion guaranty in substantially the form of Exhibit L,
with such changes as may be reasonably requested by Construction Lender or
Ground Lessee; provided that such changes do not materially or substantially
adversely affect Landlord's or Tenant's rights or obligations thereunder.

     10.2 SECURITY FOR TENANT'S CONSTRUCTION OBLIGATIONS. In order to secure
Tenant's obligations to complete the Tenant Construction Work and pay for the
costs thereof to the extent such costs may exceed the Construction Allowances,
in accordance with the Project Schedule, Tenant agrees to do the following:

              (i) Tenant shall execute on or before the closing of Landlord's
         construction loan with Construction Lender, for the benefit of
         Landlord, Construction Lender, and the Ground Lessor, a completion
         guaranty in substantially the form of Exhibit M, with such changes as
         may be reasonably requested by Construction Lender or Ground Lessor;
         provided that such changes do not materially or substantially adversely
         affect Tenant's or Landlord's rights or obligations thereunder;

              (ii) Tenant shall deliver, prior to the closing of Landlord's
         Construction Loan to the Construction Lender, in an amount equal to Ten
         Million Dollars ($10,000,000), a

                                       26

<PAGE>

         clean, unconditional and irrevocable letter of credit in form and
         substance reasonably satisfactory in all respects to the Landlord, and
         from a commercial bank having an AA rating by Standard and Poors or
         from an institution that is a wholly owned subsidiary of a bank having
         an AA rating by Standard and Poors (it being agreed that the form of
         letter of credit attached hereto as SCHEDULE C-11 and State Street
         Bank, as issuer of a letter of credit shall be substantially
         satisfactory for all purposes hereunder) (the "Tenant Work L/C"). The
         Tenant Work L/C shall be used to secure the performance by Tenant of
         the Tenant Construction Work, and the payment therefor by Tenant. The
         Tenant Work L/C shall provide for partial draws. The Tenant Work L/C
         may be replaced or amended periodically, but no more often than
         quarterly, to reduce the amount payable thereunder, as more
         specifically set forth in Section 6.5 hereof.

              (iii) Tenant shall deliver to Landlord on or before the closing of
         Landlord's construction loan, a clean, unconditional and irrevocable
         letter of credit in form and substance reasonably satisfactory in all
         respects to the Landlord, and from a commercial bank having an AA
         rating by Standard and Poors or from an institution that is a wholly
         owned subsidiary of a bank having an AA rating by Standard and Poors
         (it being agreed that the form of letter of credit attached hereto as
         SCHEDULE C-11, and State Street Bank, as issuer of a letter of credit
         shall be substantially satisfactory for all purposes hereunder), in the
         amount of Five Million Dollars ($5,000,000). The TI Security Deposit
         shall provide for partial draws. Provided that Tenant has cured any
         Material Tenant Construction Default, the TI Security Deposit shall be
         reduced to Two Million Five Hundred Thousand Dollars ($2,500,000) upon
         Tenant's achievement of Interim Project Substantial Completion (with
         the only exception being for Base Building Improvements) by the Tenant
         Work Outside Date.

              (iv) As a condition precedent to Tenant's exercising Tenant's
         Deferral Election described in Section 4.7, the Tenant shall deliver to
         Landlord or Construction Lender a clean, unconditional and irrevocable
         letter of credit in form and substance reasonably satisfactory in all
         respects to the Landlord, and from a commercial bank having an AA
         rating by Standard and Poors or from an institution that is a wholly
         owned subsidiary of a bank having an AA rating by Standard and Poors
         (it being agreed that the form of letter of credit attached hereto as
         SCHEDULE C-11, and State Street Bank, as issuer of a letter of credit
         shall be substantially satisfactory for all purposes hereunder) in an
         amount of $100 per square foot of Deferred Area, to secure the future
         build-out of the Deferred Area (the "Deferred Area L/C"). The Deferred
         Area L/C shall provide for partial draws. The Deferred Area L/C shall
         be pledged to the Construction Lender in a pledge agreement in
         substantially the form attached hereto as SCHEDULE C-10. The amount of
         the Deferred Area L/C shall be reduced, on a dollar-for-dollar basis,
         by the amount of the Deferred Area Escrow described in Option 2 of
         Section 4.7, if applicable. Also, upon Tenant's satisfaction of Interim
         Project Substantial Completion and delivery of the Deferred Area L/C,
         provided that Tenant has cured any Material Tenant Construction
         Default, then the Tenant Work L/C shall be returned to Tenant. The TI
         Security Deposit,

                                       27

<PAGE>

         the Tenant Work L/C and the Deferred Area L/C, if applicable, are
         collectively referred to as the "Letters of Credit".

     10.3 PLEDGE TO LENDER AND RETURN OF LETTERS OF CREDIT . The Landlord may
pledge its right, title and interest in and to the Letters of Credit to any
mortgagee and, in order to perfect such pledge, have such Letter of Credit held
in escrow by such mortgagee. In connection with any such pledge or grant of
security interest by the Landlord to a mortgagee ("Security Deposit Pledgee"),
Tenant covenants and agrees to cooperate as reasonably requested by the
Landlord, in order to permit the Landlord to implement the same on terms and
conditions reasonably required by such mortgagee including, without limitation,
providing in any letter of credit that the Tenant provides under this Article XI
any necessary and appropriate language that will permit the implementation of
such pledge. The Landlord and the Tenant agree that such pledge agreement shall
be in substantially the form attached hereto as SCHEDULE C-10 (but with
customary and reasonable changes as may be requested by Construction Lender
which do not materially, adversely affect Landlord's or Tenant's rights or
obligations thereunder) (the "Pledge Agreement"). No Letter of Credit shall be
available for the use of the Construction Lender or the Landlord for any reason
other than in accordance with the exercise of the Landlord's rights pursuant to
this Work Letter in the event of a Material Tenant Construction Default, as set
forth in Section 7.8. Upon completion of the Tenant's Construction Work and
payment of all costs in connection therewith, to the extent not expended in
accordance with this Work Letter, provided that there then exists no outstanding
Material Tenant Construction Default, the Letters of Credit shall be returned to
the Tenant, provided further that, upon cure of any outstanding Material Tenant
Construction Default, the Letters of Credit shall be returned to Tenant.

             End of Work Letter, other than Schedules which follow.

                                     28

<PAGE>

                                  SCHEDULE C-1

                              35 LANDSDOWNE STREET
                        MILLENNIUM PHARMACEUTICALS, INC.
          ALLOCATION OF RESPONSIBILITY BETWEEN LANDLORD AND TENANT WORK

Building architectural design to be consistent with Design Development Plans and
Specifications listed on Schedule C-2.

Building mechanical, electrical, plumbing, fire protection & alarm and similar
systems to be designed to standards mutually agreed upon by Landlord and Tenant.

<TABLE>
<CAPTION>

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

              ELEMENT                     DESCRIPTION                      LANDLORD DESIGN      LANDLORD       TENANT DESIGN
                                                                             & CONSTRUCT      DESIGN & TEN      & CONSTRUCT
                                                                                                 CONST.
<S>                    <C>                                                 <C>               <C>              <C>
---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

SITE IMPROVEMENTS      Completion of Landsdowne                                   X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Quadrangle, perimeter sidewalks, street
                       trees, street lights, street furniture and
                       landscaping accordance with current plans
                       and the University Park Design Guidelines.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

BUILDING
ENVELOPE               Brick facade articulated with                              X
                       thermally-insulated glass and curtain
                       wall systems, including light-gauge metal
                       stud back-up with insulation.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Gypsum wallboard on typical exterior walls,                                  X
                       except where required in service areas to be
                       constructed by Landlord.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Mechanical penthouse enclosure on roof,                    X
                       size to be mutually determined, designed
                       to complement architectural design of
                       building.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Screen walls to match penthouse to enclose                 X
                       exterior mechanical equipment.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Cost of all modifications to Building envelope                                                X
                       and penthouse required to accommodate Tenant
                       requirements (such as additional louvers, access
                       doors, etc.), together with installation
                       responsibility if specified too late to be
                       included in Base Building plans and
                       specifications.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

ROOFING                Mechanically adhered EPDM system, ballasted at             X
                       the roof levels visible from occupied areas on
                       sixth and seventh floors.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Roof modifications wherever located to                                                        X
                       accommodate penetrations and equipment required
                       by Tenant.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Walking pads to all rooftop-mounted mechanical                                                X
                       equipment (may not be located on fourth floor
                       roof).

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

</TABLE>

<PAGE>

<TABLE>
<CAPTION>

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

              ELEMENT                     DESCRIPTION                      LANDLORD DESIGN      LANDLORD       TENANT DESIGN
                                                                             & CONSTRUCT      DESIGN & TEN      & CONSTRUCT
                                                                                                 CONST.
<S>                    <C>                                                 <C>               <C>              <C>
---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

STRUCTURAL             All Base Building foundations.                             X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Cost of any foundation modifications required by                                              X
                       Tenant.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All steel framing for Base Building                        X
                       design, including framed openings for
                       egress stairs, elevators, and base
                       building supply air shafts.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Steel framing for up to six pre-determined                 X
                       floor penetrations (stacked on each
                       floor) to accommodate tenant laboratory
                       exhaust systems.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All floor slabs on upper floors above                      X
                       ground (designed for 100 lbs/sf live
                       load).

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       First floor structural slab (150 lbs/sf                    X
                       live load) at loading dock, main
                       electrical rooms and other primary
                       Building service rooms.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Installation of structural slab per Base                                                      X
                       Building design for remainder of ground
                       floor.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Concrete slab at penthouse and 7th floor roof for          X

                       support and sound dampening of mechanical
                       equipment.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Structural modifications to floor or roof systems          X                                  X
                       to accommodate tenant requirements, if identified     Installation                       Design and
                       in time to install with Base Building structural                                         incremental
                       system.                                                                                   cost for

                                                                                                               materials and

                                                                                                               installation

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Spray-on fireproofing (as required by                      X
                       code) for all steel installed by
                       Landlord.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All structural modifications to accommodate                                                   X

                       tenant requirements identified after
                       completion of Base Building steel package,
                       including patching of fireproofing.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

BASE BUILDING COMMON
AREA                   Building entrance doors at first floor lobbies.            X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Lobby finishes and furnishings consistent with                               X
                       Base Building Design to conform with standards
                       established in other lobbies throughout
                       University Park.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

</TABLE>

                        2

<PAGE>

<TABLE>
<CAPTION>

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

              ELEMENT                     DESCRIPTION                      LANDLORD DESIGN      LANDLORD       TENANT DESIGN
                                                                             & CONSTRUCT      DESIGN & TEN      & CONSTRUCT
                                                                                                 CONST.
<S>                    <C>                                                 <C>               <C>              <C>
---------------------- --------------------------------------------------- ----------------- ---------------- ----------------


                       Egress stairways, including all misc. metals and           X
                       concrete pans, fully installed save for
                       wall-mounted rails to be stocked for Tenant
                       installation.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Wall construction, finishes, doors and                                       X
                       rail installation for egress stairs.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Interior loading area with two truck bays                  X
                       and space for one rubbish dumpster.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Wall construction for first floor main                     X
                       electrical service rooms, water and fire
                       pump room.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Finishes, lighting, etc. for above                                           X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All core area restrooms, electrical and                                      X
                       telephone rooms, janitors closets, and
                       supply air shafts.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

CONVEYANCE             Three, 3,500 lb. passenger elevators linking                                 X
                       building lobby to all tenant office/lab floors.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Dedicated 5,000 lb. freight elevator linking                                 X
                       loading dock to all tenant floors and mechanical
                       penthouse.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Elevator cab finishes if installed by X
                       subcontractors other than elevator
                       subcontractor.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

TENANT AREAS           Partitions, ceilings, flooring, painting, other                                               X
                       finishes, doors, millwork and all related
                       office and laboratory buildout within
                       tenant premises in compliance with
                       Landlord Design Guidelines.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

WINDOW
TREATMENT              Window sills and blinds at all windows,                                      X
                       style to be mutually determined by Landlord
                       and Tenant.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

HVAC                   All Building mechanical equipment, including                                                  X

                       chilled water plant, cooling towers, hot
                       water boilers, steam boilers, water pumps
                       and piping, heat exchanges, air handling
                       equipment, exhaust systems, DDC control
                       systems, etc. System design and equipment
                       selection to be mutually approved by
                       Landlord and Tenant.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All acoustic attenuators, dampening equipment,                                                X
                       shields, etc. necessary to achieve noise control
                       standards established within City of Cambridge
                       regulations and zoning ordinances.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

</TABLE>

                        3

<PAGE>

<TABLE>
<CAPTION>

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

              ELEMENT                     DESCRIPTION                      LANDLORD DESIGN      LANDLORD       TENANT DESIGN
                                                                             & CONSTRUCT      DESIGN & TEN      & CONSTRUCT
                                                                                                 CONST.
<S>                    <C>                                                 <C>               <C>              <C>
---------------------- --------------------------------------------------- ----------------- ---------------- ----------------


                       Any modifications to Building equipment                                                       X
                       after initial design to accommodate
                       tenant requirements.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Additional air handler and associated ducts,                                                  X
                       equipment and controls if required for a Tenant
                       Animal Care Facility.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All ductwork, fire dampers, volume dampers, fan                                               X
                       boxes, registers, grills, diffusers, and
                       other components required for distribution
                       of air throughout the building.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Central computerized direct digital                                                           X
                       control. system, with links to Park
                       Management office.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Tenant exhaust system for laboratories, hoods,                                                X
                       etc. including ductwork, fans, registers,
                       grilles, shaft enclosures, structural
                       modifications for equipment mounting, and
                       controls.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Specialized tenant equipment, such as fume hoods,                                             X
                       including all related HVAC appurtenances, with
                       energy-efficient controls.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Stairwell pressurization and smoke vestibule                                                  X
                       ventilation.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

NATURAL GAS            Gas service to face of building.                           X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Gas manifolds, meter set-ups, and piping                                                      X
                       throughout Building (with separate service
                       distribution for HVAC and laboratory
                       requirements).

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

PLUMBING               Water service stubbed to Base Building                     X
                       water service room.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Sanitary service stubbed to Building, including            X
                       sampling port outside of building, stubs
                       through foundations and any lines under
                       ground floor slabs to be installed by
                       Landlord, loading dock gas/oil separator,
                       etc.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Complete storm leader system including                     X
                       roof drains, leaders through Building,
                       and connections to storm lines outside of
                       Building.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

</TABLE>

                        4

<PAGE>

<TABLE>
<CAPTION>

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

              ELEMENT                     DESCRIPTION                      LANDLORD DESIGN      LANDLORD       TENANT DESIGN
                                                                             & CONSTRUCT      DESIGN & TEN      & CONSTRUCT
                                                                                                 CONST.
<S>                    <C>                                                 <C>               <C>              <C>
---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All plumbing distribution and equipment                                                       X
                       throughout Building, including service with
                       meter-set-up and back-flow prevention (in
                       conformance with City requirements), booster
                       pumps, risers, vents, waste lines, bathroom
                       fixtures, etc.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Fuel oil storage tank, and separate transfer                                                  X
                       pumps and distribution piping for both Building
                       life safety emergency generator and equipment
                       back-up generator.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All non-potable water systems and                                                             X
                       equipment for laboratory requirements.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Production and distribution of hot water                                                      X
                       and steam for laboratory use.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Acid waste neutralization pit, equipment                                                      X
                       and waste lines, to stub provided by
                       Landlord.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Manifolds, piping, and other requirements for                                                 X
                       laboratory gases, compressed air, vacuum systems,
                       steam and humidified air.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All modifications to Base Building associated                                                 X
                       with Tenant LN2 tank such as depressed slab,
                       louvers, fill station, venting to roof, etc.
                       Location of all equipment subject to Landlord
                       approval.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

ELECTRICAL             Redundant primary service feeds from Com                   X
                       Electric's Putnam Street double ended
                       power substation, with manual transfer by
                       Com Electric
                       between feeds.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Cost of upgrade to automatic transfer from normal                                             X
                       to standby power if desired. This will reduce
                       downtime to +/- one minute.  (Note: must be
                       ordered with base building.)

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Building electrical service to provide 6000                                                   X
                       Amperes at 480/277 Volt, 3 phase, 4-wire
                       via main switchboards in main electrical
                       room (per design of Tenant engineer with
                       Landlord approval).

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Modification to primary meter system with                                                     X
                       Building-supplied transformer, if approved by

                       Landlord, with provisions for future
                       billing to multiple tenants.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

</TABLE>

                        5

<PAGE>

<TABLE>
<CAPTION>

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

              ELEMENT                     DESCRIPTION                      LANDLORD DESIGN      LANDLORD       TENANT DESIGN
                                                                             & CONSTRUCT      DESIGN & TEN      & CONSTRUCT
                                                                                                 CONST.
<S>                    <C>                                                 <C>               <C>              <C>
---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Power wiring to elevators.                                                                    X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Distribution of electrical service from                                                       X
                       main switchboard throughout building.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Tenant fit-up of panels, transformers,                                                        X
                       receptacles and lighting in tenant area.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Lighting & receptacles serving core areas.                                   X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Life safety emergency generator to provide                                                    X
                       stand-by power for code-required egress lighting,
                       fire alarm systems, elevators (one at a time) and
                       fire pump.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Stand-by generator for any tenant laboratory                                                  X
                       requirements, including transfer switch,
                       controls, acoustic enclosure, etc.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Emergency egress and exit lighting in core areas.                            X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Emergency egress and exit lighting in                                                         X
                       tenant areas.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Lightning protection system if required by                 X
                       insurance underwriters, including cabling
                       to earth ground, protection rods on all
                       base building equipment on the roof and
                       all interconnecting cables.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

FIRE PROTECTION        Complete sprinkler service including fire                                                     X
                       department connection, alarm valve flow
                       protection, stairway standpipe with flow control
                       valve stations at each floor, fire pump and all
                       related controls, and runouts and heads
                       throughout building, all per Factory Mutual and
                       City requirements.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Any extinguishing systems for special equipment.                                              X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

FIRE ALARM             Base building expandable analog/addressable fire                                              X
                       alarm system.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Detection and annunciation devices in core                                                    X
                       areas and stair entries.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Detection & annunciation devices, subpanels, and                                              X
                       all wiring in tenant areas and as required to tie
                       into base building system.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

</TABLE>

                        6

<PAGE>

<TABLE>
<CAPTION>

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

              ELEMENT                     DESCRIPTION                      LANDLORD DESIGN      LANDLORD       TENANT DESIGN
                                                                             & CONSTRUCT      DESIGN & TEN      & CONSTRUCT
                                                                                                 CONST.
<S>                    <C>                                                 <C>               <C>              <C>
---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

TELEPHONE              Conduits for tell-data access to Building ground                                              X
                       floor telephone room, and floor sleeves in
                       pre-designated locations for floor telephone
                       closets.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All telephone and data wiring, conduits                                                       X
                       and outlets for Tenant areas.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Audio-visual connections and systems for Tenant                                               X
                       areas.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

CABLE TV               Conduits for cable TV access to                            X
                       Building ground floor telephone room.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All cable TV wiring throughout Building.                                                      X

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

SECURITY               Park wide security network system for                      X
                       monitoring and access control.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Card access at Building entries and within                                                  X
                       elevators.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Card access and/or alarm systems into or                                                    X
                       within Tenant's premises.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

SIGNAGE                Building and site exterior address and                     X
                       directional signage consistent with University
                       Park standards.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Exterior corporate identity sign                                                             X
                       consistent with the lease terms.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       Code-required interior signage in building                 X
                       common areas.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

                       All other interior signage, to be designed by                                                X
                       Tenant to be consistent with building
                       signage systems.

---------------------- --------------------------------------------------- ----------------- ---------------- ----------------

</TABLE>

                                       7

<PAGE>

                                  SCHEDULE C-4

                                PROJECT SCHEDULE

                              35 LANDSDOWNE STREET

----------------------------------------------------- ------------- ------------

TASK NAME                                             TARGET DATE   OUTSIDE DATE

----------------------------------------------------- ------------- ------------

----------------------------------------------------- ------------- ------------

BASE BUILDING DESIGN

----------------------------------------------------- ------------- ------------

     Complete DD and release steel package             7/15/00

----------------------------------------------------- ------------- ------------

     Submit Building Permit set                        8/15/00           9/1/00

----------------------------------------------------- ------------- ------------

     Release 75% Base Building package for pricing     9/1/00
----------------------------------------------------- ------------- ------------

     Secure Building Permit                            9/15/00

----------------------------------------------------- ------------- ------------

     Complete 100% CDS for Base Building               10/1/00
----------------------------------------------------- ------------- ------------

BASE BUILDING CONSTRUCTION

----------------------------------------------------- ------------- ------------

     Commence Site Prep                                8/15/00

----------------------------------------------------- ------------- ------------

     Commence Foundations                              10/1/00

----------------------------------------------------- ------------- ------------

     Complete Steel Erection                           4/1/00

----------------------------------------------------- ------------- ------------

     Tenant Construction Readiness                     9/15/01       9/15/01

----------------------------------------------------- ------------- ------------

     Base Building Substantial Completion              12/1/01       3/1/02

----------------------------------------------------- ------------- ------------

     Base Building Final Completion                    5/1/02*       6/15/02*

----------------------------------------------------- ------------- ------------

     Site Improvements Final Completion                8/1/02        10/1/02

----------------------------------------------------- ------------- ------------

TENANT DESIGN

----------------------------------------------------- ------------- ------------

     Complete structural coordination with Base Bldg   6/15/00

----------------------------------------------------- ------------- ------------

     Complete Schematic Design                         8/1/00

----------------------------------------------------- ------------- ------------

     Complete Base Building MEP Permit Pkg             8/15/00      9/1/00

----------------------------------------------------- ------------- ------------

     Complete Design Development                       9/25/00

----------------------------------------------------- ------------- ------------

     Complete Construction Documents                   12/15/00

----------------------------------------------------- ------------- ------------

     Secure Building Permit                            2/1/00

----------------------------------------------------- ------------- ------------

<PAGE>

----------------------------------------------------- ------------- ------------

TENANT CONSTRUCTION

----------------------------------------------------- ------------- ------------


     Begin Equipment Deliveries & Rough-In             4/1/01**

----------------------------------------------------- ------------- ------------

     Begin Full-Scale TI Construction                  9/15/01**

----------------------------------------------------- ------------- ------------

     Base Systems & CLF Substantial Completion         6/15/02       11/1/02

----------------------------------------------------- ------------- ------------

     Tenant Improvement Substantial Completion***      9/1/02        12/1/02

----------------------------------------------------- ------------- ------------

     1st Deferred Space Substantial Completion                        9/1/03

----------------------------------------------------- ------------- ------------

     2nd Deferred Space Substantial Completion                        9/1/04

----------------------------------------------------- ------------- ------------



*    or thirty (30) days after Punchlist Notice, whichever is later.
**   or earlier if practical; not intended to be a deadline.
***  also known as Interim Project Substantial Completion if Tenant elects
     Tenant's Deferral Election.

     Pursuant to Article 3 of the Work Letter, Landlord and Tenant have agreed
to work cooperatively to prepare and review the plans and specifications for the
components of the Millennium Work which are their respective responsibilities.
In recognition of the iterative nature of the process of preparation of the
plans and specifications, and the importance of their timely approval, Landlord
and Tenant hereby agree that each party shall review and approve or provide
detailed comments upon all major submissions of plans and specifications by the
other party (i.e., schematics, design development documents, one set of interim
construction documents and final construction documents, or Major Changes as
defined herein) within ten (10) business days of delivery of the major
submission, and shall review and approve or provide detailed comments upon all
other submissions within five (5) business days of delivery of such other
submissions. Any failure by Landlord to adhere to the foregoing schedule shall
be deemed to be a "Landlord Delay", and any failure by Tenant to adhere to the
foregoing schedule shall be deemed to be a "Tenant Delay", in each case subject
to the terms of Section 7.6 of the Work Letter.

                                       2

<PAGE>

                                  SCHEDULE C-5

STANDARD INSTALLED LABORATORY EQUIPMENT

"Standard Installed Laboratory Equipment" ("SILE") shall be defined as equipment
that is integrated into the building which is consistent with and necessary for
the operation of a standard, high quality biotechnology research laboratory.
Such equipment would include, but would not be limited to, supply and exhaust
ventilation systems; fume hoods in reasonable quantity; environmental rooms in
reasonable quantity; laboratory benches and casework with associated permanent
shelving, fixtures, plumbing supply/waste lines and equipment associated
therewith, gas supply lines, a back-up electrical generator sufficient to meet
critical power requirements, etc.

Standard Installed Laboratory Equipment does not include stand-alone equipment
such as autoclaves, cagewashers, glasswashers, refrigerators, biosafety
cabinets, NMR equipment, benchtop equipment, etc.

Within thirty (30) days of Lease Commencement, Landlord and Tenant shall review
the final scope of Tenant's Improvements and develop a mutually acceptable list
of those installed equipment items that will be deemed to be Standard Installed
Laboratory Equipment, and those that, by virtue of quantity, specialization or
portability, will not be deemed to be Standard Installed Laboratory Equipment
and will therefore remain the property of Tenant, subject to the terms of this
Lease.

Landlord and Tenant agree that they shall work together in good faith to change
the definition of SILE from time to time based on changes in technologies and
standards for first-class biotechnology research laboratory facilities.

<PAGE>


                                  SCHEDULE C-10

                                PLEDGE AGREEMENT

         THIS PLEDGE AGREEMENT (this "AGREEMENT"), dated as of ____________,
200_, by and among FC 35 LANSDOWNE, INC., a Massachusetts corporation having an
address at c/o Forest City Commercial Group, Inc., 38 Sidney Street, Cambridge,
Massachusetts 02139 ("BORROWER"), ___________________________, a
____________________, having an address at ________________________________
("AGENT"), as Agent for the Lender pursuant to the terms and conditions of that
certain Loan Agreement (as defined herein), and MILLENNIUM PHARMACEUTICALS,
INC., a Delaware corporation having an address at 238 Main Street, Cambridge,
Massachusetts ("Tenant").

                               W I T N E S SE T H:

         WHEREAS, the Lender has agreed to make a loan to Borrower (the "LOAN")
pursuant to that certain Loan Agreement dated as of even date herewith, between
Borrower, Agent and the Lender (as amended, modified, or supplemented from time
to time, the "LOAN AGREEMENT," capitalized terms not otherwise defined herein
shall have the meaning of such terms in the Lease (as defined herein), or if not
therein defined, in the Loan Agreement) for the development and construction of
a first class office building and laboratory complex at 35 LANSDOWNE Street,
Cambridge, Massachusetts (the "PROJECT");

         WHEREAS, Tenant has agreed to lease the Project pursuant to that
certain lease dated __________________, 2000 between Borrower and Tenant (as
amended, modified or supplemented from time to time, the "LEASE");

         WHEREAS, pursuant to Article 10 of the Work Letter attached as Exhibit
C to the Lease (the "WORK Letter"), Tenant is required to deposit with Borrower
two letters of credit as security for the performance by Tenant of its
construction obligations under the Lease consisting of the Tenant Work L/C and
the TI Security Deposit, as defined in Article 10 of the Work Letter and
potentially a third letter of credit, consisting of the Deferred Area L/C, also
as defined in Article 10 of the Work Letter (collectively, the "LETTERS OF
CREDIT");

          WHEREAS, execution of this Agreement is a condition precedent to the
Loan Agreement;

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Borrower, Agent, and Tenant hereby
agree as follows:

         SECTION 1. PLEDGE AND ASSIGNMENT. (a) Borrower hereby pledges and
assigns to Agent for the benefit of the Lender, and grants to Agent for the
benefit of the Lender a security interest in, the Borrower's rights and interest
in the Letters of Credit and all replacements, substitutions, renewals and
proceeds of any and all of the Letters of Credit.

<PAGE>

          (b)       Borrower has delivered to Agent and Agent acknowledges
receipt of the Letters of Credit listed on Schedule I hereto, which Agent shall
hold, along with the Deferred Area L/C, if applicable, pursuant to the terms and
provisions of this Agreement. Borrower shall deliver to Agent within one (1)
Business Day of its receipt of same any replacements, substitutions and renewals
of any Letters of Credit hereafter issued in favor of Borrower or any Affiliate
of Borrower, as beneficiary, and any and all other instruments, securities,
certificates of deposits, bonds or collateral delivered to Borrower or any
Affiliate of Borrower pursuant to the Lease from and after the date hereof as
security for the Tenant's obligations thereunder.

          (c)       The pledge and assignment made pursuant to this Agreement
shall be in addition to, and shall in no way limit or impair, any rights of
Agent in and to any other Collateral pursuant to the Loan Agreement or the other
Loan Documents.

         SECTION 2. SECURITY FOR TENANT CONSTRUCTION OBLIGATIONS. This Agreement
and the pledge and assignment made hereunder secure all construction obligations
of Tenant pursuant to the Work Letter, including Tenant's obligation to pay for
the Tenant Construction Work to the extent the cost thereof exceeds the
Construction Allowances or includes non-Eligible Expenses, as defined in the
Work Letter (the "TENANT OBLIGATIONS"). In no event shall any Letter of Credit
held hereunder be drawn upon for any reason other than to cure a Material Tenant
Construction Default in accordance with Section 7.8 of the Work Letter.

         SECTION 3. AGENT'S DOMINION AND CONTROL. Subject always to the terms of
the Lease, all Letters of Credit shall be under the sole dominion and control of
Agent and, subject to the terms of this Agreement, Agent shall have the sole
right to draw upon any Letter of Credit and to exercise any rights of Agent
hereunder with respect to funds drawn thereunder.

         SECTION 4. REPLACEMENT LETTERS OF CREDIT OR AMENDMENTS. (a) Pursuant
to the Work Letter, as construction of the Tenant Work progresses, Tenant shall
have the right to periodically (but not more frequently than quarterly) replace
or modify the Tenant Work L/C so as to reduce the amount payable thereunder, as
more specifically set forth in Section 10.2 of the Work Letter. Lender agrees to
cooperate and return any superseded Tenant Work L/C for which an appropriate
substitute has been delivered, provided that there then exists no outstanding,
uncured Material Tenant Construction Default. Any such substitute shall
constitute a Letter of Credit as defined in this Agreement.

          (b)       Pursuant to the Work Letter, upon timely achievement of
Interim Project Substantial Completion, Tenant shall have the right to reduce to
$2,500,000 the amount of the TI Security Deposit, as more specifically set forth
in Section 10.2 of the Work Letter. Lender agrees to cooperate and return any
superseded TI Security Deposit for which an appropriate substitute has been
delivered, provided that there then exists no outstanding, uncured Material
Tenant Construction Default. Any such substitute shall constitute a Letter of
Credit as defined in this Agreement.

         (c) Pursuant to the Work Letter, as construction of the Tenant Work
progresses, Tenant shall have the right to periodically (but not more frequently
than quarterly) replace or


                                       2
<PAGE>

modify the Deferred Area L/C so as to reduce the amount payable thereunder, as
more specifically set forth in Section 10.2 of the Work Letter. Lender agrees to
cooperate and return any superseded Deferred Area L/C for which an appropriate
substitute has been delivered, provided that there then exists no outstanding,
uncured Material Tenant Construction Default. Any such substitute shall
constitute a Letter of Credit as defined in this Agreement.

         SECTION 5. DRAWS ON LETTERS OF CREDIT; RETURN OF LETTERS OF CREDIT. (a)
Upon notice to Agent by Borrower (i) that there has been a Material Tenant
Construction Default as defined in Section 7.8 of the Work Letter attached to
the Lease, and (ii) specifying the nature of such Material Tenant Construction
Default and specifying the amount payable to Borrower under the Work Letter and
identifying the Letter(s) of Credit to which Borrower is entitled by reason of
such Material Tenant Construction Default, together with a copy of the notice of
Material Tenant Construction Default, duly delivered to Tenant in accordance
with the Lease, then so long as no Event of Default shall have occurred and be
continuing under the Loan Agreement or other Loan Documents, Agent shall deliver
the applicable Letter(s) of Credit to Borrower and Borrower may effect a partial
draw thereunder for purposes of paying for and thereafter return the Letter(s)
of Credit to Agent. In the event that Lender has taken possession of or become
the owner of the Project, then Lender may exercise for its own behalf, the
rights of Borrower described above.

          (b)       Notwithstanding any default or Event of Default by Borrower
under the Loan Agreement or other Loan Documents, upon written notice to Agent
by Borrower that (i) Tenant Construction Work has been completed in accordance
with the Work Letter and Tenant is entitled to the return of any Letter of
Credit, or (ii) Tenant is otherwise entitled to a return of any Letter of Credit
in accordance with the terms of the Lease, and identifying the Letter(s) of
Credit to be returned, Agent shall deliver the applicable Letter(s) of Credit to
Borrower for delivery to Tenant. If Borrower has not so notified Agent, but
Tenant believes that it is entitled to the return of any Letter of Credit, then
Tenant may so notify Agent and Borrower in writing, which notice shall be
accompanied by a representation that Tenant is entitled to the return of the
applicable Letter of Credit, accompanied by such evidence of completion as is
required by the Work Letter. If Borrower fails to object in writing, within
fifteen (15) days of receipt of such notice from Tenant, then Agent shall
deliver the applicable Letter(s) of Credit to Tenant. Any dispute between
Borrower and Tenant regarding Tenant's right to a return of a Letter of Credit
shall be resolved in accordance with the Dispute Resolution Process described in
EXHIBIT J to the Lease.

          (c)       Notwithstanding any default or Event of Default by Borrower
under the Loan Agreement or other Loan Documents, upon written notice to Agent
by Borrower that pursuant to the Work Letter, Tenant is entitled to reduce the
amount of the TI Security Deposit by $2,500,000 and upon receipt by Agent of a
Letter of Credit in a face amount that is $2,500,000 less than the face amount
of the original TI Security Deposit then being held by Agent and is otherwise in
form substantially similar to the TI Security Deposit delivered to Agent on the
date hereof, Agent shall deliver to Tenant the Letter(s) of Credit then being
held by Agent.

         SECTION 6. FURTHER ASSURANCE. (a) Borrower and Tenant agree on the date
hereof and at any time from time to time hereafter to execute and deliver
promptly all instruments and documents (including any UCC (as defined below)
filing necessary to perfect Agent's liens on,


                                       3
<PAGE>

and security interests in, the Letters of Credit for the benefit of the Lender
(collectively, the "Financing Statements")), and take all further action that
may be necessary or reasonably desirable or that Agent may reasonably request,
in order to perfect and protect any security interest granted or purported to be
granted hereby or to enable Agent to exercise and enforce its rights and
remedies hereunder with respect to any Letter of Credit.

         SECTION 7. AGENT'S DUTIES; INDEMNITY. (a) The powers conferred on Agent
hereunder are solely to protect its and the Lender's interests in the Letters of
Credit and shall not impose any duty upon it to exercise any such powers except
as expressly provided herein. Except for the safe custody of any Letter of
Credit in its possession, and the accounting for moneys actually received by it
hereunder and to return the Letters of Credit to Tenant as and when Tenant is
entitled thereto under the Terms of the Work Letter. Agent shall have no duty as
to any Letter of Credit or as to the taking of any necessary steps to preserve
rights against any parties or any other rights pertaining to such Letter of
Credit. Agent shall be deemed to have exercised reasonable care in the custody
and preservation of any Letter of Credit in its possession if such Letter of
Credit is accorded treatment substantially equal to that which it accords its
own property.

          (b)       Agent shall be entitled to rely upon the authenticity of any
signature and the genuineness and/or validity of any writing received by Agent
pursuant to or otherwise relating to this Agreement, including any photocopy or
facsimile of such a writing.

          (c)       Agent and the Lender shall not be liable to either of
Borrower or Tenant for any error of judgment, mistake or act or omission
hereunder or any matter or thing arising out of its conduct hereunder, except
Agent shall be liable for Agent's willful misfeasance or gross negligence.

          (d)       Borrower agrees to indemnify and hold harmless Agent from
and against any and all costs, claims, damages or expenses (including, without
limitation, reasonable attorneys' fees and disbursements) howsoever occasioned
that may be incurred by Agent acting under this Agreement, except to the extent
arising out of Agent's willful misfeasance or gross negligence.

         SECTION 8. REMEDIES. (a) Upon the occurrence and continuation of any
Material Tenant Construction Default under the Lease with respect to which
Borrower may exercise remedies in accordance with Section 7.8 of the Work Letter
and only to the extent of the amount of such Material Tenant Construction
Default, Agent may draw upon the Letters of Credit.

          (b)       Notwithstanding anything contained in this Agreement to the
contrary, this Agreement shall not be deemed to be an election of remedies
available to Agent under the Loan Documents, at law or in equity, nor shall it
be deemed to impair, waive or otherwise modify, in any manner, any rights or
remedies of Agent under the Loan Documents or any other document or instrument
evidencing, securing or relating to the Loan, or preclude or limit Agent from
pursuing at any time whatever remedies that are available to Agent under the
Loan Documents, at law or in equity.


                                       4
<PAGE>

          SECTION 9. AMENDMENTS, WAIVER. (a) Neither this Agreement, nor any
term or provision thereof may be changed, waived, discharged or terminated
except by an instrument in writing executed by the party against which
enforcement of the change, waiver, discharge or termination shall be sought.

          (b)       No failure on the part of Agent to exercise, and no delay in
exercising, any right under this Agreement or the other Loan Documents shall
operate as a waiver thereof-, nor shall any single or partial exercise of any
such right preclude any other or further exercise thereof or the exercise of any
other right under this Agreement or the other Loan Documents. The remedies
provided in this Agreement and the other Loan Documents are cumulative and not
exclusive of any remedies provided by law.

          SECTION 10. NOTICES. All notices, approvals, consents, requests,
demands and other communications with, to, from or upon the respective parties
hereto shall be in writing and shall be hand delivered or sent by guaranteed
overnight delivery service or by registered mail, return receipt requested,
postage prepaid, addressed as follows:

                                    If to Agent or the Lender:

                                    ------------------------
                                    ------------------------
                                    ------------------------
                                    Attention:
                                               ----------------
                                    FAX:

                                    With a required copy to:

                                    ------------------------
                                    ------------------------
                                    ------------------------
                                    Attention:
                                              ------------------
                                    FAX:
                                    If to Borrower:

                                    FC 35 Lansdowne, Inc.
                                    c/o Forest City Development
                                    38 Sidney Street
                                    Cambridge, MA 02139-4234
                                    Attention: Gayle Friedland
                                    FAX:   (617) 225-0311



                                    With a required copy to:


                                       5
<PAGE>

                                    Forest City Rental Properties Corporation
                                    1100 Terminal Tower
                                    50 Public Square
                                    Cleveland, OH 44113
                                    Attention: President
                                    FAX: (216) 263-6206


                                    With a copy to:

                                    Goodwin, Procter & Hoar, LLP
                                    Exchange Place
                                    Boston, MA 02109
                                    Attention:   Elizabeth McDermott, Esq.
                                    FAX:   (617) 227-8591

                                    If to Tenant:

                                    Millennium Pharmaceuticals, Inc.
                                    75 Sidney Street
                                    Cambridge, MA 02139-4211
                                    Attention: General Counsel
                                    FAX:

                                    With a copy to:

                                    Hale and Dorr
                                    60 State Street, 2nd Fl.
                                    Boston, MA 02109
                                    Attention: Katharine E. Bachman, Esq.
                                    FAX: (617) 526-5000

or to such other address as any party may designate from time to time by notice
to the others in the manner set forth herein. All such communications shall be
deemed to be given (i) if hand delivered or sent by guaranteed overnight
delivery service, on the day received, or (ii) if mailed, on the third Business
Day following deposit thereof in the U.S. Mail.

         SECTION 11. CONTINUING SECURITY INTEREST AND RETURN OF LETTERS OF
CREDIT. This Agreement shall create a continuing security interest, subject to
the terms and conditions set forth herein, in the Letters of Credit and shall
remain in full force and effect until the performance in full of the Tenant
Obligations. Notwithstanding the foregoing, upon the performance in full of the
Tenant Obligations, the security interest granted hereby shall automatically and
completely terminate without further act on the part of any party hereto and all
rights to the Letters of Credit shall revert to Tenant or Borrower, as provided
in the Lease. Upon any such termination, Agent will, at Borrower's expense,
immediately return to Borrower such of the Letters of Credit as shall not have
been sold or otherwise applied pursuant to the terms hereof and execute and
deliver to


                                       6
<PAGE>

Borrower, at Borrower's sole cost and expense, such documents as Borrower or
Tenant shall reasonably request to evidence such termination.

         SECTION 12. SUCCESSORS AND ASSIGNS. The provisions of this Agreement
shall be binding upon and shall inure to the benefit of Borrower, Agent and
Tenant and their respective successors, permitted assigns and legal
representatives; provided , however , that Borrower shall not have the right to
assign its rights or obligations hereunder or any interest herein without the
prior written consent of Agent.

         SECTION 13. SEVERABILITY. If any one or more provisions in this
Agreement shall be held to be invalid, illegal, void or unenforceable in any
respect (a) such provision or provisions shall be given force to the fullest
possible extent that they are valid, legal and enforceable, (b) such invalidity,
illegality, or unenforceability shall not affect any other provision of this
Agreement and (c) this Agreement shall be construed as if such invalid, illegal
or unenforceable provision had never been contained herein.

         SECTION 14. GOVERNING LAW; TERMS. This Agreement shall be governed by
and construed in accordance with the laws of the Commonwealth of Massachusetts,
except the creation, perfection and enforcement of the security interest
contemplated hereby shall be governed by and construed in accordance with the
laws of the State of _____________________. Unless otherwise defined herein or
in the Loan Agreement, terms defined in the UCC are used herein as therein
defined,

         SECTION 15. AGENT APPOINTED ATTORNEY-IN-FACT. Borrower hereby
irrevocably appoints Agent its attorney-in-fact with full authority in the place
and stead of Borrower and in the name of Borrower or otherwise in Agent's
discretion, subject always to the terms of the Lease, to take any action and to
execute any instrument that Agent may deem necessary or advisable to accomplish
the purposes of this Agreement.

         SECTION 16. AGENT IN POSSESSION. Borrower hereby confirms and agrees
that notwithstanding the provisions of this Agreement, Borrower retains sole
control of the operation and maintenance of the Project, subject to the
obligations of Borrower under this Agreement and the other Loan Documents, and
Agent is not and shall not be deemed to be a mortgagee in possession.

         SECTION 18. ENTIRE AGREEMENT. This Agreement embodies the entire
understanding of the parties with respect to the subject matter hereof, and all
prior representations, promises, warranties, understandings or agreements,
expressed or implied, oral or otherwise, relating hereto, are merged into this
Agreement.

         SECTION 19. WAIVER OF JURY TRIAL. EACH OF BORROWER AND TENANT
IRREVOCABLY AND AS AN INDEPENDENT COVENANT WAIVES A JURY TRIAL AND THE RIGHT
THERETO IN ANY ACTION OR PROCEEDING BETWEEN BORROWER AND/OR TENANT AND AGENT
AND/OR THE LENDER, WHETHER HEREUNDER OR OTHERWISE, IRRESPECTIVE OF WHICH PARTY
COMMENCES SUCH ACTION OR


                                       7
<PAGE>

PROCEEDING.

          SECTION 20. HEADINGS. The headings preceding the text of the
paragraphs of this Agreement are used solely for convenience of reference and
shall not affect the meaning or interpretation of this Agreement.

          SECTION 21. COUNTERPARTS. This Agreement may be executed in any number
of counterparts. All such counterparts will be deemed to be originals and will
together constitute but one and the same instrument.

         SECTION 22. CONSENT TO JURISDICTION. All disputes arising out of or
relating to this Agreement and all actions to enforce this Agreement shall be
adjudicated in the state courts of Massachusetts or the federal courts sitting
in Boston, Massachusetts and each of Borrower and Tenant hereby irrevocably
submits to the jurisdiction of such courts in any suit, action or proceeding
arising out of or relating to this Agreement or in any action to enforce this
Agreement.
                              *        *        *
                            [SIGNATURES ON NEXT PAGE]


                                       8
<PAGE>

         IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed and delivered as of the date first above written.


                                       BORROWER:

                                       FC 35 LANDSDOWNE, INC.

                                       By:
                                           -----------------------------
                                             Name:
                                              Title:


                                       AGENT:

                                                         , AS AGENT
                                       ------------------

                                       By: -----------------------------
                                             Name:
                                             Title:


                                       TENANT:

                                       MILLENNIUM PHARMACEUTICALS, INC.

                                       By:
                                            ------------------------------
                                             Name: Janet C. Bush
                                             Title:   Vice President of Finance


                                       9
<PAGE>

                                   Schedule I

                      Security Deposits as of Closing Date


         An Irrevocable Standby Letter of Credit, which allows for partial
draws, provided by Millennium Pharmaceuticals, Inc. issued by [State Street
Bank] naming FC 35 Lansdowne, Inc. (or any successor in interest thereto as
landlord under the lease) as Beneficiary in the amount of [$10,000,000]


         An Irrevocable Standby Letter of Credit, which allows for partial
draws, provided by Millennium Pharmaceuticals, Inc. issued by [State Street
Bank] naming FC 35 Lansdowne, Inc. (or any successor in interest thereto as
landlord under the lease) as Beneficiary in the amount of [$5,000,000]


                                       10



<PAGE>

                                    EXHIBIT D

                                STANDARD SERVICES

         The building standard services shall be defined by the Landlord and its
Management Agent. A listing of services shall be as promulgated from time to
time by the Landlord and shall be further described in the Tenant Handbook.

         The following services will be provided by the Landlord:

         A.       Regular maintenance of exterior and parking lot landscaping
                  and University Park common areas.

         B.       Regular maintenance, sweeping and snow removal of building
                  exterior areas such as roadways, driveways, sidewalks, parking
                  areas and courtyard paving.

         C.       Maintenance and repair of external base building surveillance,
                  fire and environmental alarm equipment, elevators, mechanical,
                  electrical, plumbing equipment on roof and life safety
                  systems, with Landlord hereby agreeing to comply with the
                  terms of applicable warranties in performing such maintenance.

         D.       Building surveillance and alarm system operation and live
                  monitoring service to building standard specifications. Tenant
                  may connect its monitoring system to Landlord's monitoring
                  system to enable to enable Landlord and Tenant to jointly
                  monitor security.

         E.       Daily, weekday rubbish removal from dumpsters.

         F.       Maintenance of exterior lighting.

         G.       Complete interior and exterior cleaning of all windows two
                  times per year.

         The following services will be provided by the Tenant:

         H.       Daily, weekday maintenance, passenger elevators, lobby areas
                  and vestibules.

         I.       Periodic cleaning of stairwells, freight elevators, and back
                  of house areas.

         J.       Daily, weekday rubbish removal of all tenant trash receptacles
                  other than dumpsters.

         K.       Daily, weekday cleaning of Tenant space to building standard.

<PAGE>

The foregoing allocation may be modified from time to time with the mutual
consent of Landlord and Tenant, such consent not to be unreasonably withheld,
conditioned or delayed

                                    EXHIBIT E

                              RULES AND REGULATIONS

         Wherever in these Rules and Regulations the word "Tenant" is used, it
shall be taken to apply to and include the Tenant and its agents, employees,
invitees, licensees, contractors, any subtenants and is to be deemed of such
number and gender as the circumstances require. The word "Premises" is to be
taken to include the space covered by the Lease. The word "Landlord" shall be
taken to include the employees and agents of Landlord. Other capitalized terms
used but not defined herein shall have the meanings set forth in the Lease.

         The following Rules and Regulations apply to buildings that are wholly
occupied by a single tenant.

         A.       No actions shall be taken by Tenant that modify the exterior
                  appearance of the Premises or the Building, unless expressly
                  permitted by Landlord.

         B.       No blinds, shades, awnings or other window treatments shall be
                  placed or installed in windows or curtain wall glazing other
                  than those approved and installed as Building Standard.

         C.       No sign, advertisement, notice or the like, shall be posted on
                  the Building exterior, in windows visible from the exterior of
                  the building, or in the building lobby except as permitted
                  under the terms of the Lease or as reasonably necessary in the
                  operation of the Tenant's operations without prior approval of
                  Landlord.

         D.       All deliveries to the Building, other than envelopes and small
                  packages that can legitimately be delivered by hand or bicycle
                  courier service, shall be accepted only through the building
                  loading dock and not through the main lobby.

         E.       Bicycles shall not be fastened to fences, signposts, or other
                  non-designated equipment.

         F.       Tenant shall not cause or permit any unusual or objectionable
                  odors, noises or vibrations to be produced upon or emanate
                  from said Premises.

         G.       No pets or other animals, excepting those used for legitimate
                  research purposes, shall be permitted in or about the Building
                  or the grounds of the Park.

                                       2

<PAGE>

         H        Unless specifically authorized by Landlord, employees or
                  agents of Landlord shall not perform for nor be asked by
                  Tenant to perform work other than their regularly assigned
                  duties.

         I.       Canvassing, soliciting and peddling in the Building is
                  prohibited and Tenant shall cooperate to prevent the same from
                  occurring.

         J.       Access roads and loading areas, parking areas, sidewalks,
                  entrances, lobbies, halls, walkways, elevators, stairways and
                  other common area provided by Landlord shall not be obstructed
                  by Tenant, or used for other purpose than for ingress and
                  egress.

         K.       Landlord shall have the right to make such other and further
                  reasonable rules and regulations as in the judgment of
                  Landlord, may from time to time be needful for the safety,
                  appearance, care and cleanliness of the Building or the park
                  and for the preservation of good order therein. All parking,
                  building operation, or construction rules and regulations
                  which may be established from time to time by Landlord on a
                  uniform basis shall be respected and obeyed, subject to the
                  requirements of this Lease.

         L.       Landlord shall not be responsible to Tenant for any violation
                  of rules and regulations by other tenants except that Landlord
                  shall use good faith efforts to uniformly enforce such rules
                  and regulations.

                                       3

<PAGE>

                                   EXHIBIT H-1

                      FORM OF MIT NON-DISTURBANCE AGREEMENT

         Agreement dated as of ______________, 2000, by and between
MASSACHUSETTS INSTITUTE OF TECHNOLOGY, a Massachusetts educational corporation
chartered by Massachusetts law (the "Ground Lessor"), FC 35 LANDSDOWNE, INC., a
Massachusetts corporation ("Landlord") and MILLENNIUM PHARMACEUTICALS, INC., a
Delaware corporation ("Tenant").

                                   BACKGROUND

         Ground Lessor and Landlord are parties, as landlord and tenant
respectively, to a Construction and Lease Agreement ("Ground Lease") dated
________________, 2000, for certain real property located at 35 Landsdowne
Street in Cambridge, Massachusetts, as more particularly described on Exhibit A
attached hereto ("Land"). A Notice of Lease pertaining to the Ground Lease has
been recorded at the Middlesex South District Registry of Deeds and filed for
registration in the Middlesex South Registry District of the Land Court.
Landlord intends to construct a building (collectively, the "Building") on the
Land. Tenant has entered into a lease dated as of _______________, 2000
("Lease") with Landlord for premises in the Building ("Premises"), the Premises
being more particularly described in the Lease.

                                   AGREEMENTS

         1. NON-DISTURBANCE. If the Ground Lease is terminated, for any reason,
Ground Lessor shall not disturb Tenant in Tenant's possession of the Premises
and without any hindrance or interference from the Ground Lessor, shall permit
Tenant peaceably to hold and enjoy the Premises for the remainder of the
unexpired term of the Lease, together with any extension periods provided for
therein, upon and subject to the same terms, covenants and conditions as are
contained in the Lease, and shall recognize the Lease as modified hereby. The
foregoing is on the condition that Tenant is not in default under the Lease
beyond any applicable notice and grace periods contained in the Lease.

         2. ATTORNMENT. Tenant hereby agrees that if the Ground Lease is
terminated for any reason, Tenant shall attorn to Ground Lessor and shall be
liable to and recognize Ground Lessor as Landlord under the Lease for the
balance of the term of the Lease upon and subject to all of the terms and
conditions thereof. In such case, upon receipt of notice from Ground Lessor
setting forth the effective date of the termination of the Ground Lease, Tenant
shall pay to the Ground Lessor all obligations required to be paid and performed
by Tenant under the Lease arising after the date of termination. The Lease shall
continue in full force and effect as a direct lease between Ground Lessor and
Tenant.

<PAGE>

         3. ADDITIONAL CONDITIONS. Tenant agrees that Ground Lessor shall not
be: (i) liable for any act or omission of any person or party who may be
landlord under the Lease prior to any termination of the Ground Lease ("Prior
Landlord"); (ii) subject to any offsets or defenses which Tenant might have
against Prior Landlord; (iii) bound by any prepayment of rent or additional
rent, or any other charge which Tenant might have paid to Prior Landlord for
more than the then current month (other than a bona fide security deposit paid
by Tenant to Landlord under the Lease or other rent, additional rent or charge
which has been received by Ground Lessor); and (iv) bound by any amendment,
modification or termination of the Lease made without Ground Lessor's express
agreement when such agreement is required under the Ground Lease. Tenant
additionally agrees with Ground Lessor that Tenant shall not enter into any
assignment of the Lease or sublease of all or any part of the Premises in cases
where Landlord's consent is required thereto, unless Ground Lessor shall have
also given its consent thereto, which consent shall not be unreasonably withheld
or delayed. Nothing herein, however, shall constitute a waiver of tenant's
rights as against such individual or entity which is the landlord under the
Lease as of the time of any event or circumstances which may give rise to a
claim of the Tenant against such individual or entity. In addition, nothing
herein shall relieve any successor landlord under the Lease from its obligation
to comply with those obligations of a Landlord under the Lease during the period
for which it is the owner of the Landlord's interest in the Lease.

         4. LANDLORD'S DEFAULTS. Tenant hereby agrees that, if Tenant provides
Landlord with any notice of default or claimed default on the part of Landlord
under the Lease, Tenant shall concurrently therewith send a copy of such notice
to Ground Lessor. In such event, Ground Lessor shall be permitted (but not
obligated) to cure any such default within the period of time allotted thereto
in the Lease. If Landlord shall fail to cure such default within the period of
time allocated thereto in the Lease (or, if Landlord shall not within such time
period have commenced diligent efforts to remedy a default that cannot be fully
cured within such time period) then Tenant shall provide Ground Lessor with
notice of such failure. Upon receipt of such notice of Landlord's failure to
cure, Ground Lessor shall be granted an additional thirty (30) days during which
it shall be permitted (but not obligated) to cure such default. In the case of a
default, which cannot with diligence be remedied by Ground Lessor within thirty
(30) days, Ground Lessor shall have such additional period of time as may be
reasonably necessary in order for Ground Lessor to remedy such default with
diligence and continuity of effort, provided that Ground Lessor has commenced to
cure such default within such thirty (30) day period.

         5. NOTICES. Duplicates of all notices delivered by any party to another
party and required by this Agreement shall be delivered concurrently to all
other parties to this Agreement. All notices shall be written, delivered by
certified or registered mail, and sent, if to Ground Lessor, to 238 Main Street,
Suite 200, Cambridge, Massachusetts 02142, Attention: Director of Real Estate,
if to Tenant to 75 Sidney Street, Cambridge, Massachusetts 02139-4211,
Attention: General Counsel, with a copy to Hale and Dorr LLP, 60 State Street,
Boston, MA 02109, Attention: Katharine E. Bachman, Esq., and if to Landlord to
38 Sidney Street, Cambridge, MA 02139-4234, Attention: Ms. Gayle W. Friedland,
or such addresses as may, from time to time, be set forth in notices to the
other parties hereunder.

                                       2

<PAGE>

         6. EXCULPATION OF GROUND LESSOR. Ground Lessor shall not be personally
liable hereunder. Tenant agrees to look to Ground Lessor's interest in the Land
and Building only for satisfaction of any claim against Ground Lessor hereunder.

         7. SUCCESSORS AND ASSIGNS. This Agreement shall bind Tenant, its
successors and assigns, and shall benefit Tenant and only such successor and
assigns of Tenant as are permitted by the Lease and shall bind and benefit
Ground Lessor and its successors and assigns (provided that after transfer of
Ground Lessor's entire interest in the Land to another party, Ground Lessor
shall have no liability for any act or omission of such party) and shall bind
and benefit Landlord and its successors and assigns.

         EXECUTED as an instrument under seal as of the date set forth above.

                                    MASSACHUSETTS INSTITUTE OF TECHNOLOGY

                                    Ground Lessor

                                    By:
                                        --------------------------------------
                                          Name:

                                          Title:

                                    MILLENNIUM PHARMACEUTICALS, INC.

                                    Tenant

                                    By:
                                        --------------------------------------

                                    FC 35 LANDSDOWNE, INC.

                                    Landlord

                                    By:
                                        --------------------------------------

COMMONWEALTH OF MASSACHUSETTS       )

                                    )        ss:

COUNTY OF MIDDLESEX                 )

         BEFORE ME, a Notary Public in and for said County and State, personally
appeared the MASSACHUSETTS INSTITUTE OF TECHNOLOGY, by___________________, its
Director

                                       3

<PAGE>

of Real Estate and Associate Treasurer, who acknowledged that he did sign the
foregoing instrument and that the same is his free act and deed and the free act
and deed of said corporation.

         IN TESTIMONY HEREOF, I set my hand and official seal at
__________________, this _____ day of __________________, ____.


                                           ----------------------------------
                                           Notary Public

                                           My Commission Expires:
                                                                  -----------
COMMONWEALTH OF MASSACHUSETTS       )

                                    )        ss:

COUNTY OF MIDDLESEX                 )

         BEFORE ME, a Notary Public in and for said County and State, personally
appeared the above-named MILLENNIUM PHARMACEUTICALS, INC., by
______________________ who acknowledged that he/she did sign the foregoing
instrument and that the same is his/her free act and deed and the free act and
deed of said corporation.

         IN TESTIMONY HEREOF, I set my hand and official seal at
__________________, this _____ day of __________________, ____.


                                           ----------------------------------
                                           Notary Public

                                           My Commission Expires:
                                                                  -----------

COMMONWEALTH OF MASSACHUSETTS       )

                                                              )        ss:

COUNTY OF MIDDLESEX                         )

         BEFORE ME, a Notary Public in and for said County and State, personally
appeared the above-named FC 35 LANDSDOWNE, INC., by _______________________ who
acknowledged that he/she did sign the foregoing instrument and that the same is
his/her free act and deed and the free act and deed of said corporation.

         IN TESTIMONY HEREOF, I set my hand and official seal at
__________________, this _____ day of __________________, ____.


                                           ----------------------------------
                                           Notary Public

                                           My Commission Expires:
                                                                  -----------

                                       4

<PAGE>

                                    EXHIBIT I

                         FORM OF PERMANENT LENDER'S SNDA
                 [TO BE PROVIDED BY PERMANENT LENDER ONLY AFTER
                     COMPLETION OF THE MILLENNIUM PROJECT]

             SUBORDINATION, NONDISTURBANCE AND ATTORNMENT AGREEMENT

         THIS AGREEMENT made as of the ____ day of ___________, 2000 between
____________________________________, a _________________________________,
having an office at ________________________________________________________
("MORTGAGEE"), FC 35 LANDSDOWNE, INC., a Massachusetts corporation, having an
office at 1100 Terminal Tower, 50 Public Square, Cleveland, Ohio 44113
("LANDLORD"), and MILLENNIUM PHARMACEUTICALS, INC., a Delaware corporation,
having an office at 238 Main Street, Cambridge, Massachusetts ("TENANT").

                              W I T N E S S E T H:

         WHEREAS, Tenant has entered into a lease dated _____________, 2000
between Landlord, as landlord, and Tenant, as tenant, with respect to certain
space (the "DEMISED PREMISES") in the building located at 35 Landsdowne Street,
Cambridge, Massachusetts (said lease, as hereafter amended and supplemented,
subject to Paragraph 7 hereof, is hereinafter called the "LEASE"); capitalized
terms used herein and not otherwise defined herein shall have the same meanings
set forth in the Lease; and

         WHEREAS, Mortgagee is the holder of the Leasehold Mortgage with
Assignments of Rents, Security Agreement and Fixture Filing dated ____________,
2000 from Landlord to Mortgagee (the "MORTGAGEE"), which encumbers the land and
improvements more particularly described in Exhibit A hereto (the "PREMISES"),
of which the Demised Premises are a part, and Landlord's interest in the Lease;
and

         WHEREAS, Mortgagee, Landlord and Tenant desire to enter into this
Agreement upon the terms, covenants and conditions contained herein.

         NOW, THEREFORE, in consideration of the premises and the agreements of
the parties contained herein, the parties agree as follows:

         1. The Lease and all of Tenant's rights thereunder are and shall be at
all times and in all respects subject and subordinate to the lien of the
Mortgage, and to all advances now or hereafter made under or secured by the
Mortgage, and all renewals, modifications, consolidations, replacements,
substitutions, additions and extensions of the Mortgage and to any subsequent
mortgages or assignments with which the Mortgage may be spread and/or
consolidated.

<PAGE>

         2. Provided Tenant complies with this Agreement and if Tenant shall not
be in default under the Lease beyond the applicable period of grace, if any,
provided therein with respect to the default in question as of the date
Mortgagee commences a foreclosure action, (i) Tenant shall not be named as a
party in any action or proceeding to enforce the Mortgage, unless such joinder
shall be required under applicable law, and in which case Mortgagee shall not
seek affirmative relief from Tenant in such action or proceeding, nor shall the
Lease be cut off or terminated nor Tenant's possession thereunder be disturbed
in any such action or proceeding, and (ii) subject to the provisions of
Paragraph 4 of this Agreement, Mortgagee will recognize the Lease and Tenant's
rights thereunder.

         3. Upon any foreclosure of the Mortgage or other acquisition of the
leasehold estate in the Premises, Tenant shall attorn to Mortgagee or any other
party acquiring the leasehold estate in the Premises or so succeeding to
Landlord's rights (collectively, the "SUCCESSOR LANDLORD") and shall recognize
the Successor Landlord as its landlord under the Lease and Tenant shall promptly
execute and deliver any instrument that the Successor Landlord may reasonably
request in writing to evidence further said attornment.

         4. Upon such attornment or other acquisition of the leasehold estate in
the Premises, the Lease shall continue as a direct lease between the Successor
Landlord and Tenant upon all terms, covenants and conditions thereof as are then
applicable except that the Successor Landlord shall not be (i) liable for any
previous act or omission of Landlord under the Lease, (ii) subject to any
offsets, defenses, claims or counterclaims that Tenant may have against
Landlord, (iii) bound by any covenant to perform or complete any construction in
connection with the Demised Premises or the Premises or to pay any sums to
Tenant in connection therewith, (iv) bound by any prepayment of more than one
(1) month's rent or other charges under the Lease unless such payment shall have
been expressly approved in writing by Mortgagee, (v) bound by any amendment,
modification, extension, expansion, termination, cancellation or surrender of
the Lease unless approved in writing by Mortgagee, or (vi) liable for any
security deposit given by Tenant under the Lease, unless and to the extent in
Mortgagee's possession on or after the date that Successor Landlord succeeds to
the interest of Landlord under the Lease.

         5. The attornment provided for in Paragraph 3 of this Agreement shall
inure to the benefit of Mortgagee or any Successor Landlord, shall be
self-operative, and no further instrument shall be required to give effect to
the attornment. Tenant, however, upon demand of Mortgagee or any Successor
Landlord, as the case may be, agrees to execute, from time to time, instruments
in confirmation thereof, reasonably satisfactory to Mortgagee or any such
Successor Landlord, acknowledging such attornment and setting forth the terms
and conditions of its tenancy. Nothing contained in this Paragraph shall be
construed to impair any right otherwise exercisable by Mortgagee or any such
Successor Landlord.

         6. Tenant from and after the date hereof shall send a copy of any
notice of default or notice in connection with the commencement of any action to
terminate the Lease or similar

                                       2

<PAGE>

statement under the Lease to Mortgagee at the same time such notice or statement
is sent to Landlord under the Lease and agrees that, notwithstanding any
provisions of the Lease to the contrary, such notice shall not be effective
unless Mortgagee shall have been given such notice and shall have failed to cure
such default as hereinafter provided. Such notices shall be sent by certified or
registered mail, postage prepaid, return receipt requested, or shall be
delivered to Mortgagee at the following address (or at such other address as
Mortgagee shall specify in a written notice to Tenant at the address specified
above for Tenant):

                        -------------------------------

                        -------------------------------

                        -------------------------------

                        -------------------------------


Any notice to Tenant hereunder shall be sent to the address specified above,
with a copy to Hale and Dorr LLP, 60 State Street, Boston, MA 02109, Attention:
Katharine E. Bachman, Esq.

Any such notice of default shall be deemed to be given to Mortgagee on the
earlier to occur of (a) the day of receipt (as evidenced by a receipt signed by
Mortgagee or the refusal to accept delivery by Mortgagee) or (b) three (3) days
after deposit in the mail. With respect to the commencement by Tenant of any
action to terminate the Lease, Mortgagee shall have the right, but not the
obligation, to cure any default on the part of Landlord which is the basis for
such action within a reasonable time (including the time required for Mortgagee
to foreclose the Mortgage and obtain possession of the Premises if such
possession is necessary to effect such cure) after receipt of the notice by
Tenant with respect to such action, so long as Mortgagee is diligently
prosecuting such cure and/or the foreclosure of the Mortgagee if possession of
the Premises is necessary to effect such cure. Mortgagee's right to cure shall
not apply to the exercise by Tenant of its right to terminate the Lease pursuant
to Section 12.18 thereof or Article 9 of the Work Letter, and shall not affect
Tenant's rights under Section 2.5 of the Lease.

         7. Tenant shall not change, or consent to a change in, the terms,
covenants, conditions and agreements of the Lease in any manner which would be
binding on Mortgagee without the express consent in writing of Mortgagee. No
exercise by Landlord of any right to terminate the Lease, and no acceptance of
any termination, surrender or cancellation of the Lease (except as otherwise
expressly referred to in Paragraph 6 above), shall be effective unless Mortgagee
shall have expressly consented thereto in writing.

         8. Anything herein or in the Lease to the contrary notwithstanding, in
the event that Successor Landlord shall acquire title to the Premises, Successor
Landlord shall have no obligation, nor incur any liability, beyond Successor
Landlord's then interest, if any, in the Premises and Tenant shall look
exclusively to such interest of Successor Landlord, if any, in the Premises for
the payment and discharge of any obligations imposed upon Successor Landlord
hereunder or under the Lease and Successor Landlord is hereby released or
relieved of any other liability hereunder and under the Lease. Tenant agrees
that with respect to any money judgment

                                       3

<PAGE>

which may be obtained or secured by Tenant against Successor Landlord, Tenant
shall look solely to the estate or interest owned by Successor Landlord in the
Premises and Tenant will not collect or attempt to collect any such judgment (i)
from any officer, director, shareholder, partner, employee, agent or
representative of Successor Landlord or (ii) out of any assets of Successor
Landlord other than Successor Landlord's estate or interest in the Premises.

         9. (a) Tenant acknowledges that it has notice that Landlord's interest
under the Lease and the rent and all other sums due thereunder have been
assigned to Mortgagee, pursuant to the Mortgage as part of the security for the
obligations secured by the Mortgage. In the event that Mortgagee notifies Tenant
of a default under the Mortgage and demands that Tenant pay its rent and all
other sums due under the Lease to Mortgagee, Tenant agrees that it shall pay its
rent and all other sums due under the Lease to Mortgagee or as Mortgagee shall
direct in writing to Tenant at its address for notices set forth in Section 12.2
of the Lease.

            (b) Tenant hereby acknowledges receipt of a notice from Landlord
directing Tenant to pay, INTER ALIA, all Annual Fixed Rent and Additional Rent
directly to account #__________ at ______________________________. Tenant shall
not make any payment under the Lease contrary to the aforementioned direction
without the written consent of Mortgagee.

            (c) Landlord hereby indemnifies and holds Tenant harmless from and
against all claims, loss or damage of whatever nature arising from Tenant's
compliance with Paragraphs 9(a) and (b) above.

         10. Landlord hereby consents to the terms and provisions of this
Agreement, including, without limitation, Paragraph 9 hereof.

         11. This Agreement may not be modified, amended or terminated unless in
writing and duly executed by the party against whom the same is sought to be
asserted and constitutes the entire agreement between the parties with respect
to the subject matter hereof.

         12. This Agreement shall bind and inure to the benefit of the parties
hereto and their respective successors and assigns.

         13. Notwithstanding anything to the contrary contained in Section 12.10
of the Lease, the Lease shall not terminate upon or as a result of the
termination of the Ground Lease if Mortgagee exercises its rights under the
Ground Lease to request that MIT (or its successors and assigns) enter into a
"new lease" for the Premises. The entering into of any such "new lease" shall
have the same effect as the acquisition of the leasehold estate in the Premises
for purposes of Paragraphs 3 and 4 hereunder.

                                       4

<PAGE>

         14. Tenant agrees that Tenant shall from time to time, upon not less
than fifteen (15) days' prior written request by Mortgagee, execute,
acknowledge, deliver and certify to Mortgagee a statement in the form set forth
in Section 10.4 of the Lease.

                                        5

<PAGE>

         IN WITNESS WHEREOF, the parties hereto have duly executed this
Agreement as of the day and year first above written.

                        MORTGAGEE:

                        By:
                            --------------------------------------------------
                            Name:
                            Title:

                        TENANT:

                        MILLENNIUM PHARMACEUTICALS, INC.

                        By:
                           ---------------------------------------------------
                           Name:
                           Title:

                        LANDLORD:

                        FC 35 LANDSDOWNE, INC.

                        By:
                           ---------------------------------------------------

                             By:
                                 ---------------------------------------------
                                 Name:
                                 Title:

                          [ACKNOWLEDGMENTS TO BE ADDED]

                                       6

<PAGE>

                                    EXHIBIT J

                           DISPUTE RESOLUTION PROCESS

         Any dispute or determination by a party hereto which, pursuant to the
terms of this Lease, may be resolved by the "Dispute Resolution Process," shall
be undertaken in accordance with the following provisions:

         (a) In the event of any such dispute, the complaining party (the
"Claimant") shall serve upon the other party (the "Respondent") by registered
mail or hand delivery a written demand for arbitration (the "Dispute Notice"),
setting forth with particularity the nature of the dispute. The Claimant shall
simultaneously serve any request (the "Document Request") for production of
relevant documents from the Respondent. The service of such Dispute Notice and
Document Request shall be effective upon receipt thereof. Failure to serve a
Document Request shall constitute a waiver by the Claimant of any right to
demand documents from the Respondent, except as provided in Subparagraph (c)
below. The Dispute Notice shall also be delivered to J.A.M.S./Endispute, 73
Tremont Street, 4th floor, Boston, Massachusetts 02108, for mediation as part of
that firm's national mediation services expertise. J.A.M.S./Endispute shall
select a member of the company to conduct the arbitration (hereinafter the
"Arbitrator"), the choice of which shall be binding on the parties. If
J.A.M.S./Endispute believes it has a material conflict of interest with any of
the parties, it shall select an alternative nationally recognized firm to
conduct the arbitration, within ten (10) business days of receipt of the Dispute
Notice. If J.A.M.S./Endispute shall cease to exist and/or shall decline to serve
under this Lease as to all or any particular dispute submitted thereto for
arbitration, and within ten (10) business days of receipt of a Dispute Notice,
shall fail to select an alternative nationally recognized firm, then, and in any
such event, the parties shall mutually select an alternative arbitrator for
their dispute(s) and, in the absence of agreement within a period of ten (10)
days, either party shall have the right, on notice to the other, to apply to the
President of the Boston Bar Association for selection of an independent
arbitrator.

         (b) RESPONSE BY RESPONDENT. Within ten (10) business days of receipt of
a Dispute Notice and Document Request, the Respondent shall serve a detailed
written response to the Dispute Notice, including any arbitrable counterclaims,
and shall produce all non-privileged documents called for in the Document
Request. At the same time, Respondent shall serve any Document Request on
Claimant, failing which Respondent shall be deemed to have waived any right to
demand documents from Claimant. Within two (2) business days of delivery of the
response, all undisputed amounts shall be paid by Respondent by wire transfer.

          (c) RESPONSE BY CLAIMANT. Within ten (10) business days of receipt of
such written response, the Claimant shall serve a reply to any counterclaims
asserted by Respondent and shall produce all non-privileged documents requested
by Respondent. At the same time, the Claimant may serve a second Document
Request limited to documents relevant to Respondent's

<PAGE>

counterclaim. Within two (2) business days of delivery of the reply to any
counterclaims, all undisputed amounts shall be paid by the Claimant by wire
transfer.

         (d) RESPONSE BY RESPONDENT. Respondent shall produce all non-privileged
documents called for in any such second Document Request within ten (10)
business days of service thereof.

         (e) Appearance Before Arbitrator. Within thirty-five (35) business days
of service of the Dispute Notice, any arbitrable dispute shall be submitted to
the Arbitrator, whose decision shall be final, binding and non-appealable, and
may be entered and enforced as a judgment by any court of competent
jurisdiction. The Arbitrator shall consider and determine only matters properly
subject to arbitration pursuant to this Lease.

         The Arbitrator shall, in consultation with the parties, establish such
further procedures, including hearings, as he or she deems appropriate,
provided, however, that a decision of the dispute (including counterclaims)
shall be rendered no later than sixty (60) business days after service of the
Dispute Notice.

         (f) Final Decision; Fees and Expenses. The Arbitrator's decision shall
be in writing, and shall include findings of fact and a concise explanation of
the reasons for the decision. The decision shall be delivered to the parties
immediately. The Arbitrator's fees and expenses shall be borne by one or both of
the parties in accordance with the direction of the Arbitrator, who shall be
guided in such determination by the results of the arbitration. If any party
refuses to appear before the Arbitrator or to respond as required in
subparagraphs (a) through (e) above, the Arbitrator shall decide the matter as
by default against the non-appearing party, and such decision shall be final,
binding and non-appealable to the same extent as a decision rendered with the
full participation of such party.

                                       2

<PAGE>

                                    EXHIBIT K

             SUBORDINATION, NONDISTURBANCE AND ATTORNMENT AGREEMENT

         THIS AGREEMENT made as of the _____ day of _____________, 2000, between
_______________________________ ("Mortgagee"), FC 35 LANDSDOWNE, INC., a
Massachusetts corporation, having an office at c/o Forest City Development, 38
Sidney Street, Cambridge, Massachusetts 02139 ("Landlord"), and MILLENNIUM
PHARMACEUTICALS, INC., a Delaware corporation, having an office at 75 Sidney
Street, Cambridge, Massachusetts 02139 ("Tenant").

                              W I T N E S S E T H:

         WHEREAS, Tenant has entered into a lease dated _______________, 2000
between Landlord, as landlord, and Tenant, as tenant, with respect to certain
space (the "Demised Premises") in the buildings located at 35 Landsdowne Street,
Cambridge, Massachusetts, (said lease, as hereafter amended and supplemented,
subject to Paragraph 7 hereof, is hereinafter called the "Lease"), capitalized
terms used herein and not otherwise defined herein shall have the same meanings
set forth in the Lease); and

         WHEREAS, Mortgagee is the holder of the Leasehold Mortgage with
Assignments of Rents, Security Agreement and Fixture Filing dated _____________,
2000, from Landlord to Mortgagee (the "Mortgage"), which encumbers the land and
improvements more particularly described in Exhibit __ hereto (the "Premise"),
of which the Demised Premises are a part, and Landlord's interest in the Lease;
and

         WHEREAS, Mortgagee, Landlord and Tenant desire to enter into this
Agreement upon the terms, covenants and conditions contained herein;

         NOW, THEREFORE, in consideration of the premises and the agreements of
the parties contained herein, the parties agree as follows:

         1. The Lease and all of Tenant's rights thereunder are and shall be at
all times and in all respects subject and subordinate to the lien of the
Mortgage, and to all advances now or hereafter made under or secured by the
Mortgage, and all renewals, modifications, consolidations, replacements,
substitutions, additions and extensions of the Mortgage and to any subsequent
mortgages or assignments with which the Mortgage may be spread and/or
consolidated.

         2. Provided Tenant complies with this Agreement and if Tenant shall not
be in default under the Lease beyond the applicable period of grace, if any,
provided therein with respect to the default in question as of the date
Mortgagee commences a foreclosure action,

<PAGE>

(i) Tenant shall not be named as a party in any action or proceeding to enforce
the Mortgage, unless such joinder shall be required under applicable law, and in
which case Mortgagee shall not seek affirmative relief from Tenant in such
action or proceeding, nor shall the Lease be cut off or terminated nor Tenant's
possession thereunder be disturbed in any such action or proceeding, and (ii)
subject to the provisions of Paragraph 4 of this Agreement, Mortgagee will
recognize the Lease and Tenant's rights thereunder and hereunder.

         3. Upon any foreclosure of the Mortgage or other acquisition of the
leasehold estate in the Premises, Tenant shall attorn to Mortgagee or any other
party acquiring the leasehold estate in the Premises or so succeeding to
Landlord's rights (collectively, the "Successor Landlord") and shall recognize
the Successor Landlord as its landlord under the Lease and Tenant shall promptly
execute and deliver any instrument that the Successor Landlord may reasonably
request in writing to evidence further said attornment.

         4. (a) Upon such attornment or other acquisition of the leasehold
estate in the Premises, the Lease shall continue as a direct lease between the
Successor Landlord and Tenant upon all terms, covenants and conditions thereof
as are then applicable except that the Successor Landlord shall not be (i)
liable for any previous act or omission of Landlord under the Lease, (ii)
subject to any offsets, defenses, claims or counterclaims that Tenant may have
against Landlord, (iii) except as provided in Paragraph 4(b) below, bound by any
covenant to perform or complete any construction in connection with the Demised
Premises or the Premises or to pay any sums to Tenant in connection therewith,
(iv) bound by any prepayment of more than one (1) month's rent or other charges
under the Lease unless such payment shall have been expressly approved in
writing by Mortgagee, (v) bound by any amendment, modification, extension,
expansion, termination, cancellation or surrender of the Lease unless approved
in writing by Mortgagee, or (vi) liable for any security deposit given by Tenant
under the Lease, except as provided in a Pledge Agreement between Landlord,
Tenant, and Mortgagee of even date herewith.

             (b) In the event Landlord shall have failed to complete and
pay for the Base Building Improvements or to pay the Leasehold Improvement
Allowance as and to the extent required under the Lease (collectively, the
"Construction/TI Obligations") on or prior to the date that Successor Landlord
succeeds to the interest of Landlord under the Lease (such date being the
"Succession Date"), Successor Landlord shall be obligated, from and after the
Succession Date, to perform the Construction/TI Obligations.

         5. The attornment provided for in Paragraph 3 of this Agreement shall
inure to the benefit of Mortgagee or any Successor Landlord and shall be
self-operative, and no further instrument shall be required to give effect to
the attornment. Tenant, however, upon demand of Mortgagee or any Successor
Landlord, as the case may be, agrees to execute, from time to time, instruments
in confirmation thereof, reasonably satisfactory to Mortgagee or any such
Successor Landlord, acknowledging such attornment and setting forth the terms
and conditions of its tenancy. Nothing contained in this Paragraph shall be
construed to impair any right otherwise exercisable by Mortgagee or any such
Successor Landlord.

                                       3

<PAGE>

         6. Tenant from and after the date hereof shall send a copy of any
notice of default or notice in connection with the commencement of any action to
terminate the Lease or similar statement under the Lease to Mortgagee at the
same time such notice or statement is sent to Landlord under the Lease and
agrees that, notwithstanding any provisions of the Lease to the contrary, such
notice shall not be effective unless Mortgagee shall have been given such notice
and shall have failed to cure such default as hereinafter provided. Such notices
shall be sent by certified or registered mail, postage prepaid, return receipt
requested, or shall be delivered to Mortgagee at the following address (or at
such other address as Mortgagee shall specify in a written notice to Tenant at
the address specified above for Tenant):

                  -----------------------------------

                  -----------------------------------

                  -----------------------------------

                  Attention:
                             ------------------------

Any such notice of default shall be deemed to be given to Mortgagee on the
earlier to occur of (a) the day of receipt (as evidenced by a receipt signed by
Mortgagee or the refusal to accept delivery by Mortgagee) or (b) three (3) days
after deposit in the mail. With respect to the commencement by Tenant of any
action to terminate the Lease, Mortgagee shall have the right, but not the
obligation, to cure any default on the part of Landlord which is the basis for
such action within a reasonable time (including the time required for Mortgagee
to foreclose the Mortgage and obtain possession of the Premises if such
possession is necessary to effect such cure) after receipt of the notice by
Tenant with respect to such action, so long as Mortgagee is diligently
prosecuting such cure and/or the foreclosure of the Mortgage if possession of
the Premises is necessary to effect such cure. Mortgagee's right to cure shall
not apply to the exercise by Tenant of its right to terminate the Lease pursuant
to Section 12.18 thereof and shall not affect Tenant's rights under Section 2.5
of the Lease or Section 4(b) of this Agreement.

         7. Tenant shall not change, or consent to a change in, the terms,
covenants, conditions and agreements of the Lease in any manner which would be
binding on Mortgagee without the express consent in writing of Mortgagee. No
exercise by Landlord of any right to terminate the Lease, and no acceptance of
any termination, surrender or cancellation of the Lease (except as expressly set
forth in the last sentence of Paragraph 6 above), shall be effective unless
Mortgagee shall have expressly consented thereto in writing.

         8. Anything herein or in the Lease to the contrary notwithstanding, in
the event that Successor Landlord shall acquire title to the Premises, Successor
Landlord shall have no obligation, nor incur any liability, beyond Successor
Landlord's then interest, if any, in the Premises and Tenant shall look
exclusively to such interest of Successor Landlord, if any, in the Premises for
the payment and discharge of any obligations imposed upon Successor Landlord
hereunder or under the Lease and Successor Landlord is hereby released and
relieved of any other liability hereunder and under the Lease. Tenant agrees
that with respect to any money

                                       4

<PAGE>

judgment which may be obtained or secured by Tenant against Successor Landlord,
Tenant shall look solely to the estate or interest owned by Successor Landlord
in the Premises and Tenant will not collect or attempt to collect any such
judgment (i) from any officer, director, shareholder, partner, employee, agent
or representative of Successor Landlord or (ii) out of any assets of Successor
Landlord other than Successor Landlord's estate or interest in the Premises.

         9. (a) Tenant acknowledges that it has notice that Landlord's interest
under the Lease and the rent and all other sums due thereunder have been
assigned to Mortgagee pursuant to the Mortgage as part of the security for the
obligations secured by the Mortgage. In the event that Mortgagee notifies Tenant
of a default under the Mortgage and demands that Tenant pay its rent and all
other sums due under the Lease to Mortgagee, Tenant agrees that it shall pay its
rent and all other sums due under the Lease to Mortgagee or as Mortgagee shall
direct in writing to Tenant at its address for notices set forth in Section 12.2
of the Lease.

            (b) Tenant hereby acknowledges receipt of a notice from Landlord
directing Tenant to pay, inter ali, all Annual Fixed Rent and Additional Rent
directly to account # __________________ at ___________________________.
Tenant shall not make any payment under the Lease contrary to the
aforementioned direction without the written consent of Mortgagee.

            (c) Landlord hereby indemnifies Tenant and holds Tenant harmless
from and against all claims, losses or damages of whatever nature arising from
Tenant's compliance with Paragraphs 9(a) and (b) above.

         10. Landlord hereby consents to the terms and provisions of this
Agreement, including, without limitation, Paragraph 9 hereof.

         11. This Agreement may not be modified, amended or terminated unless in
writing and duly executed by the party against whom the same is sought to be
asserted and constitutes the entire agreement between the parties with respect
to the subject matter hereof.

         12. This Agreement shall bind and inure to the benefit of the parties
hereto and their respective successors and assigns.

         13. Notwithstanding anything to the contrary contained in Section 12.10
of the Lease, the Lease shall not terminate upon or as a result of the
termination of the Ground Lease if Mortgagee exercises its rights under the
Ground Lease to request that MIT (or its successors and assigns) enter into a
"new lease" for the Premises. The entering into of any such "new lease" shall
have the same effect as the acquisition of the leasehold estate in the Premises
for purposes of Paragraphs 3 and 4 hereof.

                                       5

<PAGE>

         14. Tenant agrees that Tenant shall from time to time, upon not less
than fifteen (15) days' prior written request by Mortgagee, execute,
acknowledge, deliver and certify to Mortgagee a statement in the form set forth
in Section 10.4 of the Lease.

         15. This Agreement may be executed in any number of counterparts. All
such counterparts will be deemed to be originals and will together constitute
but one and the same instrument.

         IN WITNESS WHEREOF, the parties hereto have duly executed this
Agreement as of the day and year first above written.

                                      MORTGAGEE:

                                      ------------------------------

                                      By:
                                         ---------------------------
                                         Name:
                                         Title:

                                      TENANT:

                                      MILLENNIUM PHARMACEUTICALS, INC.

                                      By:
                                         ---------------------------
                                         Name:
                                         Title:

                                      LANDLORD:

                                      FC 35 LANDSDOWNE, INC.

                                      By:
                                         ---------------------------

                                          By:
                                             -----------------------
                                             Name:
                                             Title:

                                       6

<PAGE>

State of                   )
                           ) ss.:

County            )

         On the ___ day of _____________ in the year 2000, before me, the
undersigned, a Notary Public in and for said State, personally appeared
_____________________, personally known to me or proved to me on the basis of
satisfactory evidence to be the individual(s) whose name(s) is (are) subscribed
to the within instrument and acknowledged to me that he/she/they executed the
same in his/her/their capacity(ies) and as his/her/their free act and deed, and
that by his/her/their signature(s) on the instrument, the individual(s), or the
person upon behalf of which the individual(s) acted, executed the instrument.

(STAMP/SEAL)                                Notary Public

My Commission expires:


<PAGE>


State of                   )
                           ) ss.:

County            )

         On the ___ day of ____________ in the year 2000, before me, the
undersigned, a Notary Public in and for said State, personally appeared
_____________________, personally known to me or proved to me on the basis of
satisfactory evidence to be the individual(s) whose name(s) is (are) subscribed
to the within instrument and acknowledged to me that he/she/they executed the
same in his/her/their capacity(ies) and as his/her/their free act and deed, and
that by his/her/their signature(s) on the instrument, the individual(s), or the
person upon behalf of which the individual(s) acted, executed the instrument.

(STAMP/SEAL)                                Notary Public

My Commission expires:


<PAGE>

State of                   )
                           ) ss.:

County            )

         On the ___ day of _____________ in the year 2000, before me, the
undersigned, a Notary Public in and for said State, personally appeared
_____________________, personally known to me or proved to me on the basis of
satisfactory evidence to be the individual(s) whose name(s) is (are) subscribed
to the within instrument and acknowledged to me that he/she/they executed the
same in his/her/their capacity(ies) and as his/her/their free act and deed, and
that by his/her/their signature(s) on the instrument, the individual(s), or the
person upon behalf of which the individual(s) acted, executed the instrument.

(STAMP/SEAL)                                Notary Public

My Commission expires:


<PAGE>

                                    EXHIBIT L

                          FOREST CITY ENTERPRISES, INC.
                               1100 TERMINAL TOWER

                                50 PUBLIC SQUARE
                            CLEVELAND, OH 44113-2203

                                      ____________, 2000

Millennium Pharmaceuticals, Inc.
75 Sidney Street
Cambridge, MA  02139

         Re:      Completion Guaranty of Forest City Enterprises, Inc. regarding
                  that certain lease, dated July __, 2000 (the "Lease") between
                  FC 35 Landsdowne, Inc. ("Landlord") and Millennium
                  Pharmaceuticals, Inc. ("Tenant") for premises located at 35
                  Landsdowne Street, Cambridge, MA

         As an inducement to your entering into the Lease, and in consideration
for your entering into the Lease with Landlord, which constitutes valuable
consideration, the receipt and sufficiency of which is hereby acknowledged, the
undersigned covenants and agrees as follows:

         1. Except as expressly provided in paragraph 2 below, if demand is made
upon the undersigned under a completion guaranty ("Bank Completion Guaranty") to
Landlord's Construction Lender (the "Bank") the form of which is anticipated to
substantially follow the form of guaranty delivered in connection with the
financing of the project known as 45/75 Sidney Street, a copy of which is
attached hereto, to perform any construction obligations relating to the Base
Building Improvements, or the undersigned can otherwise require the Bank to make
advances of the loan to which reference is made in the Bank Completion Guaranty
(the "Loan") by performing such construction obligations relating to the Base
Building Improvements, in both cases including satisfaction of all of the
applicable conditions precedent to advances of the Loan except to the extent the
undersigned is not required to satisfy the same under the Bank Completion
Guaranty, the undersigned shall so perform the construction obligations relating
to the Base Building Improvements. For purposes of interpreting any language in
the Bank Completion Guaranty to the effect that advances of the Loan shall be
made "in accordance with the terms of the Loan Agreement," the undersigned
represents and warrants that such language refers to administrative and
procedural requirements governing the process of disbursing the proceeds of the
Loan.

                                      L-1

<PAGE>

         2. In no event shall you have any right to enforce the completion
guaranty memorialized hereby (this "Millennium Completion Guaranty") following
any default in (i) your obligations under the Lease not cured within any
applicable notice and grace periods or (ii) your obligations in the completion
guaranty delivered by you to the undersigned of even date herewith. Furthermore,
neither shall you have any right to enforce this Millennium Completion Guaranty
under the circumstances where Landlord under the Lease is entitled to terminate
the Lease by virtue of the penultimate paragraph of that certain side letter
agreement between Landlord and Tenant, of even date herewith, regarding
"Potential 35 Landsdowne Lease Termination", whether or not Landlord elects to
terminate the Lease as provided therein.

         3. For purposes of interpreting the Bank Completion Guaranty, the
undersigned represents and warrants that the construction work, completion of
which is guaranteed by the undersigned for the benefit of the Bank includes the
Base Building Improvements, the Base Systems, and the Core/Lobby/Finish Work and
those of the Leasehold Improvements contemplated to be paid for by application
of the Leasehold Improvement Allowance, as such terms are defined in the Lease.

         4. The undersigned represents and warrants that, prior to drawing down
the Loan, an equity contribution of at least Ten Million Dollars ($10,000,000)
shall have been made by Landlord, and the undersigned covenants and agrees that,
to the extent such contribution has not yet been made, it will so be made.

         5. The undersigned expressly acknowledges and agrees that the benefit
of this Millennium Completion Guaranty to Tenant shall not be diminished by the
prepayment in full of the principal amount of, and all interest on, the Loan by
either Landlord or the undersigned and the resulting termination of the Bank
Completion Guaranty prior to Substantial Completion of the Base Building
Improvements, and disbursement of the Construction Allowances, and therefore the
undersigned represents and warrants that any such prepayment shall either (x) be
made only contemporaneously with a refinancing of the Loan or (y) constitute an
extension of financing by the undersigned, and in either case neither the
Guaranteed Completion Obligations of the undersigned nor the Landlord's right to
advances of Loan proceeds shall be in any way diminished. The undersigned
further expressly acknowledges and agrees that the benefit of this Millennium
Completion Guaranty to Tenant shall not be diminished by a purchase of the Loan
by either Landlord or the undersigned or any affiliate thereof.

         6. Any communications concerning this Millennium Completion Guaranty
shall be in writing and addressed as follows:

                                      L-2

<PAGE>

                  If to Guarantor:      Forest City Enterprises, Inc.
                                        1130 Terminal Tower
                                        50 Public Square
                                        Cleveland, OH  44113-2203
                                        Attention:  James A. Ratner

                  With a copy to:       Forest City Enterprises, Inc.
                                        1100 Terminal Tower
                                        50 Public Square
                                        Cleveland, OH  44113-2203
                                        Attention:  General Counsel

                  and

                                        Goodwin, Procter & Hoar  LLP
                                        Exchange Place
                                        Boston, MA  02109-2881
                                        Attention: Elizabeth McDermott Esq.

                  If to Millennium:     Millennium Pharmaceuticals, Inc.
                                        75 Sidney Street
                                        Cambridge, MA  02139
                                        Attention: General Counsel

                  With a copy to:       Hale and Dorr, LLP
                                        60 State Street
                                        Boston, MA  02109

                                        Attention: Katharine E. Bachman, Esq.

         7. This Millennium Completion Guaranty memorialized shall be governed
by, and construed in accordance with, the laws of the Commonwealth of
Massachusetts applicable to agreements made and to be performed solely entirely
within the same. No amendment of any provision hereof shall be effective unless
the same shall be in writing and signed by the undersigned.

                                      L-3

<PAGE>



         Executed under seal as of the date and year first above written.

                                           Very truly yours,

                                           FOREST CITY ENTERPRISES, INC.

                                           By:

                                           Its:


                                    L-4

<PAGE>

                                    EXHIBIT M

                        MILLENNIUM PHARMACEUTICALS, INC.
                                75 SIDNEY STREET
                               CAMBRIDGE, MA 02139

                                         ____________, 2000

Forest City Enterprises, Inc.
1100 Terminal Tower
50 Public Square
Cleveland, OH  44113-2203

         Re:      Completion Guaranty of Millennium Pharmaceuticals, Inc.
                  ("Tenant") for the benefit of Forest City Enterprises, Inc.
                  ("FCE") regarding that certain lease, dated June __, 2000 (the
                  "Lease") between FC 35 Landsdowne, Inc. ("Landlord") and
                  Tenant for premises located at 35 Landsdowne Street,
                  Cambridge, MA

         As an inducement to your entering into (a) the completion guaranty (the
"Bank Completion Guaranty") of even date herewith for the benefit of
_____________________ ("Bank") and (b) the completion guaranty of even date
herewith for the benefit of Tenant, which constitutes valuable consideration,
the receipt and sufficiency of which are hereby acknowledged, the undersigned
covenants and agrees as follows:

         1. Except as expressly provided in paragraph 2 below, if demand is made
upon FCE under a completion guaranty of even date herewith (the "Bank Completion
Guaranty") to the Bank the form of which is anticipated to substantially follow
the form of guaranty delivered in connection with the financing of the project
known as 45/75 Sidney Street, a copy of which is attached hereto, to perform any
construction obligations relating to the Base Systems, the undersigned shall so
perform the construction obligations relating to the Base Systems. For purposes
of interpreting any language in the Bank Completion Guaranty to the effect that
advances of the Loan shall be made "in accordance with the terms of the Loan
Agreement," you have represented and warranted that such language refers to
administrative and procedural requirements governing the process of disbursing
the proceeds of the Loan.

                                      M-1

<PAGE>

         2. In no event shall you have any right to enforce the completion
guaranty memoralized hereby (this "FCE Completion Guaranty") following (i) any
default in your obligations in the completion guaranty delivered by you to the
undersigned of even date herewith, or (ii) any default by the Landlord in its
obligations under the Lease not cured within applicable grace periods.
Furthermore, neither shall you have any right to enforce this Millennium
Completion Guaranty under the circumstances where Landlord under the Lease is
entitled to terminate the Lease by virtue of the penultimate paragraph of that
certain side letter agreement between Landlord and Tenant, of even date
herewith, if Landlord elects to terminate the Lease as provided therein.

         3. For purposes of interpreting the Bank Completion Guaranty, you
represent and warrant that the construction work, completion of which is
guaranteed by the undersigned includes the Base Building Improvements, the Base
Systems and the Core/Lobby/Finish Work, and those of the Leasehold Improvements
contemplated to be paid for by application of the Leasehold Improvement
Allowance, as such terms are defined in the Lease.

         4. You have represented and warranted that, prior to drawing down the
Loan, an equity contribution of at least Ten Million Dollars ($10,000,000) shall
have been made by Landlord, and you have covenanted and agreed that, to the
extent such contribution has not yet been made, it will so be made.

         5. Any communications concerning this FCE Completion Guaranty shall be
in writing and addressed as follows:

                  If to FCE:           Forest City Enterprises, Inc.
                                       1130 Terminal Tower
                                       50 Public Square
                                       Cleveland, OH  44113-2203
                                       Attention:  James A. Ratner

                  With a copy to:      Forest City Enterprises, Inc.
                                       1100 Terminal Tower
                                       50 Public Square
                                       Cleveland, OH  44113-2203
                                       Attention:  General Counsel

                  and

                                       Goodwin, Procter & Hoar  LLP
                                       Exchange Place
                                       Boston, MA  02109-2881
                                       Attention: Elizabeth McDermott Esq.

                                      M-2

<PAGE>

                  If to Guarantor:     Millennium Pharmaceuticals, Inc.
                                       75 Sidney Street
                                       Cambridge, MA  02139
                                       Attention: General Counsel

                  With a copy to:      Hale and Dorr, LLP
                                       60 State Street
                                       Boston, MA  02109
                                       Attention: Katharine E. Bachman, Esq.

         6. This FCE Completion Guaranty shall be governed by, and construed in
accordance with, the laws of the Commonwealth of Massachusetts applicable to
agreements made and to be performed solely entirely within the same. No
amendment of any provision hereof shall be effective unless the same shall be in
writing and signed by the undersigned.

         Executed under seal as of the date and year first above written.

                                       Very truly yours,

                                       MILLENNIUM PHARMACEUTICALS, INC.

                                       By:

                                       Its:


                                      M-3

<PAGE>


                                    EXHIBIT A

                            Bank Completion Guaranty


<PAGE>


                             FC 35 LANDSDOWNE, INC.
                           C/O FOREST CITY DEVELOPMENT
                                38 SIDNEY STREET
                               CAMBRIDGE, MA 02139


                                                     August 4, 2000


Millennium Pharmaceuticals, Inc.
75 Sidney Street
Cambridge, MA  02139

         Re:      POTENTIAL 35 LANDSDOWNE LEASE TERMINATION

         Reference is hereby made to the lease, dated August 4, 2000 (the
"Lease") between FC 35 Landsdowne, Inc. ("Landlord") and Millennium
Pharmaceuticals, Inc. ("Tenant") for premises located at 35 Landsdowne
Street, Cambridge, MA (the "Building"). Capitalized terms not defined
hereinbelow shall have the respective meanings ascribed to them in the Lease.

         The purpose of this letter is provide Landlord with the right to
terminate the Lease, upon the occurrence of the circumstances contemplated
below.


         Landlord and Tenant additionally agree that Landlord shall be
obligated, in connection with its construction of the Building under the Lease,
to expend up to, but not in excess of, Ten Million Dollars ($10,000,000) to
cause the remediation of environmental contamination at the Premises. Should the
performance by Landlord of the remediation of environmental contamination
require that costs in excess of Ten Million Dollars ($10,000,000) be incurred,
in order to fulfill the requirements of environmental laws and regulations, as a
consequence of existing contamination other than by virtue of the negligence or
willful misconduct of Landlord or Landlord's contractors, agents or employees,
then Landlord may terminate the Lease, by notice given to Tenant prior to the
Commencement Date under the Lease, whereupon the Lease shall terminate and
neither Landlord nor Tenant shall have any further obligations to each other
under the Lease and this letter.

         Tenant acknowledges that Landlord may not exercise the aforesaid right
to terminate the Lease without the consent of its construction lender unless the
construction lender's mortgage encumbering the Premises has been repaid in full.
Therefore, Tenant may not rely on the effectiveness of Landlord's termination of
the Lease as aforesaid unless Tenant has been furnished with a written consent
of any mortgagee with whom Tenant has executed a Subordination, Non-Disturbance
and Attornment Agreement, or evidence reasonably satisfactory to Tenant that the
mortgage held by such mortgagee has been repaid in full.

<PAGE>

Millennium Pharmaceuticals, Inc.
August 4, 2000
Page 3

         This Agreement shall expire and have no further force or effect from
and after the Rent Commencement Date.

                                       Very truly yours,

                                       FC 35 LANDSDOWNE, INC.


                                       By:  /s/ Gayle Friedland
                                            ------------------------------------

                                       Its: Vice President
                                            ------------------------------------

Above Agreed to:

MILLENNIUM PHARMACEUTICALS, INC.

By:  /s/ Kevin Starr
     -----------------------------

Its: CFO
     -----------------------------