Document:

<PAGE>
                                                                   Exhibit 10.17

                                      LEASE

                                 by and between

                    Trustees of Fort Washington Realty Trust,

                                   Landlord,

                                       and

                       Vertex Pharmaceuticals Incorporated

                                     Tenant

<PAGE>

                                    ARTICLE I

                                 REFERENCE DATA

1.1   SUBJECTS REFERRED TO

ANNUAL FIXED RENT RATE:      Subject to Sections 2.3 and 4.1(b) hereof, (i)
                             $10/r.s.f. for the Basement of the Building,
                             $25/r.s.f. for each of Levels 1, 2 and 3 of the
                             Building, and $ 18/r.s.f. for Level 4 of the
                             Building or (ii) if Landlord advances the Tenant
                             Allowance, the Annual Fixed Rent Rate set forth
                             above shall be $23/r.s.f. for the Basement of the
                             Building, $38/r.s.f. for Levels 1, 2 and 3 of the
                             Building and $31/r.s.f. for Level 4 of the
                             Building.

APPROXIMATE TERM:            10 years and 4 months or 15 years and 4 months if
                             Landlord advances the Tenant Allowance.

BUILDING:                    The Building known as and numbered 200 Sidney
                             Street, Cambridge, Massachusetts, to be constructed
                             by Landlord and containing approximately 191,904
                             rentable square feet ("r.s.f.")

BUILDING ADDRESS:            200 Sidney Street
                             Cambridge, Massachusetts

INITIAL ESTIMATED ANNUAL
ADDITIONAL RENT:             $1,527,555.84 based upon $7.96/r.s.f.

LANDLORD:                    David E. Clem and David M. Roby, Trustees of Fort
                             Washington Realty Trust u/d/t dated June 19, 1995,
                             recorded with the Middlesex (South) District
                             Registry of Deeds in Book 25422, Page 360 and filed
                             with the Middlesex (South) Registry District of the
                             Land Court as Document No. 976230

LANDLORD'S ARCHITECT:        Arrowstreet, Inc.

                                       2
<PAGE>

LANDLORD'S ADDRESS:          c/o Lyme Properties LLC
                             101 Main Street
                             Cambridge, Massachusetts 02142

LANDLORD'S CONTRACTOR:       William A. Berry & Son, Inc.

LANDLORD'S REPRESENTATIVE:   David Clem

LEASE YEAR:                  Each consecutive period of twelve (12) calendar
                             months commencing on the Commencement Date if it
                             occurs on the first day of a calendar month and
                             otherwise commencing on the first day of the month
                             immediately following the month in which the
                             Commencement Date occurs, and each anniversary of
                             such date, except that the first Lease Year shall
                             be approximately one year and four months so as to
                             include the period from the Commencement Date
                             through the Rent Commencement Date or until the
                             first day of the following month in the event that
                             the Rent Commencement Date does not occur on the
                             first day of a calendar month.

LOT:                         The land shown on Exhibit A and more particularly
                             described on Exhibit A-1 attached hereto.

MANAGING AGENT:              Hall Keen Management, Inc.

OPTIONS TO EXTEND:           Two (2) Options to Extend the Term of this Lease
                             for successive periods of ten (10) years each, in
                             accordance with Section 10.12 hereof.

PERMITTED USES:              General office, research and development,
                             laboratory and light manufacturing.

PREMISES:                    Approximately 191,904 r.s.f. of space in the
                             Building to be constructed by Landlord on the Lot
                             as shown on Exhibit A.

PREMISES DESIGN FLOOR AREA:  191,904 r.s.f, comprised of 37,211 r.s.f. in the
                             Basement of the Building, 43,515 r.s.f. on Level 1
                             of the Building, 44,266 r.s.f. on

                                       3
<PAGE>

                             Level 2 of the Building, 41,693 r.s.f. on Level 3
                             of the Building, and 25,219 r.s.f. on Level 4 of
                             the Building, which Level 4 space includes a 17,552
                             r.s.f. enclosed high bay mechanical penthouse to
                             accommodate mechanical, electrical and plumbing
                             equipment to be installed by Landlord and Tenant.

PUBLIC LIABILITY
INSURANCE LIMITS:            Bodily injury:     $ 10,000,000
                             Property Damage:   $ 10,000,000

SCHEDULED SUBSTANTIAL
COMPLETION DATE:             May 1, 2000

SCHEDULED RENT
COMMENCEMENT DATE:           September 1, 2000

SCHEDULED TERM
COMMENCEMENT DATE:           May 1, 2000

SECURITY DEPOSIT:            As further described in Section 10.11 hereof, a
                             letter of credit or cash in the initial amount of
                             $4,062,902 plus an amount equal to the Initial
                             Estimated Annual Additional Rent ($1,527,555.84),
                             subject to adjustment in accordance with Sections
                             2.3 and 10.11 hereof.

TENANT:                      Vertex Pharmaceuticals Incorporated.

TENANT'S ADDRESS (For        130 Waverly Street
Notice and Billing):         Cambridge, Massachusetts 02139-4242

TENANT ALLOWANCE:            Subject to Section 2.3 hereof, $12,890,320
                             ($67.17/r.s.f), comprised of $10/r.s.f. for
                             basement and penthouse space and $90/r.s.f. for
                             lab/office space

TENANT'S ARCHITECT:          Tsoi/Kobus & Associates, Inc.

TENANT'S PROPORTIONATE
FRACTION:                    100%

                                        4
<PAGE>

TENANT'S REPRESENTATIVE:                Alfred Vaz, Jr.

TERM EXPIRATION DATE:                   August 31, 2010 or August 31, 2015 if
                                        Landlord advances the Tenant Allowance,
                                        subject in either event to two (2)
                                        Options to Extend for successive periods
                                        of ten (10) years each, in accordance
                                        with Section 10.12.

1.2   EXHIBITS.

      The Exhibits listed below in this section are incorporated in this Lease
by reference and are to be construed as a part of this Lease:

EXHIBIT A.                              Plan showing Lot

EXHIBIT A-1.                            Legal Description

EXHIBIT B.                              Landlord's Plans

EXHIBIT C.                              Rules and Regulations

EXHIBIT D.                              Schedule of Floor Load Limits

                                        5
<PAGE>

1.3   TABLE OF CONTENTS

<TABLE>
<S>                                                                                    <C>
ARTICLE I.......................................................................        2
   1.1      SUBJECTS REFERRED TO................................................        2
   1.2      EXHIBITS............................................................        5
   1.3      TABLE OF CONTENTS...................................................        6

ARTICLE II......................................................................        9
   2.1      PREMISES............................................................        9
   2.2      TERM................................................................        9
   2.3      MODIFICATION OF CERTAIN DEFINITIONS; CERTIFICATE
            REGARDING COMMENCEMENT DATE.........................................       10

ARTICLE III.....................................................................       10
   3.1      INITIAL CONSTRUCTION................................................       10
   3.2      PREPARATION OF PREMISES FOR PERFORMANCE OF TENANT'S
            WORK................................................................       11
   3.3      GENERAL PROVISIONS APPLICABLE TO CONSTRUCTION.......................       15
   3.4      REPRESENTATIVES.....................................................       17
   3.5      CORRECTION OF LANDLORD'S WORK.......................................       17

ARTICLE IV......................................................................       18
   4.1      FIXED RENT..........................................................       18
   4.2      ADDITIONAL RENT.....................................................       18
    4.2.1    Real Estate Taxes..................................................       19
    4.2.2    Insurance..........................................................       20
     4.2.2.1  Insurance Taken Out by Tenant.....................................       20
     4.2.2.2  Insurance Taken Out by Landlord...................................       20
     4.2.2.3  Tenant Reimbursement of Insurance Taken Out by Landlord...........       21
     4.2.2.4  Certain Requirements Applicable to Insurance Policies.............       21
     4.2.2.5  Waiver of Subrogation.............................................       22
    4.2.3    Utilities..........................................................       22
    4.2.4    Common Area Maintenance and Expenses...............................       23
    4.2.5    Payments on Account of Taxes, Insurance and Utilities..............       25
   4.3      LATE PAYMENT OF RENT................................................       26

ARTICLE V.......................................................................       27
   5.1      AFFIRMATIVE COVENANTS...............................................       27
    5.1.1    Perform Obligations................................................       27
    5.1.2    Occupancy and Use..................................................       27
    5.1.3    Repair and Maintenance.............................................       28
    5.1.4    Compliance with Law................................................       28
    5.1.5    Tenant's Work......................................................       31
    5.1.6    Indemnity..........................................................       31
    5.1.7    Landlord's Right to Enter..........................................       32
    5.1.8    Personal Property at Tenant's Risk.................................       32
    5.1.9    Payment of Landlord's Cost of Enforcement..........................       32
    5.1.10   Yield Up...........................................................       32
    5.1.11   Estoppel Certificate...............................................       33
</TABLE>

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<PAGE>

<TABLE>
<S>                                                                                    <C>
    5.1.12   Landlord's Expenses Re Consents....................................       33
    5.1.13   Rules and Regulations..............................................       33
    5.1.14   Loading............................................................       34
    5.1.15   Holdover...........................................................       34
   5.2      NEGATIVE COVENANTS..................................................       34
    5.2.1    Assignment and Subletting..........................................       34
    5.2.2    Nuisance...........................................................       36
    5.2.3    Installation, Alterations or Additions.............................       36

ARTICLE VI......................................................................       36
   6.1      TERMINATION.........................................................       36
   6.2      RESTORATION.........................................................       36
   6.3      AWARD...............................................................       37

ARTICLE VII.....................................................................       37
   7.1      EVENTS OF DEFAULT...................................................       37
   7 2      REMEDIES............................................................       38
   7.3      REMEDIES CUMULATIVE.................................................       39
   7.4      LANDLORD'S RIGHT TO CURE DEFAULTS...................................       39
   7.5      EFFECT OF WAIVERS OF DEFAULT........................................       39
   7.6      NO ACCORD AND SATISFACTION..........................................       40

ARTICLE VIII....................................................................       40
   8.1      RIGHTS OF MORTGAGE HOLDERS..........................................       40
   8.2      SUPERIORITY OF LEASE; OPTION TO SUBORDINATE.........................       41
   8.3      LEASE AMENDMENTS....................................................       41

ARTICLE IX......................................................................       42
   9.1      AFFIRMATIVE COVENANTS...............................................       42
    9.1.1    Perform Obligations................................................       42
    9.1.2    Repairs............................................................       42
    9.1.3    Compliance with Law................................................       42
    9.1.4    Indemnity..........................................................       43
    9.1.5    Estoppel Certificate...............................................       43
    9.1.6    Landlord's Title...................................................       43
    9.1.7    Utilities..........................................................       44

ARTICLE X.......................................................................       44
   10.1     NOTICES FROM ONE PARTY TO THE OTHER.................................       44
   10.2     QUIET ENJOYMENT.....................................................       44
   10.3     EASEMENTS; CHANGES TO LOT LINES.....................................       45
   10.4     LEASE NOT TO BE RECORDED............................................       45
   10.5     BIND AND INURE; LIMITATION OF LANDLORD'S LIABILITY..................       45
   10.6     ACTS OF GOD.........................................................       45
   ]0.7     LANDLORD'S DEFAULT..................................................       46
   10.8     BROKERAGE...........................................................       46
   10.9     APPLICABLE LAW AND CONSTRUCTION.....................................       47
   10.10    SUBMISSION NOT AN OFFER.............................................       47
   10.11    SECURITY DEPOSIT....................................................       47
   10.12    OPTIONS TO EXTEND...................................................       48
</TABLE>

                                        7
<PAGE>

<TABLE>
<S>                                                                                    <C>
   10.13    INTENTIONALLY OMITTED...............................................       50
   10.14    PARKING.............................................................       50
   10.15    CONFIDENTIAL INFORMATION............................................       50
   10.16    SIGNAGE.............................................................       50
</TABLE>

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<PAGE>

                                   ARTICLE II

                               PREMISES AND TERM

2.1   PREMISES.

      Landlord hereby leases and demises to Tenant and Tenant hereby leases from
Landlord, subject to and with the benefit of the terms, covenants, conditions
and provisions of this Lease, the Premises. Tenant shall have, as appurtenant to
the Premises, the right to use in common with others, if any, entitled thereto
(i) the common facilities, if any, included in the Building or on the Lot, (ii)
the building service fixtures and equipment serving the Premises, and (iii)
subject to Section 10.14 hereof, the right to use one hundred sixty-six (166)
parking spaces ("Tenant's Parking Spaces") in the parking garage located at 47
Erie Street, Cambridge, Massachusetts (the "Garage").

      Landlord reserves the right from time to time, without unreasonable
interference with Tenant's use, (a) to install, repair, replace, use, maintain
and relocate for service to the Premises and to other parts of the Building or
either, building service fixtures and equipment wherever located in the
Building, provided, however, that the Annual Fixed Rent, Additional Rent (as
defined in Section 4.2 hereof) and other charges payable hereunder by Tenant
shall be proportionally reduced in the event that any such installation or
relocation of service materially reduces the usable floor area of the Premises
(other than a temporary reduction to accommodate installation, repair,
replacement, maintenance and relocation of such service); and (b) to alter or
relocate any common facilities and/or Tenant's Parking Spaces on or (subject to
Section 10.14 hereof) off the Lot, provided that in all events (1) substitutions
are in compliance with applicable zoning laws, (2) substitutions are
substantially equivalent, and (3) the total number of parking spaces available
to Tenant on or off the Lot is not decreased.

2.2   TERM.

      To have and to hold for a period (the "Term") commencing on the date which
is the later of the Scheduled Term Commencement Date or the Substantial
Completion Date (whichever of said dates is appropriate being hereafter referred
to as the "Commencement Date"), and continuing until the Term Expiration Date,
unless sooner terminated as provided in Section 3.2 or in Article VII, and
subject to extension in accordance with the terms of Section 10.12 hereof. As
used herein the term "Rent Commencement Date" shall mean that date which is four
(4) calendar months after the Commencement Date. Tenant shall have the right to
access the Premises prior to the Commencement Date for purposes of installing
equipment and furnishings and performing Tenant's Work (as hereinafter defined)
in accordance with and subject to the provisions of Section 3.2.

                                        9
<PAGE>

2.3   MODIFICATION OF CERTAIN DEFINITIONS; CERTIFICATE REGARDING COMMENCEMENT
      DATE.

      Landlord and Tenant acknowledge that the actual rentable square footage of
the Premises may, upon completion of construction of the Base Building
Improvements (as hereinafter defined) be different than the Premises Design
Floor Area as set forth in Article I hereof. Accordingly, after completion of
construction of the Base Building Improvements, Landlord will notify Tenant of
the actual rentable square footage of the Premises, which shall be measured in
accordance with the ANSI/BOMA 265.1-1996 Standard Method for Measuring Building
Rentable Area,, approved June 7, 1996, and, if necessary, Landlord and Tenant
will execute an amendment to this Lease modifying the definitions of the
Premises Design Floor Area, the Annual Fixed Rent Rate, the Initial Estimated
Annual Additional Rent, the amount of the Security Deposit (as specified in
Section 10.11) and the Tenant Allowance, and such other terms and provisions, if
any, of this Lease as may be necessary. Tenant shall have a period of forty-five
(45) days after the date hereof within which to notify Landlord in writing
whether Tenant intends to have Landlord advance the Tenant Allowance (the
"Tenant Allowance Notice"). In the absence of a Tenant Allowance Notice by such
date, Tenant shall be deemed to have elected not to have Landlord advance the
Tenant Allowance, in which event the last paragraph of Section 3.2.1 hereof
shall be of no force or effect. If Tenant elects to have Landlord advance the
Tenant Allowance, Landlord and Tenant shall execute an amendment to this Lease
evidencing such election, acknowledging an increase in the amount of the
Security Deposit (as specified in Section 10.11) and the Term of this Lease, and
amending such other terms and provisions, if any, of this Lease as may be
necessary. Landlord and Tenant will also execute, upon request of either,
certificates acknowledging the Commencement Date of this Lease as provided for
in Section 2.2 hereof, after such commencement date has occurred (as described
in Section 3.2 hereof).

                                   ARTICLE III

                                  IMPROVEMENTS

3.1   INITIAL CONSTRUCTION.

      (a) Plans. Landlord has developed and Tenant hereby approves construction
drawings and specifications described in Exhibit B hereto ("Landlord's Work")
for the Premises showing all items of work and materials to be performed and
supplied by Landlord ("Base Building Improvements"). Landlord has obtained a
building permit for the Base Building Improvements. Tenant acknowledges that a
number of additional licenses, permits and approvals may be required in order
for Landlord to commence and complete construction of the Base Building
Improvements and Tenant agrees to cooperate with Landlord in obtaining the same.
Landlord agrees to provide Tenant with copies of all change orders for the Base
Building Improvements. Tenant shall have five (5) business days after the
receipt thereof to review and approve or disapprove of any such change orders.
Tenant may disapprove a change order only if such change order would materially
deviate from Landlord's Work. If Tenant does not notify

                                       10
<PAGE>

Landlord of Tenant's disapproval within such five-business day period, Tenant
shall be deemed to have approved such change order.

      (b) if Tenant requests any change order for the Base Building
Improvements, Including any change relating to the construction of a covered
walkway, utility and electric conduits and the like between the Building and the
building known as and numbered 130 Waverly Street, Cambridge, Massachusetts (the
"Phase I Building"), Landlord shall submit all such change orders with plans,
specifications, pricing and a schedule of values if appropriate to Tenant for
Its review and approval. No change order for the Base Building Improvements
requested by Tenant shall be effective unless approved by Landlord's
Representative and Tenant's Representative in writing, such approval not to be
unreasonably withheld or delayed. Tenant shall pay as amount equal to all costs
directly incurred by Landlord as a result of any change orders signed by Tenant
and Landlord affecting Landlord's Plans or the Base Building Improvements,
including the cost to Landlord of Landlord's Contractor's overhead and profit
equal to 10% of those costs exclusive of overhead and profit. Amounts due and
payable on account of such change orders shall be paid within fifteen (15) days
of billing therefor, and in all events by the Substantial Completion Date. If,
however, Tenant has elected to have Landlord advance the Tenant Allowance,
amounts due and payable by Tenant on account of such change orders may be
applied to the extent of the unadvanced balance, if any, of the Tenant
Allowance, After completion of the Base Building Improvements, Landlord shall
deliver a set of as built plans for the Base Building Improvements to Tenant
upon Landlord's receipt thereof.

      Except for the Base Building Improvements to be performed by Landlord in
accordance with the plans Bad specifications described in Exhibit B hereto, all
of Tenant's initial interior improvements, fixtures, finishes, furnishings,
furniture, telephones, movable equipment and signs visible from the exterior of
the Building (collectively, "Tenant's Work"), shall be performed el the sole
cost and expense of Tenant, provided however, that subject to Section 3.2.1
hereof Landlord shall advance up to the amount of the Tenant Allowance if Tenant
elects to have Landlord advance the Tenant Allowance, Tenant's Representative
shall serve as construction manager for Tenant's Work. Tenant's performance of
Tenant's Work shall be coordinated with any work being performed by Landlord in
such manner as to maintain harmonious labor relations during the performance of
the Base Building Improvements and not to damage the Building or Lot or
interfere with Building or Lot operations. All work described in Tenant's Work
shall be performed by Landlord's Contractor (if Tenant enters into a contract
with Landlord's Contractor for the initial Tenant's Work) or by a contractor
selected by Tenant and approved by Landlord, which approval shall not be
unreasonably withheld ("Tenant's Contractor"). The performance of Tenant's Work
in accordance with this Lease shall not be deemed to be a violation of the
Permitted Uses of the Premises. Except as set forth in Sections 3.3 and 5.1.10
hereof and Exhibit E hereto, all Tenant's Work shall become a part of the
Premises and upon termination of this Lease shall be considered to be the
property of the Landlord.

3.2   PREPARATION OF PREMISES FOR PERFORMANCE OF TENANT'S WORK.

landlord agrees to use reasonable efforts to have the Premises ready for the
performance of Tenant's Work on or before the Scheduled Substantial Completion
Date which shall, however,

                                       11
<PAGE>

be extended for a period equal to that of (a) any delays due to Acts of God, or
by labor disputes, fire, unusual delays In deliveries, unavoidable casualties or
other causes beyond Landlord's reasonable control (collectively, "Force Majeure
Events") and (b) any delays due to (i) any changes in the Base Building
Improvements requested by Tenant, (ii) any act or neglect of Tenant, or of any
employee, agent, or separate contractor of Tenant or (iii) the concurrent
performance of the Base Building Improvements and Tenant's Work (collectively,
"Tenant Delay"). The Premises shall be deemed ready for the performance of
Tenant's Work on the Substantial Completion Date (as hereinafter defined). As
used herein, the term "Substantial Completion Date" shall mean and refer to the
date on which: (i) the Base Building Improvements are substantially complete as
certified by Landlord's Architect and verified by Tenant's Architect, with the
exception of minor items which can be fully completed by Landlord within thirty
(30) days without material interference with Tenant and other items which
because of the season or weather or the nature of the item are not practicable
to do at the time, provided that none of said items is necessary to perform
Tenant's Work (collectively "Punch List Items"), (ii) if Tenant's Work has not
then commenced, a Certificate of Occupancy from the City of Cambridge (or a
Temporary Certificate of Occupancy with conditions which can be satisfied
without material interference with the performance of Tenant's Work) shall have
been obtained, (iii) the Premises is broom clean and free of debris except to
the extent, if any, resulting from Tenant's Work, and (iv) all utilities
required for the use of the Premises have been brought by Landlord to the
Utility Switching Points (as defined in Section 4.2.3 hereof); provided,
however, that if the Substantial Completion Date does not occur on or before the
Scheduled Substantial Completion Date due to any Tenant Delay then the Base
Building Improvements shall be deemed to be substantially completed, and the
Substantial Completion Date shall be deemed to have occurred, on the date on
which the Substantial Completion Date would have occurred taking into account
any Force Majeure Events but without taking into account any Tenant Delay. If
Landlord's Architect has certified that the Base Building Improvements are
substantially complete but Tenant's Architect does not verify the same within
five (5) business days thereafter, Landlord's Architect and Tenant's Architect
shall immediately select a third independent architect who shall conclusively
determine whether the Base Building Improvements are substantially complete.
Landlord and Tenant shall share equally the costs of such third architect.
Landlord's obligations under Sections 3.1 and 3.2 shall be deemed to have been
performed on the Substantial Completion Date except for Punch List Items and
items which do not conform with the requirements of Sections 3.1 and 3.2 and as
to which Tenant or Tenant's Architect shall have given written notice to
Landlord prior to such date. If Tenant or Tenant's Architect does not provide
such written notice prior to the Substantial Completion Date, a certificate of
substantial completion by a licensed architect or registered engineer shall be
conclusive evidence that Landlord has performed all such obligations except for
Punch List Items and items stated in such certificate to be incomplete or not in
conformity with such requirements.

      Notwithstanding the foregoing, Tenant shall have the right to terminate
this Lease upon notice given to Landlord on or before the date which is 180 days
after the Scheduled Substantial Completion Date in the event that the
Substantial Completion Date, as extended due to the occurrence of any Tenant
Delay, has not occurred on or before the date which is 180 days after the
Scheduled Substantial Completion Date, as so extended for Tenant Delay.

                                       12
<PAGE>

      3.2.1. PERFORMANCE OF TENANT'S WORK.

      Tenant shall not effect any Tenant's Work (or any alterations or additions
to the Premises after performance of Tenant's Work) that might (i) diminish the
value of the Premises for laboratory/office use, or (ii) require any unusual
expense to re-adapt the Premises for any laboratory/office use.

      Tenant's Work shall be performed in accordance with complete, consistent,
final construction drawings and specifications ("Construction Documents")
approved in advance by Landlord in writing, which approval shall not be
unreasonably withheld. The Construction Documents shall be prepared and stamped
by Tenant's Architect and approved by Landlord in writing. Landlord reserves the
right to reject, in whole or in part, the Construction Documents which in its
reasonable opinion fail to comply with the provisions of this Lease within
fifteen (15) business days of its receipt thereof (the "Review Period"). The
Review Period shall not commence unless and until Tenant delivers a complete set
of Construction Documents. If Landlord shall disapprove the Construction
Documents, it shall state specifically the reasons therefor, and Tenant shall
promptly revise and resubmit the Construction Documents. If Landlord fails to
respond to Tenant's request for approval of the Construction Documents within
the Review Period then the Construction Documents shall be deemed approved.

      Tenant shall be solely responsible for the liabilities of and expenses of
all architectural and engineering services relating to Tenant's Work and for the
adequacy, accuracy, and completeness of the Construction Documents approved by
Landlord unless Tenant elects that Landlord advance the Tenant Allowance (and if
so elected, then only to the extent of the Tenant Allowance). The Construction
Documents (i) shall set forth in detail the requirements for construction of the
Tenant's Work (including all architectural, mechanical, electrical and
structural drawings and detailed specifications), (ii) shall be fully
coordinated with one another and with field conditions as they exist in the
Premises and elsewhere in the Building, and (iii) shall show all work necessary
to complete the Tenant's Work including all cutting, fitting, and patching and
all connections to the mechanical and electrical systems and components of the
Building. Tenant agrees to hold Landlord harmless if any Tenant's Work described
in the Construction Documents (a) fails to comply with all applicable laws,
regulations, building codes, and building design standards, (b) in any manner
affects any structural component of the Building (including, without limitation,
exterior walls, exterior windows, core walls, roofs or floor slabs), (c) in any
respect is incompatible with the electrical and mechanical components and
systems of the Building, (d) affects the exterior of the Building, (e) fails to
conform to floor loading limits, and (f) with respect to all materials,
equipment and special designs, processes, or products, infringes on any patent
or other proprietary rights of others. Landlord's approval or deemed approval of
the Construction Documents and the performance of Tenant's Work pursuant to the
Construction Documents shall not result in any liability of Landlord, except to
the extent that Tenant elects to have Landlord advance the Tenant Allowance, and
Landlord's approval of Construction Documents shall signify only Landlord's
consent to Tenant's Work shown thereon and shall not result in any
responsibility of Landlord concerning compliance of Tenant's Work with laws,
regulations, or codes, coordination of any aspect of Tenant's Work

                                       13
<PAGE>

with any other aspect of Tenant's Work, or the feasibility of constructing
Tenant's Work without material damage or harm to the Building, all of which
shall be the sole responsibility of Tenant.

      After Tenant's Contractor has been approved, then the same may thereafter
be used by Tenant until Landlord notifies Tenant that Tenant's Contractor is no
longer approved due to Tenant's Contractor's failure to comply in any material
respect with the requirements of the Construction Documents and/or this Lease.
Tenant shall procure all necessary governmental permits, licenses and approvals
before undertaking any Tenant's Work. Tenant shall perform all Tenant's Work at
Tenant's risk in compliance with all applicable laws, codes and regulations and
in a good and workmanlike manner employing new materials of good quality. When
any Tenant's Work is in progress, Tenant shall cause to be maintained (i)
insurance as may be required by Landlord covering any additional hazards due to
such Tenant Work, and (ii) a statutory lien bond pursuant to M.G.L. c.254,
Section 12 or any successor statute (or such other protection of Landlord's
interest in the Building and Lot against liens as Landlord may reasonably
require), in each case for the benefit of Landlord. It shall be a condition of
Landlord's approval of any Tenant's Work that certificates of such insurance and
a lien bond in recordable form, both issued by responsible insurance companies
qualified to do business in Massachusetts and reasonably approved by Landlord,
shall have been deposited with Landlord, that Tenant has provided Tenant's
certification of the insurable value of the work in question for casualty
insurance purposes, and that all of the other conditions of the Lease have been
satisfied. Tenant shall reimburse up to $10,000.00 for Landlord's reasonable
costs of reviewing proposed Tenant's Work and Inspecting installation of the
same provided however, that if Tenant elects to have Landlord advance the Tenant
Allowance, such costs may be paid from the Tenant Allowance. At all times while
performing Tenant's Work, Tenant shall require any Tenant's Contractor to comply
with all applicable laws, regulations, permits and policies relating to such
work. In performing Tenant's Work, each Tenant's Contractor shall comply with
Landlord's requirements set forth in Section 3.2.1, the first paragraph of
Section 3.3, Section 5.1.5 and Section 5.2.3 hereof relating to the time and
methods for such work, use of delivery elevators and other Building facilities
and each Tenant's Contractor shall not interfere or disrupt Landlord's
Contractor. Each Tenant's Contractor shall in all events work on the Premises
without causing labor disharmony., coordination difficulties, or delay or impair
any guaranties, warranties or obligations of any contractors of Landlord. If any
Tenant's Contractor uses any Building services or facilities, such Contractor,
jointly and severally with Tenant, shall agree to reimburse Landlord for the
cost thereof based on Landlord's schedule of charges established from time to
time (and if no such charges have been established, then based on Landlord's
reasonable charge established at the time). Each Tenant's Contractor shall, by
entry into the Building, be deemed to have agreed to indemnify and hold Landlord
harmless from any claim, loss or expense arising in whole or in part out of any
act or neglect committed by such person while in the Building, to the same
extent as Tenant has so agreed in this Lease, the indemnities of Tenant and
Tenant's Contractor to be joint and several.

      Tenant shall pay on or prior to date when any such payment is due, either
from its own funds or from the Tenant Allowance if Tenant elects to have
Landlord advance the Tenant Allowance, the entire cost of all Tenant's Work so
that the Premises shall always be free of liens for labor or materials. If any
mechanic's lien (which term shall include all similar liens relating

                                       14
<PAGE>

to the furnishing of labor and materials) is filed against the Premises or the
Building or any part thereof which is claimed to be attributable to Tenant, its
agents, employees or contractors, Tenant shall promptly discharge the same by
payment or filing any necessary bond within thirty (30) days after Tenant has
notice (from any source) of such mechanic's lien. Landlord may, as a condition
of Its approval of any Tenant's Work, require Tenant to deposit with Landlord a
bond, letter of credit or other similar security in the amount of Landlord's
reasonable estimate of the value of such Work securing Tenant's obligations to
make payments for such Work.

      Landlord shall permit Tenant and Tenant's Contractor access to the
Premises prior to the Commencement Date for the performance of Tenant's Work if
Tenant employs Landlord's Contractor or another contractor approved by Landlord
for the performance of Tenant's Work and if the concurrent performance of the
Base Building Improvements and Tenant's Work will not delay the Substantial
Completion Date. Subject to the foregoing, Landlord shall cooperate with
Tenant's Contractor in connection with Tenant's Work.

      If Tenant elects to have Landlord advance the Tenant Allowance, Tenant
shall provide Landlord with a budget and copies of all contracts entered into
with respect to Tenant's Work and such ether information as Landlord reasonably
may request. The Tenant Allowance shall be advanced to Tenant by Landlord no
more frequently than monthly against costs then incurred but unpaid by Tenant
with respect to Tenant's Work. The Tenant Allowance shall be advanced to Tenant
in the proportion which the Tenant Allowance bears to Tenant's budget, as the
same may be updated, for Tenant's Work. Tenant shall make application to
Landlord for an advance of the Tenant Allowance at least ten (10) business days
prior to the date upon which an advance is to be made. Such application shall be
on such form or forms as Landlord reasonably may require, and shall be
accompanied by invoices, receipts, lien waivers and such other documents as
Landlord reasonably may require.

3.3   GENERAL PROVISIONS APPLICABLE TO CONSTRUCTION.

      All construction work required or permitted by this Lease, whether by
Landlord or by Tenant, shall be done in a good and workmanlike manner and in
compliance with all applicable laws and all lawful ordinances, regulations and
orders of any governmental authority or insurer of the Building. Either party
may inspect the work of the other at reasonable times and shall give notice of
observed defects. Landlord shall not be responsible for any loss, damage, or
injury resulting from the installation of any components, fixtures, or equipment
provided they were appropriately specified and installed in accordance with the
manufacturer's or supplier's instructions; provided, however, that Landlord
shall assign any and all contractor's, manufacturer's and supplier's warranties
with respect to all components, fixtures, or equipment, including, without
limitation, Landlord's Contractor's warranty, to Tenant for the Term of this
Lease, upon the expiration or sooner termination of which such warranties shall
automatically revert to Landlord.

      After the performance of Tenant's Work, Tenant will not make any
alterations or additions to the Premises without Landlord's approval, which
approval shall not be unreasonably withheld or delayed provided that Landlord
and Tenant shall agree in writing whether Tenant

                                       15
<PAGE>

will be required to, permitted to or forbidden to, at Tenant's sole cost and
expense, remove any such alteration or addition upon the expiration or
termination of this Lease. Landlord's approval of any alteration or addition
which is not a Minor Alteration (as defined below in this Section 3.3) shall be
deemed to have been given if Landlord fails to notify Tenant of its objection
thereto within fifteen (15)business days after Tenant's request for such
approval. In circumstances in which Tenant desires the right to remove additions
or alterations at the expiration or termination of this Lease, Landlord shall
reasonably agree, and such agreement shall not be unreasonably withheld or
delayed (and shall be deemed to have been given if Landlord fails to notify
Tenant of its objection thereto within fifteen (15) business days after Tenant's
request for such agreement), to permit such removal (i) where items installed by
Tenant are in the nature of equipment, but are so affixed to Building that such
items may be construed as fixtures or (ii) where additions or alterations of
Tenant Include specific items that after removal from the Building will have in
the aggregate for each such alteration or addition a fair market value of
$25,000.00 or greater. Tenant's rights to remove additions or alterations
hereunder shall not apply to replacement of items included in Tenant's Work that
are replaced due to the fact that such items have worn out or become
substantially obsolete. In the event the Tenant is required to or permitted to
remove any such alteration or addition, as a condition to Landlord's approval of
such alteration or addition, Tenant shall agree in writing to readapt, repair
and restore the Premises to the condition the same were in prior to such
alteration or addition. After the performance of Tenant's Work, all changes and
additions shall be part of the Building except such items as by writing at the
time of approval the parties agree either shall be removed by Tenant on
termination of this Lease, or shall be removed or left at Tenant's election.

      During the Term hereof and the term of the Phase I Lease (as hereinafter
defined), Tenant, as part of Tenant's Work or as an alteration or addition to
the Premises after the performance of Tenant's Work, shall have the right to
make interconnections between the Building and the Phase I Building, including
installation of a covered walkway, electrical connections, utility connections
and the like (collectively "Interconnections"). Each Interconnection proposed by
Tenant shall require Landlord's prior written consent, which consent shall not
be unreasonably withheld and shall otherwise be subject to the terms of this
Lease, including the preceding paragraph hereof, except that the original
installation of any Interconnection shall not be considered a Minor Alteration
(as hereinafter defined). Landlord's consent to any Interconnection pursuant to
the terms of this Lease shall also constitute Landlord's consent thereto
pursuant to the Phase I Lease so long as Landlord is the landlord pursuant to
the Phase I Lease. Landlord and Tenant agree that Landlord may (and prior to the
Subdivision (as defined in Section 9.1.6) Landlord shall) establish an area or
areas to be shown on the Plan (as defined in Section 9.1.6 hereof) within which
Interconnections may be made, and that except for the covered walkway and
Interconnections therein, Landlord may require subsurface installation of
Interconnections.

      Notwithstanding the foregoing, the parties hereby agree that for any
non-structural alterations or additions to the Premises which do not involve
modifications to the Building operating systems and for which the cost may be
reasonably estimated to be less than $50,000 and which is not the original
installation of an Interconnection (each a "Minor Alteration"): (i) Landlord's
prior written consent shall not be required unless such Minor Alteration
requires a

                                       16
<PAGE>

building permit from the City of Cambridge, in which case Landlord's reasonable
consent shall be required, provided that such consent shall be deemed to have
been given if Landlord fails to notify Tenant of its objection to such Minor
Alteration within 2 days after Tenant's request for Landlord's consent with
respect thereto, and (ii) upon the expiration or termination of this Lease,
Tenant shall readapt, repair and restore the Premises to the condition the same
were in prior to such Minor Alteration, regardless of whether Landlord's consent
was required or obtained with respect thereto. Additionally, Tenant shall give
prior written notice to Landlord of any Minor Alteration for which the cost may
be reasonably estimated to be less than $50,000 but greater than $25,000 and
regardless of whether Landlord's consent is required.

      The parties further agree that after the performance of Tenant's Work (a)
any request for consent to any alteration or addition (including, without
limitation, any Minor Alteration and any Interconnection) shall be accompanied
by drawings and specifications in reasonable detail given the size and scope of
the proposed alteration or addition, and (b) Tenant shall furnish Landlord
as-built drawings showing any and all alterations or additions (including,
without limitation, any and all Minor Alterations and Interconnections) made by
Tenant or any assignee, sublessee or licensee of Tenant within 30 days after
completion of the same.

3.4   REPRESENTATIVES.

      Each party authorizes the other to rely in connection with their
respective rights and obligations under this Article III upon approval and other
actions on the party's behalf by Landlord's Representative in the case of
Landlord and Tenant's Representative in the case of Tenant or by any person
hereafter designated in substitution or addition by notice to the party relying.

3.5   CORRECTION OF LANDLORD'S WORK.

      If within one year after the Substantial Completion Date (i) any item of
Base Building Improvements does not conform with the plans and specifications
described in Exhibit B or (ii) there is any defect in the Base Building
Improvements caused by faulty workmanship performed on behalf of Landlord or
materials installed on behalf of Landlord, Landlord, upon written notice thereof
from Tenant prior to the expiration of such one-year period, shall forthwith
cause the contractor(s) who or which performed such work to correct such
nonconformity or defect without cost or expense to Tenant. Nothing set forth in
this Section shall affect or impair any warranties specified in Section 3.3
hereof or any right of Tenant to pursue any action, right or remedy otherwise
available to Tenant due to a breach by Landlord of its obligations pursuant to
this Lease.

                                       17
<PAGE>

                                   ARTICLE IV

                                      RENT

4.1   FIXED RENT.

      (a) Monthly Installments; Definitions. Commencing on the Rent Commencement
Date, Tenant covenants and agrees to pay rent to Landlord, without any offset or
reduction whatsoever (except as may be made in accordance with the express
provisions of this Lease), at the Original Address of Landlord or at such other
place or to such other person or entity as Landlord may by notice to Tenant from
time to time direct, at the Annual Fixed Rent Rate set forth in Article I, in
equal installments equal to 1/12th of the Annual Fixed Rent Rate in advance on
the first day of each calendar month included in the Term; and for any portion
of a calendar month at the beginning or end of the Term, at that rate payable in
advance for such portion.

      (b) Adjustment for CPI. On the fifth anniversary of the Rent Commencement
Date, on the tenth anniversary of the Rent Commencement Date (if Tenant elects
to have Landlord advance the Tenant Allowance), and on the fifth anniversary of
the commencement of each Extension Period (as such term is defined in Section
10.12 hereof) (each an "Adjustment Date") the Annual Fixed Rent Rate shall be
increased by multiplying said rate by the lesser of (i) a fraction, the
numerator of which shall be the Price Index (as hereinafter defined) most
recently established prior to the Adjustment Date, and the denominator of which
shall be the Base Price Index (as hereinafter defined), or (ii) one hundred four
percent (104%) per year, compounded annually over the then prior five (5) years
of the Term of this Lease. As used herein, the term "Price Index" shall mean and
refer to the "Consumer Price Index for Urban Wage Earners and Clerical Workers,
for the Boston, Massachusetts area, All Items (1982-84=100)" published by the
Bureau of Labor Statistics of the United States Department of Labor or successor
or substitute index appropriately adjusted, and the term "Base Price Index"
shall mean and refer to the Price Index most recently established prior to (a)
the Commencement Date or (b) with respect to any Extension Period, the
commencement date of such Extension Period, as applicable. In the event the
Price Index (or a successor or substitute index) shall not be published for the
City of Boston, or for the months indicated above, the corresponding index for
the United States City Average (and if this is not available, a reliable
governmental or other nonpartisan publication evaluating similar or equivalent
information as used in the Price Index) shall be used. In the event the Price
Index ceases to use the 1982-84 average of 100 as the basis of calculation, or
if a substantial change is made in the terms or numbers of items contained in
the Price Index, then the Price Index shall be adjusted to the figure that would
have been arrived at had the manner of computing the Price Index in effect at
the date of this Lease not been changed.

4.2   ADDITIONAL RENT.

      In order that the Fixed Rent shall be absolutely net to Landlord,
commencing on the Commencement Date. Tenant covenants and agrees to pay, as
Additional Rent, without any offset or reduction whatsoever, taxes, municipal or
state betterment assessments, insurance costs,

                                       18
<PAGE>

utility charges and Annual Maintenance Charges with respect to the Premises as
provided in this Section 4.2 as follows:

      As used herein, the term "Estimated Annual Additional Rent" shall mean and
refer to Landlord's estimate of the total amount of Additional Rent which may be
due from Tenant for any particular Lease Year. Landlord shall furnish Tenant
with a statement within sixty (60) days after the commencement of each Lease
Year setting forth the amount of Landlord's Estimated Annual Additional Rent for
such Lease Year. Landlord's good faith estimate of the Estimated Annual
Additional Rent for the first fiscal year of the Term is set forth in Section
1.1 as the "Initial Estimated Annual Additional Rent".

      4.2.1 REAL ESTATE TAXES. Tenant shall pay directly to the Landlord: (i)
all taxes, assessments (special or otherwise), levies, fees, water and sewer
rents and charges, and all other government levies and charges, general and
special, ordinary and extraordinary, foreseen and unforeseen (and Tenant's
Proportionate Fraction of any such taxes, assessments, levies, fees and charges
if they are assessed against the entire Building or Lot), which are, at any time
prior to or during the Term hereof, imposed or levied upon or assessed against
(A) the Premises or the Building or the Lot, (B) any Fixed Rent, Additional Rent
or other sum payable hereunder or (C) this Lease, or the leasehold estate hereby
created, or which arise in respect of the operation, possession or use of the
Premises or the Building or the Lot; (ii) all gross receipts or similar taxes
imposed or levied upon, assessed against or measured by any Fixed Rent,
Additional Rent or other sum payable hereunder, (iii) all sales, value added,
use and similar taxes at any time levied, assessed or payable on account of the
acquisition, leasing or use of the Premises (and Tenant's Proportionate Fraction
of any such taxes if they are levied, assessed or payable on account of the
acquisition, leasing or use of the entire Building or Lot); and (iv) all charges
for utilities furnished to the Premises (and Tenant's Proportionate Fraction of
all charges for utilities furnished to the entire Building or Lot) which may
become a lien on the Building or the Lot or the Premises (collectively "taxes
and assessments" or if singular "tax or assessment"). For each tax or assessment
period, or installment period thereof, wholly included in the Term, all such
payments shall be made by Tenant not less than five (5) days prior to the last
date on which the same may be paid without interest or penalty. For any fraction
of a tax or assessment period, or installment period thereof, included in the
Term at the beginning or end thereof, Tenant shall pay to Landlord, within 20
days after receipt of invoice therefor, the fraction of taxes and assessments so
levied or assessed or becoming payable which is allocable to such included
period. At Landlord's option, Tenant shall pay taxes and assessments in
accordance with Section 4.2.5 hereof. Anything herein to the contrary
notwithstanding, if and to the extent that the Lot is not a separately assessed
parcel, Landlord shall make a fair and reasonable allocation of any taxes and
assessments between the Lot and the remaining parcel of land of which the Lot is
a part.

      In the event that Tenant requests that Landlord apply for any abatement
of, or otherwise contest, any tax or assessment, Landlord shall file such
abatement or otherwise contest such tax or assessment and shall diligently
pursue the same to completion, provided that (i) Landlord receives notice of
such request from Tenant be made under applicable law, and (ii) the expenses of
such proceedings, including, without limitation, any penalties, interest, late
fees or charges,

                                       19
<PAGE>

and attorneys' fees incurred as a result thereof, shall be included in the
Annual Maintenance Charge of the then current fiscal year.

      Nothing contained in this Lease shall, however, require Tenant to pay any
income taxes, excess profits taxes, excise taxes, franchise taxes ("Excluded
Taxes"), estate, succession, inheritance or transfer taxes, 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 Landlord a capital
levy or other tax on the gross rents received with respect to the Building or
the Lot, or all of them, or a federal, stale, county, municipal, or other local
income, franchise, excise or similar tax, assessment, levy or charge (distinct
from any now in effect) measured by or based, whole or in part, upon gross
rests, then any and all of such taxes, assessments, levies or charges, to the
extent so measured or based ("Substitute Taxes"), shall be payable by Tenant;
provided, however, that (i) Tenant's obligation with respect to the aforesaid
Substitute Taxes shall be limited to the amount thereof as computed at the rates
that would be payable if the Premises were the only property of Landlord, and
(ii) only that portion of the Substitute Taxes in excess of the Excluded Taxes
shall, be payable by Tenant. Landlord shall furnish to Tenant a copy of any
notice of any public, special or betterment assessment received by Landlord
concerning the Premises.

      4.2.2 ISSURANCE.

            4.2.2.1 INSURANCE TAKEN OUT BY TENANT.

                        Tenant shall take out and maintain throughout the Term
the following insurance:

                        (a) Comprehensive liability insurance indemnifying
Landlord and Tenant against all claims and demands for (i) injury to or death of
any person or damage to or loss of property, on the Premises or adjoining walks,
streets or ways, or connected with the use, condition or occupancy of any
thereof unless caused by the negligence of Landlord or its servants or agents,
(ii) violation of this Lease, or (iii) any act, fault or omission, or other
misconduct of Tenant or its agents, contractors, licensees, sublessees or
invitees, in amounts which shall, at the beginning of the Term, be at least
equal to the limits set forth in Section 1.1, and, from time to time during the
Term, shall be for such higher limits, if any, as are customarily carried in the
area in which the Premises are located on property similar to the Premises and
used for similar purposes; and shall be written on the "Occurrence Basis" and
include Host Liquor liability insurance; and

                        (b) Worker's compensation insurance with statutory
limits covering all of the Tenant's employees working on the Premises.

            4.2.2.2 INSURANCE TAKEN OUT BY LANDLORD.

                        Landlord shall take out and maintain throughout the Term
the following insurance:

                                       20
<PAGE>

                        (a) Comprehensive liability insurance for the Building
and Lot of the same nature and type as described in Section 4.2.2.1(a) of this
Lease, and with the same policy limits; and

                        (b) All risk, fire and casualty insurance on a 100%
replacement cost basis, together with rental loss coverage and, if the Building
is located in a flood zone, flood coverage to the extent the same is available,
insuring the Building and its rental value; and

                        (c) Insurance against loss or damage from sprinklers and
from leakage or explosions or cracking of boilers, pipes carrying steam or
water, or both, pressure vessels or similar apparatus, in the so-called "broad
form", in such amounts as are customary and commercially reasonable for
buildings in the Cambridge, Massachusetts area which are of like kind and
quality to the Building and have laboratory uses, and insurance against such
other hazards and in such amounts as may from time to time be required by any
bank, insurance company or other lending institution holding a first mortgage on
the Building and Lot.

                  Landlord shall have no obligation to insure Tenant's personal
property or chattels, including without limitation, Tenant's trade fixtures.

            4.2.2.3 TENANT REIMBURSEMENT OF INSURANCE TAKEN OUT BY LANDLORD.

                        Tenant shall from time to time reimburse Landlord within
thirty days of Landlord's invoice for Tenant's Proportionate Fraction of
Landlord's costs incurred in providing the insurance provided pursuant to
Section 4.2.2.2 of this Lease, equitably prorated in the case of blanket
policies to reflect the insurance coverage reasonably attributable to the
Premises, and provided further that Tenant shall reimburse Landlord for all of
Landlord's costs incurred in providing such insurance which is attributable to
any special endorsement or increase in premium resulting from the business or
operations of Tenant, and any special or extraordinary risks or hazards
resulting therefrom, including without limitation, any risks or hazards
associated with the generation, storage and disposal of medical waste. At
Landlord's option, Tenant shall reimburse Landlord for insurance costs in
accordance with Section 4.2.5 hereof.

            4.2.2.4 CERTAIN REQUIREMENTS APPLICABLE TO INSURANCE POLICIES.

                        Policies for insurance provided for under the provisions
of Sections 4.2.2.2(b) and 4.2.2.2(c) shall, in case of loss, be first payable
to the holders of any mortgages on the Building and Lot tinder a standard
mortgagee's clause, and shall be deposited with the holder of any mortgage or
with Landlord, as Landlord may elect. All policies for insurance required to be
obtained by either party under the provisions of Section 4.2.2 shall be obtained
from responsible companies qualified to do business in the state in which the
Premises are located and in good standing therein, which companies and the
amount of insurance allocated thereto shall be subject to Landlord's approval.
Each party agrees to furnish the other with certificates of all such insurance
which such party is obligated to obtain pursuant to Section 4.2.2 prior to the
beginning of the Term hereof and with renewal certificates at least 30 days
prior to the expiration of the

                                       21
<PAGE>

policy they renew. In addition, Tenant agrees to furnish Landlord with any
policies of insurance which Tenant is obligated to obtain hereunder, including
any renewal policies, upon request of any of Landlord's mortgagees (provided
that Tenant may redact from such policies any Confidential Information, as
defined in Section 10.15 hereof). Each such policy required to be maintained by
Tenant shall name Landlord and Landlord's Managing Agent as additional insureds
and shall be noncancellable with respect to the interest of Landlord, Landlord's
Managing Agent and such mortgagees without at least 30 days' prior written
notice thereto.

            4.2.2.5 WAIVER OF SUBROGATION.

                        All insurance which is carried by either party with
respect to the Premises or to furniture, furnishings, fixtures or equipment
therein or alterations or improvements thereto, whether or not required, shall
include provisions which either designate the other party as one of the insured
or deny to the insurer acquisition by subrogation of rights of recovery against
the other party to the extent such rights have been waived by the insured party
prior to occurrence of loss or injury, insofar as, and to the extent that such
provisions may be effective without making it impossible to obtain insurance
coverage from responsible companies qualified to do business in the state in
which the Premises are located (even though extra premium may result therefrom)
and without voiding the insurance coverage in force between the insurer and the
insured party. In the event that extra premium is payable by either party as a
result of this provision, the other party shall reimburse the party paying such
premium the amount of such extra premium. If at the request of one party, this
non-subrogation provision is waived, then the obligation of reimbursement shall
cease for such period of time as such waiver shall be effective, but nothing
contained in this Section 4.2.2.5 shall derogate from or otherwise affect
releases elsewhere herein contained of either party for claims. Each party
hereby waives all rights of recovery against the other for loss or injury
against which the waiving party is protected by insurance containing said
provisions, reserving, however, any rights with respect to any excess of loss or
injury over the amount recovered by such insurance. Tenant shall not acquire as
insured under any insurance carried on the Premises under the provisions of this
Section 4.2.2 any right to participate in the adjustment of loss or to receive
insurance proceeds and agrees upon request promptly to endorse and deliver to
Landlord any checks or other instruments in payment of loss in which Tenant is
named as payee.

      4.2.3 UTILITIES.

            Tenant shall pay directly to the proper authorities charged with the
collection thereof all charges for water, sewer, gas, electricity, telephone and
other utilities or services used or consumed on the Premises, whether called
charge, tax, assessment, fee or otherwise, including, without limitation, water
and sewer use charges and taxes, if any, all such charges to be paid as the same
from time to time become due. If Tenant is not charged directly by the
respective utility for any of such utilities or services, Tenant shall from time
to time, within 20 days of receipt of Landlord's invoice therefor, pay to
Landlord Tenant's Proportionate Fraction of the total of such charges for the
Building and Lot provided that, at Landlord's option, all such charges shall be
payable by Tenant in accordance with Section 4.2.5. It is understood and agreed
that (i) Landlord shall be responsible for bringing such utilities to a common
switching point(s)

                                       22
<PAGE>

at the Building, which, in the case of electricity shall mean the switch gear
and not the transformer (collectively, the "Utility Switching Points") as shown
on plans described in Exhibit B at Landlord's cost and expense; (ii) Tenant
shall pay for any and all costs to connect such utilities from such Utility
Switching Points to the Building; (iii) Landlord shall be under no obligation to
furnish any utilities to the Premises (beyond the foregoing responsibility to
bring such utilities to the Utility Switching Points and as may be shown on the
plans described in Exhibit B); and (iv) subject to Section 3.2 hereof, Landlord
shall not be liable for any interruption or failure in the supply of any such
utilities to the Premises; provided, however, that in the event such loss or
failure is due to Landlord's negligence or willful misconduct, Landlord shall be
responsible for restoring the supply of such utilities to the Premises but
otherwise shall have no liability to Tenant. Without limitation of the
foregoing, in the event of a Casualty or Taking, if Landlord and Tenant
reasonably determine and agree that utilities will not be repaired or restored
so as to be available at the Utility Switching Points within one year after the
occurrence of such Casualty or Taking, then Tenant shall have the right to
terminate this Lease by notice given within 30 days after the date of such
determination.

      4.2.4 COMMON AREA MAINTENANCE AND EXPENSES.

            Landlord shall maintain the Lot and the exterior of the Building in
a clean and orderly condition including without limitation, keeping the Lot and
exterior of the Building clean and free of debris, keeping the sidewalks,
driveways and parking areas reasonably clear of snow and ice, and maintaining
the exterior landscaping, lighting, parking areas and sidewalks of the Lot.
Tenant shall maintain the interior of the Building, including the mechanical,
electrical and plumbing systems of the Building in good order, repair and
condition (provided that if Tenant shall fail to effect such repairs or
maintenance, at Landlord's option, Landlord may effect such repairs or
maintenance and charge the entire cost thereof to Tenant as Additional Rent).
Notwithstanding the foregoing, it is expressly understood and agreed that
Landlord shall have no liability or responsibility for the storage, containment
or disposal of any hazardous or medical waste generated, stored or contained by
Tenant, Tenant hereby agreeing to store, contain and dispose of any and all such
hazardous or medical waste at Tenant's sole cost and expense in accordance with
the provisions of Article V hereof. Tenant shall pay to Landlord as Additional
Rent the Annual Maintenance Charge computed and payable as follows:

      (1)   The Annual Maintenance Charge shall be equal to the sum of the
            Annual Lot Maintenance Charge and the Annual Building Maintenance
            and Operation Charge as hereinafter defined.

                (a) The Annual Lot Maintenance Charge shall be equal to the
            costs incurred by Landlord during the fiscal year (as hereinafter d
            defined) for which the Annual Maintenance Charge is being computed
            (the "Current Fiscal Year") in providing Lot maintenance, including
            without limitation landscaping, street lighting, security (if
            required, in Landlord's judgment), maintenance and snow plowing,
            maintenance of Lot signage, maintenance of utilities, management
            fees and amortization of equipment to the extent used for Lot
            maintenance.

                                       23
<PAGE>

                (b) The Annual Building Maintenance and Operation Charge shall
            be equal to Tenant's Proportionate Fraction of the reasonable costs
            incurred by Landlord during the Current Fiscal Year in providing
            Building maintenance, including without limitation maintenance and
            repair of all heating, plumbing, electrical, air conditioning and
            mechanical fixtures and equipment serving the Building or the
            Premises), elevators, trash dumpster rental, trash removal,
            remediation, performance of such other tasks as Tenant shall request
            and Landlord shall agree to perform, management fees (exclusive of
            leasing and sale commissions, fees paid in connection with tenant
            improvement costs, and such other fees or commissions paid in
            connection with the leasing, re-leasing, extension or renewal of
            leases for the Building or the Lot) and amortization of equipment to
            the extent used for Building maintenance.

      Notwithstanding the foregoing, management fees included in the Annual
Maintenance Charge shall not exceed those customarily charged for single tenant
buildings within a three-mile radius of the Lot which are used for purposes
similar to the use of the Building by Tenant and in the event that any capital
repair, improvement or replacement to the common areas and facilities of the
Building and the Lot made by Landlord has a useful life of over one year (as
determined in accordance with generally accepted accounting practices
consistently applied), then only the amortized cost of such repair, improvement
or replacement over said useful life shall be included in the Annual Lot
Maintenance Charge or the Annual Building Maintenance Charge, as applicable.

      Tenant shall make payments on account of the Annual Maintenance Charge
monthly in advance on the first day of each calendar month during the Term. At
the beginning of every fiscal year, Landlord shall deliver to Tenant its
reasonable estimate of the Annual Maintenance Charge (the "Estimated Annual
Maintenance Charge") for the said fiscal year which estimate may include a
reasonable contingency of up to 5%, and Tenant shall make payments on account of
the Annual Maintenance Charge monthly in advance on the first day of each
calendar month during the Term in the amount of one-twelfth of the Estimated
Annual Maintenance Charge. Landlord reserves the right to reasonably re-estimate
and modify the Estimated Annual Maintenance Charge by notice to Tenant once
annually on or about July 1 of each Lease Year (the "Additional Rent Adjustment
Date"), and Tenant's payments shall thereupon be adjusted accordingly. Not later
than sixty (60) days after the end of each fiscal year during the Term and after
Lease termination, Landlord shall render a statement ("Landlord's Statement") in
reasonable detail and according to usual accounting practices certified by
Landlord and showing for the preceding fiscal year or fraction thereof, as the
case may be, the actual Annual Maintenance Charges for the said fiscal year or
fraction thereof, and thereupon any balance owed by Tenant or excess paid by
Tenant under this Section shall be paid to Landlord, or credited to Tenant, as
the case may be, on the next rent payment date. Landlord shall furnish Tenant
with copies of all reasonable documentation and records for the Annual
Maintenance Charges for any fiscal year upon Tenant's request for the same;
provided, however, that Landlord shall not be required to furnish such copies
for any fiscal year if Tenant has not requested such copies within two (2) years
after the expiration of such fiscal year.

                                       24
<PAGE>

      For purposes of this Lease, the first "fiscal year" shall be the annual
period commencing on the Commencement Date and ending on December 31 of the year
in which the Commencement Date occurs; subsequently, the term "fiscal year"
shall mean each consecutive annual period thereafter, commencing on the day
following the end of the preceding fiscal year. Landlord shall have the right
from time to time to change the periods of accounting under this Section 4.2.4
to any annual period other than a fiscal year, and upon any such change all
items referred to in this Section shall be appropriately apportioned. In all
Landlord's Statements rendered under this Section, amounts for periods partially
within and partially without the accounting periods shall be appropriately
apportioned, and any items which are not determinable at the time of a
Landlord's Statement shall be included therein on the basis of Landlord's
estimate, and with respect thereto Landlord shall render promptly after
determination a supplemental Landlord's Statement, and appropriate adjustment
shall be made according thereto. All of Landlord's Statements shall be prepared
on an accrual basis of accounting.

      Notwithstanding any other provision of this Section 4.2.4, if the Term
expires or is terminated as of a date other than the last day of a fiscal year,
then for such fraction of a fiscal year at the end of the Term, Tenant's last
payment to Landlord under this Section 4.2.4 shall be made on the basis of
Landlord's best estimate of the items otherwise includable in Landlord's
Statement and shall be made on or before the later of (a) 10 days after Landlord
delivers such estimate to Tenant or (b) the last day of the Term. Landlord shall
thereafter prepare a Landlord's Statement showing the actual Annual Maintenance
Charge for such fiscal year, as hereinabove provided, and an appropriate payment
or refund shall thereafter promptly be made upon submission of such Landlord's
Statement to Tenant.

      Notwithstanding the foregoing, Landlord hereby agrees that HVAC service
and all other utilities shall be available to Tenant 24 hours per day, seven (7)
days per week, subject to the provisions hereof with respect to loss or
interruption of utilities and other services.

      4.2.5 PAYMENTS ON ACCOUNT OF TAXES, INSURANCE AND UTILITIES.

      Tenant shall make payments on account of the Annual Tax, Insurance and
Utility Charge (as hereinafter defined) monthly in advance on the first day of
each calendar month during the Term, which payments shall initially be in the
amount of the sum of the Initial Tax Charge, the Initial Insurance Charge and
the Initial Utility Charge (the "Estimated Initial Tax, Insurance and Utility
Charges"). At the beginning of every fiscal year, Landlord shall deliver to
Tenant its reasonable estimate of the Annual Tax, Insurance and Utility Charge
("the Estimated Annual Tax, Insurance and Utility Charge") for said fiscal year,
and, in lieu of payments of one twelfth of the Estimated Initial Tax, Insurance
and Utility Charge, Tenant shall make payments on account of the Annual Tax,
Insurance and Utility Charge monthly in advance on the first day of each
calendar month during the Term in the amount of one-twelfth of the Estimated
Annual Tax, Insurance and Utility Charge. Landlord reserves the right to
reasonably re-estimate and modify the Estimated Annual Tax, Insurance and
Utility Charge by notice to Tenant once annually on the Additional Rent
Adjustment Date (as defined in Section 4.2.4 hereof), and Tenant's payments
shall thereupon be adjusted accordingly.

                                       25
<PAGE>

      Not later than sixty (60) days after the end of each fiscal year during
the Term and after Lease termination, Landlord shall render a statement in
reasonable detail and according to usual accounting practices certified by
Landlord and showing for the preceding fiscal year or fraction thereof, as the
case may be, the actual Annual Tax, Insurance and Utility Charge for the said
fiscal year or fraction thereof, and thereupon any balance owed by Tenant or
excess paid by Tenant under this Section shall be paid to Landlord, or credited
to Tenant, as the case may be, on the next rent payment date. As used herein,
the term "Annual Tax, Insurance and Utility Charge" shall mean and refer to the
amount of funds paid by Tenant pursuant to Section 4.2.1, 4.2.2 and 4.2.3 for
the fiscal year in question for costs actually incurred by Landlord (without any
mark-up for Landlord's overhead or profit). All payments under this Section
shall to the extent thereof relieve Tenant of its obligations under said
Sections 4.2.1, 4.2.2 and 4.2.3 hereof. Notwithstanding any other provision
hereof to the contrary, the Annual Tax, Insurance and Utility Charge and the
Annual Maintenance Charge payable by Tenant pursuant to this Lease shall not
include any amount payable by Tenant pursuant to a lease, as amended, by and
between David M. Roby and David E. Clem, Trustees of Fort Washington Realty
Trust, as landlord, and Tenant, as tenant, for premises known as and numbered
130 Waverly Street and/or 40 Erie Street, Cambridge, Massachusetts (the "Phase I
Lease"), provided however, that Landlord may equitably allocate real estate
taxes until the Lot is separately assessed.

      Landlord shall have the right from time to time to change the periods of
accounting under this Section 4.2.5 to any annual period other than a fiscal
year, and upon any such change all items referred to in this Section shall be
appropriately apportioned. In all Landlord's annual statements rendered under
this Section, amounts for periods partially within and partially without the
accounting periods shall be appropriately apportioned, and any items which are
not determinable at the time of such a statement shall be included therein on
the basis of Landlord's estimate, and with respect thereto Landlord shall render
promptly after determination a supplemental statement, and an appropriate
adjustment shall be made according thereto. All of landlord's statements under
this Section shall be prepared on an accrual basis of accounting.

      Notwithstanding any other provision of this Section 4.2.5, if the Term
expires or is terminated as of a date other than the last day of a fiscal year,
then for such fraction of a fiscal year at the end of the Term, Tenant's last
payment to Landlord under this Section 4.2.5 shall be made on the basis of
Landlord's best estimate of the items otherwise includable in the annual
statement rendered by Landlord under this Section and shall be made on or before
the later of (a) 10 days after Landlord delivers such estimate to Tenant or (b)
the last day of the Term, with an appropriate payment or refund to be made upon
submission of Landlord's statement.

4.3   LATE PAYMENT OF RENT.

      If any installment of rent is paid after the date the same was due, it
shall bear interest from the due date at the prime commercial rate of
BankBoston, N.A. or its successor(s), as it may be adjusted from time to time,
plus 4% per annum, but in no event more than the highest rate of interest
allowed by applicable law. Any amounts due under this Section 4.3 shall be
Additional Rent. The foregoing provisions of this Section 4.3 shall not apply to
the first two installments of

                                       26
<PAGE>

rent paid after the date the same were due during each twelve consecutive month
period during the Term.

                                   ARTICLE V

                         TENANT'S ADDITIONAL COVENANTS

5.1   AFFIRMATIVE COVENANTS.

      Tenant covenants at its expense at all times during the Term and for such
further time as Tenant occupies the Premises or any part thereof:

      5.1.1 PERFORM OBLIGATIONS.

            To perform promptly all of the obligations of Tenant set forth in
this Lease; and to pay when due the Fixed Rent and Additional Rent and all
charges, rates and other sums which by the terms of this Lease are to be paid by
Tenant.

      5.1.2 OCCUPANCY AND USE.

            Continuously from the Commencement Date, to use and occupy the
Premises only for the Permitted Uses, and from time to time to procure all
licenses and permits necessary therefor at Tenant's sole expense.

            Without limitation, Tenant shall strictly comply with all federal,
state, and municipal laws, ordinances, and regulations governing the use of
Tenant's laboratory, scientific experimentation and the generation, storage,
containment and disposal of medical waste. Tenant shall be solely responsible
for procuring and complying at all times with any and all necessary permits
directly relating or incident to: the conduct of its office and research
activities on the Premises; its scientific experimentation, transportation,
storage, handling, use and disposal of any chemical or radioactive or
bacteriological or pathological substances or organisms or other hazardous
wastes or environmentally dangerous substances or materials or medical waste.
Within ten (10) days of a request by Landlord, which request shall be made not
more than once during each period of twelve (12) consecutive months during the
Term hereof, unless otherwise requested by any mortgagee of Landlord, Tenant
shall furnish Landlord with copies of all such permits which Tenant possesses or
has obtained together with a certificate certifying that such permits are all of
the permits which Tenant possesses or has obtained with respect to the Premises.
Tenant shall be entitled to redact any Confidential Information from the copies
of such permits and accompanying certificates of Tenant. Tenant shall promptly
give notice to Landlord of any warnings or violations relative to the above
received from any federal, state, or municipal agency or by any court of law and
shall promptly cure the conditions causing any such violations. Tenant shall not
be deemed to be in default of its obligations under the preceding sentence to
promptly cure any condition causing any such violation in the event that, in
lieu of such cure, Tenant shall contest the validity of such violation by
appellate or other proceedings permitted under applicable law, provided that:
(i) any such contest is made reasonably and in

                                       27
<PAGE>

good faith, (ii) Tenant makes provisions, including, without limitation, posting
bond(s) or giving other security, acceptable to Landlord to protect Landlord,
the Building and the Lot from any liability, costs, damages or expenses arising
in connection with such violation and failure to cure, (iii) Tenant shall agree
to indemnify, defend (with counsel reasonably acceptable to Landlord) and hold
Landlord harmless from and against any and all liability, costs, damages, or
expenses arising in connection with such condition and/or violation, (iv) Tenant
shall promptly cure any violation in the event that its appeal of such violation
is overruled or rejected, and (v) Tenant shall certify to Landlord's
satisfaction that Tenant's decision to delay such cure shall not result in any
actual or threatened bodily injury or property damage to Landlord, any tenant or
occupant of the Building or the Lot, or any other person or entity. Landlord
agrees that any Confidential Information gained or obtained by Landlord pursuant
to this Section 5.1.2 shall be kept confidential in accordance with Section
10.15 hereof.

      5.1.3 REPAIR AND MAINTENANCE.

            Except as otherwise provided in Article VI, to keep the Premises
including, without limitation, all fixtures and equipment now or hereafter on
the Premises, or exclusively serving the Premises, but excluding the exterior
(exclusive of glass and doors) and structural elements of the Building and the
grounds and parking lot, which Landlord shall maintain and repair unless such
repairs are required because of Tenant's willful misconduct or negligence, in
good order, condition and repair and at least as good order, condition and
repair as they are in on the Commencement Date or may be put in during the Term,
reasonable use and wear only excepted; to keep in a safe, secure and sanitary
condition all trash and rubbish temporarily stored at the Premises; and to make
all repairs and replacements and to do all other work necessary for the
foregoing purposes whether the same may be ordinary or extraordinary, foreseen
or unforeseen. Tenant shall be responsible for heating and air-conditioning
systems serving the Premises to the extent that such systems are not a part of
Base Building Improvements, and Tenant shall secure, pay for and keep in force
contracts with appropriate and reputable service companies providing for the
regular maintenance of the heating and air-conditioning systems serving the
Premises to the extent that such systems are not a part of Base Building
Improvements, and copies of such contracts shall be furnished to Landlord. It is
further agreed that the exception of reasonable use and wear shall not apply so
as to permit Tenant to keep the Premises in anything less than suitable,
tenantlike, and efficient and usable condition considering the nature of the
Premises and the use reasonably made thereof, or in less than good and
tenantlike repair.

      5.1.4 COMPLIANCE WITH LAW.

            To make all repairs, alterations, additions or replacements to the
Premises required by any law or ordinance or any order or regulation of any
public authority other than major capital repairs, alterations, additions or
replacements to the foundations and structural elements of the Building which
are not required because of Tenant's failure to comply with the provisions of
Section 5.1.3 hereof; to keep the Premises equipped with all safety appliances
so required; to pay all municipal, county, or state taxes assessed against the
leasehold interest hereunder, or against personal property of any kind on or
about the Premises; and to comply with

                                       28
<PAGE>

the orders and regulations of all governmental authorities with respect to
zoning, building, fire, health and other codes, regulations, ordinances or laws
applicable to the Premises.

            Tenant shall not use, generate, manufacture, produce, handle, store,
release, discharge or dispose of in, on, under or about the Premises or
transport to or from the Premises, or allow its employees, agents, contractors,
invitees or any other person or entity to do so, any oil, hazardous or toxic
materials or hazardous or toxic wastes or medical waste (collectively,
"hazardous materials") except to the extent that the following conditions
regarding the use, generation, manufacture, production, handling, storing,
releasing, discharging, disposal or transport (individually or collectively, the
"Use") of hazardous materials shall be satisfied: (i) the Use shall be directly
related to the operation of Tenant's business as permitted herein, (ii) Tenant
shall first provide Landlord with the list of the types and quantities of such
proposed hazardous materials which Tenant is required to furnish to the
applicable governmental authorities for purposes of compliance with the Resource
Conservation and Recovery Act, as amended (42 U.S.C. Section 9601,et seq.) (the
"RCRA List") (or, in the event that the RCRA List ceases to be required to be
filed under such law, a list containing the same information required to be
listed on the RCRA List as of the date hereof), and shall update such list as
necessary for continuing accuracy, and such other information reasonably
satisfactory to Landlord as Landlord may reasonably require concerning such Use,
and (iii) such Use shall be in strict compliance (at Tenant's expense) with all
applicable laws, regulations, licenses and permits. Landlord hereby covenants
and agrees that the information contained in any list, or update thereof,
referred to in the foregoing clause (ii) shall be kept confidential in
accordance with Section 10.15 hereof. Notwithstanding the foregoing, Tenant
hereby agrees to consult and coordinate with Landlord prior to transporting any
hazardous materials to or from the Premises whenever (i) such transportation is
not of the kind regularly made during the ordinary course of business by a
person or entity operating a laboratory facility for the Permitted Uses or (ii)
Tenant has reason to believe that such transportation may result in a public
demonstration, protest or other similar disturbance at the Building or the Lot.
If the transportation, generation, manufacture, production, handling, release,
storage, use or disposal of any hazardous materials anywhere on the Premises in
connection with the Tenant's use of the Premises results in (1) contamination of
the soil, surface or ground water or (2) loss or damage to person(s) or
property, then Tenant agrees to respond in accordance with the following
paragraph:

            Tenant agrees (i) to notify Landlord immediately of any
            contamination, claim of contamination, loss or damage, (ii) after
            consultation and approval by Landlord, to clean up the contamination
            in full compliance with all applicable statutes, regulations and
            standards, and (iii) to indemnify, defend (with counsel acceptable
            to Landlord) and hold Landlord harmless from and against any claims,
            suits, causes of action, costs and fees, including attorneys' fees,
            arising from or connected with any such contamination, claim of
            contamination, loss or damage. No consent or approval of Landlord
            shall in any way be construed as imposing upon Landlord any
            liability for the means, methods, or manner of removal, containment
            or other compliance with applicable law for and with respect to the
            foregoing.

                                       29
<PAGE>

            Tenant shall promptly notify Landlord upon Tenant's receipt of any
inquiry, notice, or threat to give notice by any government authority or any
other third party with respect to any hazardous materials. Notwithstanding the
foregoing, Tenant shall not be liable to Landlord hereunder for any
contamination, claim of contamination, loss or damage arising in connection with
hazardous materials to the extent the same is the result of (A) hazardous
materials existing in the Building and the Lot prior to Tenant's use or
occupancy of the Premises, (B) migration of hazardous materials from any site
onto the Lot not caused by Tenant, (C) the generation,, manufacture, production,
handling, release, storage, use or disposal of any hazardous materials at the
Building or the Lot by Landlord, any other tenant or occupant, or any so-called
"mid-night dumpers" or (D) the Use (as defined above in this Section) by any
party other than Tenant of hazardous materials at the Building or the Lot after
the date upon which Tenant has completely vacated the same, including removal of
all of its property (to the extent permitted herein) and hazardous materials.
Tenant's indemnification obligations under this Section shall survive the
expiration or earlier termination of this Lease.

            Prior to vacating the Premises at the expiration of the Term hereof,
Tenant at its sole cost and expense shall provide Landlord with an environmental
audit by a qualified environmental engineering firm satisfactory to Landlord,
which audit shall include reasonable subsurface testing if requested by
Landlord. The aforesaid environmental audit shall affirmatively certify that the
Premises are free from any and all contaminants, pollutants, radioactive
materials, hazardous wastes or materials, medical waste, bacteriological agents
or organisms which would render the Premises in violation of M.G.L. c. 21E, the
Comprehensive Environmental Response, Compensation, and Liability Act of 1980,
as amended, 42 U.S.C. Section 9602 et seq, or any other applicable laws, rules,
regulations or orders, as they may be amended or supplemented by administrative
regulations, from time to time.

            Nothing herein contained shall be construed to limit or impair
Tenant's obligation to comply with any law, code, rule or regulation which
requires Tenant to notify any governmental authority or any other person
concerning the Use (as defined above in this Section) of hazardous materials by
Tenant at the Premises.

            Tenant agrees that, with respect to the Premises, it shall be
responsible for compliance with the Americans with Disabilities Act (42 U.S.C.
Section 12101 et seq.) and the regulations and Accessibility Guidelines for
Buildings and Facilities issued pursuant thereto (collectively, the "ADA
Requirements"), except to the extent that the Base Building Improvements are
subject to ADA Requirements, in which event Landlord shall be responsible for
compliance of the Base Building Improvements therewith.

            Tenant covenants and agrees that its use of the Premises shall not
cause a discharge of more than its pro rata share on a square foot basis of the
design flow gallonage per day of sanitary (non-industrial) sewage allowed under
the sewage discharge permit(s) for the Building. Discharges in excess of that
amount, and any discharge of industrial sewage, shall only be permitted if
Tenant, at its sole expense, shall have obtained all necessary permits and

                                       30
<PAGE>

licenses therefor, including without limitation permits from state and local
authorities having jurisdiction thereof.

      5.1.5 TENANT'S WORK.

            To procure at Tenant's sole expense all necessary permits and
licenses before Tenant undertakes any work on the Premises; to do all such work
in compliance with the applicable provisions of Sections 3.2.1, 3.3 and 5.2.3
hereof; to do all such work in a good and workmanlike manner employing materials
of good quality and so as to conform with all applicable zoning, environmental,
building, fire, health and other codes, regulations, ordinances and laws and the
ADA Requirements; to furnish to Landlord prior to the commencement of any such
work, or any other such work for which the cost may reasonably be estimated to
exceed $100,000, a bond or other security acceptable to Landlord assuring that
any work commenced or continued by Tenant will be completed in accordance with
specifications approved in advance in writing by Landlord; to keep the Premises
at all times free of liens for labor and materials; to employ for such work one
or more responsible contractors whose labor will work without Interference with
other labor working on the Premises; to require such contractors employed by
Tenant to carry worker's compensation insurance in accordance with statutory
requirements and comprehensive public liability insurance covering any general
contractors on or about the Premises in amounts that at least equal the limits
set forth in Section 1.1 and to submit certificates evidencing such coverage to
Landlord prior to the commencement of such work; and to save Landlord harmless
and indemnified from all injury, loss, claims or damage to any person or
property occasioned by or growing out of such work. Notwithstanding any other
provision of this Section 5.1.5 or any other provision of this Lease to the
contrary, Landlord shall not, subject to applicable law, prohibit Tenant from
using non-union labor to perform any work at the Premises, including Tenant's
Work. Without limitation of the foregoing, Landlord shall not require Tenant's
employees to unionize.

      5.1.6 INDEMNITY.

            To defend, with counsel approved by Landlord, all actions against
Landlord, any partner, trustee, stockholder, officer, director, employee or
beneficiary of Landlord, holders of mortgages secured by the Premises or the
Building and Lot and any other party having an interest in the Premises
("Indemnified Parties") with respect to, and to pay, protect, indemnify and save
harmless, to the extent permitted by law, all Indemnified Parties from and
against, any and all liabilities, losses, damages, costs, (including reasonable
attorneys' fees and expenses), causes of action, suits, claims, demands or
judgments of any nature arising from (i) injury to or death of any person, or
damage to or loss of property, on the Premises or on adjoining streets or ways
connected with the use or occupancy thereof by Tenant or its agents,
contractors, licensees, employees, sublessees or invitees, unless and to the
extent caused by the negligence of Landlord or its servants or agents, (ii)
violation of this Lease by Tenant or its agents, contractors, licensees,
employees, sublessees or invitees, or (iii) any act, fault, omission, or other
misconduct of Tenant or its agents, contractors, licensees, employees,
sublessees or invitees.

                                       31
<PAGE>

      5.1.7 LANDLORD'S RIGHT TO ENTER.

            To permit Landlord and its agents to enter into the Premises at
reasonable times and upon at least 24 hours advance notice (except in case of
emergency in which event no prior notice shall be required) to examine the
Premises, make such repairs and replacements as Landlord may elect, without
however, any obligation to do so except as may be otherwise expressly set forth
in this Lease, and show the Premises to prospective purchasers and lenders, and,
during the last twelve months of the Term, to show the Premises to prospective
tenants and to keep affixed in suitable places notices of availability of the
Premises. Landlord's right to enter the Premises in accordance with the
foregoing shall be subject to Landlord's obligations pursuant to Section 10.15
hereof. Notwithstanding the foregoing, Landlord agrees that in the event that
Landlord shows the Premises to any prospective purchaser or tenant, Landlord
shall: (i) provide at least three (3) days' notice to Tenant identifying the
prospective purchaser or tenant, (ii) only show the Premises to such purchaser
or tenant if Landlord believes in good faith that such person or entity is a
bona fide prospective purchaser or tenant, (iii) conduct such showing in
compliance with such reasonable requests and instructions as Tenant may make for
purposes of protecting Tenant's Confidential Information.

      5.1.8 PERSONAL PROPERTY AT TENANT'S RISK.

            All of the furnishings, fixtures, equipment, effects and property of
every kind, nature and description owned or leased by Tenant or by any person
claiming by, through or under Tenant which, during the continuance of this Lease
or any occupancy of the Premises by Tenant or anyone claiming under Tenant, may
be on the Premises (collectively, "Tenant's Property"), shall, as between the
parties, be at the sole risk and hazard of 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 to
be borne by Landlord, except that Landlord shall in no event be indemnified or
held harmless or exonerated from any liability to Tenant or to any other person,
for any injury, loss, damage or liability to the extent (i) such injury, loss,
damage or liability is the result of the negligence or willful misconduct of
Landlord, its contractors, agents or employees, or (ii) such indemnification,
agreement to hold harmless or exoneration is prohibited by law.

      5.1.9 PAYMENT OF LANDLORD'S COST OF ENFORCEMENT.

            To pay on demand Landlord's expenses, including reasonable
attorney's fees, incurred in enforcing any obligation of Tenant under this Lease
or in curing any default by Tenant under this Lease as provided in Section 7.4.

      5.1.10 YIELD UP.

            Subject to Section 3.3 hereof, at the expiration of the Term or
earlier termination of this Lease: to surrender all keys to the Premises; to
remove all of its trade fixtures and personal property in the Premises; to
remove such installations and improvements made by

                                       32
<PAGE>

Tenant as Landlord may request and all Tenant's signs wherever located; to
repair all damage caused by such removal; and to yield up the Premises
(including all installations and improvements made by Tenant except for trade
fixtures and such of said installations or improvements as Landlord shall
request Tenant to remove), broom-clean and in the same good order and repair in
which Tenant is obliged to keep and maintain the Premises by the provisions of
this Lease. Subject to Section 3.3 hereof, any property not so removed shall be
deemed abandoned and may be removed and disposed of by Landlord in such manner
as Landlord shall determine and Tenant shall pay Landlord the entire cost and
expense incurred by Landlord in effecting such removal and disposition and in
making any incidental repairs and replacements to the Premises and for use and
occupancy during the period after the expiration of the Term and prior to
Tenant's performance of its obligations under this Section 5.1.10. Tenant shall
further indemnify Landlord against all loss, cost and damage resulting from
Tenant's failure and delay in surrendering the Premises as above provided.

      5.1.11 ESTOPPEL CERTIFICATE.

            Upon not less than 10 days' prior notice by Landlord, to execute,
acknowledge and deliver to Landlord a statement in writing certifying that this
Lease is unmodified and in full force and effect and that except as stated
therein Tenant has no knowledge of any defenses, offsets or counterclaims
against its obligations to pay the Fixed Rent and Additional Rent and any other
charges end to perform its other covenants under this Lease (or, if there have
been any modifications that the Lease is in full force and effect as modified
and stating the modifications and, if there are any defenses, offsets or
counterclaims, setting them forth in reasonable detail), the dates to which the
Fixed Rent and Additional Rent and other charges have been paid and a statement
that, to the best of Tenant's knowledge, Landlord is not in default hereunder
(or if in default, the nature of such default, in reasonable detail) and such
other matters reasonably required by Landlord or any prospective purchaser or
mortgagee of the Premises. Any such statement delivered pursuant to this Section
5.1.11 may be relied upon by any prospective purchaser or mortgagee of the
Premises, or any prospective assignee of any such mortgage.

      5.1.12 LANDLORD'S EXPENSES RE CONSENTS.

            To reimburse Landlord promptly on demand for all reasonable legal
expenses incurred by Landlord in connection with all requests by Tenant for
consent or approval under this Lease. Notwithstanding the foregoing, Tenant
shall not be liable for any reasonable legal expenses incurred by Landlord for
the first two (2) such requests made by Tenant during each period of twelve (12)
consecutive calendar months during the Term.

      5.1.13 RULES AND REGULATIONS.

            To comply with the Rules and Regulations set forth in Exhibit C, as
the same may be amended from time to time by Landlord to provide for the
beneficial operation of the Building and/or Lot, provided that such amendments
do not materially interfere with Tenant's right of use and enjoyment of the
Premises pursuant to this Lease.

                                       33
<PAGE>

      5.1.14 LOADING.

            Not to place Tenant's Property, as defined in Section 5.1.8, upon
the Premises so as to exceed the floor load limits set forth in Exhibit D
attached hereto and not to move any safe, vault or other heavy equipment in,
about or out of the Premises except in such manner and at such times as Landlord
shall in each instance approve; Tenant's business machines and mechanical
equipment which cause vibration or noise that may be detectable outside of the
Building shall be placed or maintained by Tenant in settings of cork, rubber,
spring, or other types of vibration eliminators sufficient to reduce such
vibration or noise to a level reasonably acceptable to Landlord.

      5.1.15 HOLDOVER.

            To pay to Landlord (i) the greater of twice (a) the then fair market
rent as reasonably determined by Landlord or (b) the total of the Fixed Rent,
Additional Rent, and all other payments then payable hereunder, for each month
or portion thereof Tenant shall retain possession of the Premises or any part
thereof after the termination of this Lease, whether by lapse of time or
otherwise, and (ii) all damages sustained by Landlord on account thereof;
provided, however, that any payments made by Tenant under the foregoing clause
(i) in excess of the then fair market rent for the Premises as so reasonably
determined by Landlord shall be applied against any damages under the foregoing
clause (ii). The provisions of this subsection shall not operate as a waiver by
Landlord of the right of re-entry provided in this Lease.

5.2   NEGATIVE COVENANTS.

      Tenant covenants at all times during the Term and for such further time as
Tenant occupies the Premises or any part thereof:

      5.2.1 ASSIGNMENT AND SUBLETTING.

            Not without the prior written consent of Landlord to assign this
Lease, to make any sublease, or to permit occupancy of the Premises or any part
thereof by anyone other than Tenant, voluntarily or by operation of law, except
as hereinafter provided; as Additional Rent, to reimburse Landlord promptly for
reasonable legal and other expenses incurred by Landlord in connection with any
request by Tenant for consent to assignment or subletting (subject to the
provisions of Section 5.1.12 hereof); no assignment or subletting shall affect
the continuing primary liability of Tenant (which, following assignment, shall
be joint and several with the assignee); no consent to any of the foregoing in a
specific instance shall operate as a waiver in any subsequent instance.
Landlord's consent to any proposed assignment or subletting is required both as
to the terms and conditions thereof and as to the consistency of the proposed
assignee's or subtenant's business with other uses and tenants in the Building.
In addition, as to any assignee (but not as to any sublessee) Landlord's consent
shall be required as to the reasonable creditworthiness of the proposed assignee
in view of market conditions then prevailing for leases having terms and
conditions comparable to this Lease. Landlord's consent to any assignment or
subletting by Tenant shall not be unreasonably withheld, provided that

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<PAGE>

Tenant is not then in default under this Lease and that such assignee or
subtenant pays therefor the greater of the Fixed Rent, Additional Rent, and all
other payments then payable hereunder, or the then fair market rent for the
Premises. If Tenant requests Landlord's consent to assign this Lease or to
sublet any portion of the Premises such that Tenant shall not occupy at least
40,000 r.s.f. of the Premises after the date of commencement of such sublease,
Landlord shall have the option, exercisable by written notice to Tenant given
within 10 days after receipt of such request, to terminate this Lease as of the
date of commencement the proposed sublease or assignment; provided, however,
that Tenant shall have the right to rescind any such request in the event
Landlord elects to so terminate this Lease by notice given to Landlord within
five (5) days after the date of such termination notice from Landlord, in which
event such termination notice shall be of no further force or effect;

      If, at any time during the Term of this Lease, Tenant is:

                  (i) a corporation or a trust (whether or not having shares of
beneficial interest) and there shall occur any change in the identity of any of
the persons then having power to participate in the election or appointment of
the directors, trustees or other persons exercising like functions and managing
the affairs of Tenant; or

                  (ii) a partnership or association or otherwise riot a natural
person (and is not & corporation or a trust) and there shall occur any change in
the identity of any of the persons who then are members of such partnership or
association or who comprise Tenant;

Tenant shall so notify Landlord and Landlord may terminate this Lease by notice
to Tenant given within 90 days thereafter if, in Landlord's reasonable judgment,
the credit of Tenant is thereby impaired. This paragraph shall not apply if the
initial Tenant named herein is a corporation and the outstanding voting stock
thereof is listed on a recognized securities exchange.

      Notwithstanding the foregoing provisions of this Section 5.2.1, Tenant may
assign this Lease or sublet any portion of the Premises without Landlord's
consent to (i) any successor of Tenant resulting from an acquisition of all or
substantially all of Tenant's assets or a merger or consolidation of Tenant and
(ii) any Affiliate of Tenant (as hereinafter defined) whose net worth is equal
to or greater than the net worth of Tenant as of the date hereof, provided that
Tenant provides Landlord at least thirty (30) days prior notice of such
assignment or subletting pursuant to either of the foregoing clauses (i) or
(ii). As used herein, the term "Affiliate of Tenant" shall mean, and refer to
any entity controlled by, controlling or under common control with Tenant.

      In the event that any assignee or subtenant pays to Tenant any amounts in
excess of the Fixed Rent, Additional Rent, and all other payments then payable
hereunder, or pro rata portion thereof on a square footage basis for any portion
of the Premises (such excess being hereinafter referred to as "Sublease
Profits"), Tenant shall promptly pay fifty percent (50%) of said Sublease
Profits to Landlord as and when received by Tenant after deduction of Tenant's
Sublease Costs (as hereinafter defined). The term "Sublease Costs" shall mean
and refer to Tenant's reasonable legal, brokerage and construction costs and
expenses incurred in good faith in view of the size

                                       35
<PAGE>

and expected term of any applicable sublease or assignment. Sublease Costs shall
be amortized over the term of the applicable sublease or assignment.

      5.2.2 NUISANCE.

            Not to injure, deface or otherwise harm the Premises; nor commit any
nuisance; nor permit the emission of any noise, vibration or odor which is
contrary to any law or ordinance; nor make, allow or suffer any waste; nor make
any use of the Premises which is improper, offensive or contrary to any law or
ordinance or which will invalidate any of Landlord's insurance,

      5.2.3 INSTALLATION, ALTERATIONS OR ADDITIONS.

            Subject to the provisions of Section 3.2.1, 3.3 and Section 5.1.5
hereof, not to make any installations, alterations, or additions in, to or on
the Premises (including, without limitation, buildings, lawns, planted areas,
walks, roadways, parking and loading areas, but expressly excluding the initial
Tenant's Work, provided the same is approved by Landlord, such approval not to
be unreasonably withheld or delayed), nor, except for Tenant's Work approved by
Landlord, to permit the making of any apertures in the walls, partitions,
ceilings or floors without on each occasion obtaining the prior written consent
of Landlord and then only pursuant to plans and specifications approved by
Landlord in advance in each instance.

                                   ARTICLE VI

                               CASUALTY OR TAKING

6.1   TERMINATION.

      In case during the period which is thirty (30) months prior to the
expiration of the Term all or any substantial part of the Premises or of the
Building or of the Lot or any one or more of them shall be taken by any public
authority or for any public use, or shall be destroyed or damaged by fire or
casualty, or by the action of any public authority, or Landlord receives
compensable damage by reason of anything lawfully done in pursuance of public or
other authority, (hereinafter referred to as the "Casualty or Taking"), then
this Lease may be terminated at the election of Landlord. Such election, which
may be made notwithstanding the fact that Landlord's entire interest may have
been divested, shall be made by the giving of notice by Landlord to Tenant
within 30 days after the Casualty or Taking.

6.2   RESTORATION.

      If Landlord does not exercise said election (or is not entitled to
exercise said election in the case of a Casualty or Taking occurring more than
thirty (30) months prior to the expiration of the Term of this Lease), this
Lease shall continue in force and a just proportion of the rent reserved,
according to the nature and extent of the damages sustained by the Premises, but
not in excess of an equitable proportion of the net proceeds of insurance
recovered by Landlord under

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<PAGE>

the rental insurance coverage carried pursuant to Section 4.2.2.2, shall be
abated from the date of the Casualty or Taking until the Premises, or what may
remain thereof, shall be put by Landlord in proper condition for use subject to
zoning and building laws or ordinances then in existence, which, unless Landlord
has exercised its option to terminate pursuant to Section 6.1, Landlord
covenants to do with reasonable diligence at Landlord's expense, provided that
Landlord's obligations with respect to restoration shall not require Landlord to
expend more than the net proceeds of insurance recovered or damages awarded for
such Casualty or Taking. "Net proceeds of insurance recovered or damages
awarded" refers to the gross amount of such insurance or damages less the
reasonable expenses of Landlord in connection with the collection of the same,
including without limitation, fees and expenses for legal and appraisal
services.

      Notwithstanding the foregoing, in the event that Landlord's architect
reasonably determines that the Premises will not be repaired or restored (to
the extent permitted by the net proceeds of insurance recovered or damages
awarded from such Casualty or Taking) within one year after the occurrence of
such Casualty or Taking then Tenant shall have the right to terminate this Lease
by notice given within thirty (30) days after the date of such determination.

6.3   AWARD.

      Irrespective of the form in which recovery may be had by law, all rights
to damages or compensation shall belong to Landlord in all cases except as set
forth below in this Section 6.3. Tenant hereby grants to Landlord all of
Tenant's rights to such damages and compensation and covenants to deliver such
further assignments thereof as Landlord may from time to time request. It is
agreed and understood, however, that Landlord does not reserve to itself, and
Tenant does not assign to Landlord, any damages payable for (i) movable trade
fixtures installed by Tenant or anybody claiming under Tenant, at its own cost
and expense or (ii) relocation expenses or damages for loss of business (in
excess of any such damages attributable to the value of this lease) recoverable
by Tenant from such authority in a separate action.

                                  ARTICLE VII

                                    DEFAULTS

7.1   EVENTS OF DEFAULT.

      (a) If Tenant shall default in the performance of any of its obligations
to pay the Fixed Rent or Additional Rent hereunder and if such default shall
continue for 10 days after notice from Landlord to Tenant (provided, however,
that Landlord shall not be required to provide such notice more than two (2)
times in any period of twelve (12) consecutive calendar months) or if within 30
days after notice from Landlord to Tenant specifying any other default or
defaults Tenant has not commenced diligently to correct the default or defaults
so specified or has not thereafter diligently pursued such correction to
completion, or (b) if any assignment for the benefit of creditors shall be made
by Tenant, or by any guarantor of Tenant, or (c) if Tenant's leasehold interest
shall be taken on execution or other process of law in any action against
Tenant, or (d) if a lien or other involuntary encumbrance is filed against
Tenant's leasehold

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<PAGE>

interest, and, is not discharged within thirty (30) days thereafter, or (e) if a
petition is filed by Tenant or any guarantor of Tenant for liquidation, or for
reorganization or an arrangement or any other relief under any provision of the
Bankruptcy Code as then in force and effect, or (f) if an involuntary petition
under any of the provisions of said Bankruptcy Code is filed against Tenant or
any guarantor of Tenant and such involuntary petition is not dismissed within
ninety (90) days thereafter, or (g) if Tenant fails to maintain the insurance
required under Section 4.2.2.1 hereof, then, and in any of such cases, Landlord
and the agents and servants of Landlord lawfully may, in addition to and not in
derogation of any remedies for any preceding breach of covenant, immediately or
at any time thereafter and without demand or notice, at Landlord's election, do
any one or more of the following: (1) give Tenant written notice stating that
the Lease is terminated, effective upon the giving of such notice or upon a date
stated in such notice, as Landlord may elect, in which event the Lease shall be
irrevocably extinguished and terminated as stated in such notice without any
further action, or (2) with or without process of law, in a lawful manner and
without illegal force, enter and repossess the Premises as of Landlord's former
estate, and expel Tenant and those claiming through or under Tenant, and remove
its and their effects, without being guilty of trespass, in which event the
Lease shall be irrevocably extinguished and terminated at the time of such
entry, or (3) pursue any other rights or remedies permitted by law. Any such
termination of the Lease shall be without prejudice to any remedies which might
otherwise be used for arrears of rent or prior breach of covenant, and in the
event of such termination Tenant shall remain liable under this Lease as
hereinafter provided. Tenant hereby waives all statutory rights of redemption
and Landlord, without notice to Tenant, may store Tenant's effects, and those of
any person claiming through or under Tenant, at the expense and risk of Tenant,
and, if Landlord so elects, may sell such effects at public auction or private
sale and apply the net proceeds to the payment of all sums due to Landlord from
Tenant, if any, and pay over the balance, if any, to Tenant.

7.2   REMEDIES.

      In the event that this Lease is terminated under any of the provisions
contained in Section 7.1 or shall be otherwise terminated for breach of any
obligation of Tenant, Tenant covenants to pay forthwith to Landlord, as
compensation, the excess of the total rent reserved for the residue of the Term
over the fair market rental value of the Premises for said residue of the Term.
In calculating the rent reserved there shall be included, in addition to the
Fixed Rent and Additional Rent, the value of all other considerations agreed to
be paid or performed by Tenant during said residue. Tenant further covenants (as
additional and cumulative obligations) after any such termination to pay
punctually to Landlord all the sums and to perform all the obligations which
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.
In calculating the amounts to be paid by Tenant pursuant to the next preceding
sentence Tenant shall be credited with any amount paid to Landlord as
compensation as in this Section 7.2 provided and also with the net proceeds of
any rent obtained by Landlord by reletting the Premises, after deducting all of
the Landlord's reasonable 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, it
being agreed by Tenant that Landlord may (i) relet the Premises or any part or
parts thereof, for a term or terms which may at Landlord's option be equal to or
less

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<PAGE>

than or exceed the period which would otherwise have constituted the balance of
the Term and may grant such concessions and free rent as Landlord in its
reasonable judgment considers advisable or necessary to relet the same and (ii)
make such alterations, repairs and decorations in the Premises as Landlord in
its reasonable judgment considers advisable or necessary to relet the same, and
no action of Landlord in accordance with the foregoing or failure to relet or to
collect rent under reletting shall operate or be construed to release or reduce
Tenant's liability as aforesaid.

      In lieu of any other damages or indemnity and in lieu of full recovery by
Landlord of all sums payable under all the foregoing provisions of this Section
7.2, Landlord may by notice to Tenant at any time after this Lease is terminated
under any of the provisions contained in Section 7.1 or is otherwise terminated
for breach of any obligation of Tenant and before such full recovery, elect to
recover, and Tenant shall thereupon pay, as liquidated damages, an amount equal
to the aggregate of the Fixed Rent and Additional Rent accrued in the 12 months
ended next prior to such termination, plus the amount of rent of any kind
accrued and unpaid at the time of termination and less the amount of any
recovery by Landlord under the foregoing provisions of this Section 7.2 up to
the time of payment of such liquidated damages.

      Nothing contained in this Lease shall, however, limit or prejudice the
right of Landlord to prove for and obtain in proceedings for bankruptcy or
insolvency 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 than, equal to, or less than the amount of the loss or
damages referred to above.

7.3   REMEDIES CUMULATIVE.

      Any and all rights and remedies which either Landlord or Tenant may have
under this Lease, and at law and equity, shall be cumulative and shall not be
deemed inconsistent with each other, and any two or more of all such rights and
remedies may be exercised at the same time insofar as permitted by law.

7.4   LANDLORD'S RIGHT TO CURE DEFAULTS.

      Landlord may, but shall not be obligated to, cure, at any time, following
10 days' prior notice to Tenant, except in cases of emergency when no notice
shall be required, any default by Tenant under this Lease; and whenever Landlord
so elects, all costs and expenses incurred by Landlord, including reasonable
attorneys' fees, in curing a default shall be paid by Tenant to Landlord as
Additional Rent on demand, together with interest thereon at the rate provided
in Section 4.3 from the date of payment by Landlord to the date of payment by
Tenant.

7.5   EFFECT OF WAIVERS OF DEFAULT.

      Any consent or permission by Landlord or Tenant to any act omission by the
other party which otherwise would be a breach of any covenant or condition
herein, or any waiver by

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<PAGE>

Landlord or Tenant of the breach of any covenant or condition herein by the
other party, shall not in any way be held or construed (unless expressly so
declared) to operate so as to impair the continuing obligation of any covenant
or condition herein, or otherwise, except as to the specific instance, operate
to permit similar acts or omissions.

      The failure of Landlord or Tenant to seek redress for violation of, or to
insist upon the strict performance of, any covenant or condition of this Lease
by the other party 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 Landlord, or
the payment by Tenant, as the case may be, of rent with knowledge of the breach
of any covenant of this Lease shall not be deemed to have been a waiver of such
breach by Landlord or Tenant as the case may be. No consent or waiver, express
or implied, by Landlord or Tenant, as the case may be, 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.

7.6   NO ACCORD AND SATISFACTION.

      No acceptance by Landlord of a lesser sum than the Fixed Rent, Additional
Rent or any other charge then due shall be deemed to be other than on account of
the earliest installment of such rent or charge due, unless Landlord elects by
notice to Tenant to credit such sum against the most recent installment due, nor
shall any endorsement or statement on any check or any letter accompanying any
check or payment as rent or other charge be deemed an accord and satisfaction,
and Landlord may accept such check or payment without prejudice to Landlord's
right to recover the balance of such installment or pursue any other remedy in
this Lease provided.

                                  ARTICLE VIII

                                    MORTGAGES

8.1   RIGHTS OF MORTGAGE HOLDERS.

      The word "mortgage" as used herein includes mortgages, deeds of trust or
other similar instruments evidencing other voluntary liens or encumbrances, and
modifications, consolidations, extensions, renewals, replacements and
substitutes thereof. The word "holder" shall mean a mortgagee, and any
subsequent holder or holders of a mortgage. Until the holder of a mortgage shall
enter and take possession of the Premises for the purpose of foreclosure, such
holder shall have only such rights of Landlord as are necessary to preserve the
integrity of this Lease as security. Upon entry and taking possession of the
Premises for the purpose of foreclosure, such holder shall have all the rights
of Landlord. Notwithstanding any other provision of this Lease to the contrary,
including without limitation Section 10.5, no such holder of a mortgage shall be
liable either as mortgagee or as assignee to perform, or be liable in damages
for failure to perform, any of the obligations of Landlord unless and until such
holder shall enter and take possession of the Premises for the purpose of
foreclosure, and such holder shall not in any event be liable to perform or for
failure to perform the obligations of Landlord

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<PAGE>

under Section 3.1. Upon entry for the purpose of foreclosure, such holder shall
be liable to perform all of the obligations of Landlord (except for the
obligations under Article III), subject to and with the benefit of the
provisions of Section 10.5, provided that a discontinuance of any foreclosure
proceeding shall be deemed a conveyance under said provisions to the owner of
the equity of the Premises. No Fixed Rent, Additional Rent or any other charge
shall be paid more than 10 days prior to the due dates thereof and payments made
in violation of this provision shall (except to the extent that such payments
are actually received by a mortgagee in possession or in the process of
foreclosing its mortgage) be a nullity as against such mortgagee and Tenant
shall be liable for the amount of such payments to such mortgagee.

      The covenants and agreements contained in this Lease with respect to the
rights, powers and benefits of a holder of a mortgage (including, without
limitation, the covenants and agreements contained in this Section 8.1)
constitute a continuing offer to any person, corporation or other entity, which
by accepting a mortgage subject to this Lease, assumes the obligations herein
set forth with respect to such holder; such holder is hereby constituted a party
of this Lease as an obligee hereunder to the same extent as though its name were
written hereon as such; and such holder shall be entitled to enforce such
provisions in its own name. Tenant agrees on request of Landlord to execute and
deliver from time to time any agreement which may be necessary to implement the
provisions of this Section 8.1.

8.2   SUPERIORITY OF LEASE; OPTION TO SUBORDINATE.

      This Lease shall be superior to and shall not be subordinate to any future
mortgage or other voluntary lien or other encumbrance of the Lot and Building;
provided, however, that Landlord shall have the option to subordinate this Lease
to any such mortgage of the Lot and Building provided that Landlord obtains from
the holder of record of any existing or future mortgage an agreement with Tenant
by the terms of which such holder will agree (a) to recognize the rights of
Tenant under this Lease, (b) to perform Landlord's obligations hereunder arising
after the date of such holder's acquisition of title as hereinafter described,
expressly excluding, however, Landlord's obligations under Article III of this
Lease, and (c) to accept Tenant as tenant of the Premises under the terms and
conditions of this Lease in the event of acquisition of title by such holder
through foreclosure proceedings or otherwise, provided that Tenant will agree to
recognize the holder of such mortgage as Landlord in such event, which agreement
shall be made expressly to bind and inure to the benefit of the successors and
assigns of Tenant and of the holder and upon anyone purchasing said Premises at
any foreclosure sale. Tenant and Landlord agree to execute and deliver any
appropriate instruments necessary to carry out the agreements contained In this
Section 8.2. Any such mortgage to which this Lease shall subordinate may contain
such terms, provisions and conditions as the holder deems usual or customary.

8.3   LEASE AMENDMENTS.

Tenant agrees to make such changes in this Lease as may be reasonably required
by the holder of any mortgage of which the Premises are a part, or any
institution which may purchase all or a substantial part of Landlord's interest
in the Premises, provided that such changes may not

                                       41
<PAGE>

increase the Fixed Rent or other payments due hereunder or otherwise materially
affect the obligations of Tenant hereunder, and provided further that such
changes do not (i) materially interfere with Tenant's right of use and enjoyment
of the Premises pursuant to this Lease, (ii) limit, impair or delay Tenant's
rights to sublease or assign all or portion of this Lease pursuant to Section
5.2.1 hereof, (iii) limit, impair or delay Tenant's right to obtain a reduction
or abatement of rent pursuant to Section 6.2, (iv) limit, impair or delay
Tenant's right to terminate this Lease pursuant to Section 3.2 or Section 6.2 or
(v) otherwise unreasonably limit, impair or delay Tenant's rights hereunder.

                                   ARTICLE IX

                         LANDLORD'S ADDITIONAL COVENANTS

9.1   AFFIRMATIVE COVENANTS.

      Landlord covenants at all times during the Term:

      9.1.1 PERFORM OBLIGATIONS.

            To perform promptly all of the obligations of Landlord set forth in
this Lease, including, without limitation, furnishing, through Landlord's
employees or independent contractors, the services (the cost of which is to be
included in the Annual Maintenance Charge) listed in Exhibit J and completing
construction of the Phase I Space and the Phase II Space in accordance with
Article III;

      9.1.2 REPAIRS.

            Except as otherwise provided in Article VI, to make such repairs
(the cost of which is to be included in the Annual Maintenance Charge) to the
roof, exterior walls, exterior windows and waterproofing, floor slabs, other
structural components, parking areas, walks, landscaping, courtyard and any
other common areas and facilities of the Building as may be necessary to keep
them in good, serviceable and neat condition. Landlord shall be responsible for
the maintenance and repair of the heating and air-conditioning systems and the
components thereof serving the Building to the extent that such systems and
components are included in Base Building Improvements.

      9.1.3 COMPLIANCE WITH LAW.

            To make all repairs, alterations, additions or replacements to the
Building or the Lot (the costs of which are to be included in the Annual
Maintenance Charge) required by any law, ordinance or order or regulation of any
public authority including repairs, alterations, additions or replacements to
the foundations and structural elements of the Building, except as required
because of Tenant's failure to comply with the provisions of Section 5.1.3
hereof; to keep the Building equipped with all safety appliances so required
(the costs of which are to be included in the Annual Maintenance Charge);
subject to Section 4.2.1, to pay all municipal,

                                       42
<PAGE>
county, or state taxes assessed against the Building or the Lot, or against
Landlord's personal property of any kind on or about the Building or the Lot;
and to comply with the orders and regulations of all governmental authorities
with respect to zoning, building, fire, health and other codes, regulations,
ordinances or laws applicable to the Building or the Lot, including the ADA
Requirements (as defined in Section 5.1.4 hereof) and any codes, regulations,
ordinances or laws relating to hazardous materials (as defined in Section
5.1.4), subject to, and without limitation of, Tenant's obligations with respect
to such codes, regulations, ordinances or laws. The costs incurred by Landlord
in connection with the foregoing compliance obligations shall be included in the
Annual Maintenance Charge. All of the foregoing covenants and obligations are
subject to, and without limitation of, all of Tenant's obligations under this
Lease, including, without limitation, those set forth in Sections 4.2 and 5.1.4.

      9.1.4 INDEMNITY.

            To defend, with counsel reasonably approved by Tenant, all actions
against Tenant, any partner, trustee, stockholder, officer, director, employee
or beneficiary of Tenant ("Tenant's Indemnified Parties") with respect to, and
to pay, protect, indemnify and save harmless, to the extent permitted by law,
all Tenant's Indemnified Parties from and against any and all liabilities,
losses, damages, costs, expenses (including reasonable attorneys' fees and
expenses), causes of action, suits, claims, demands or judgments of any nature
to which any of Tenant's Indemnified Parties is subject arising from and to the
extent of any negligent or willful act, fault, omission, or other misconduct of
Landlord or its agents, contractors or employees.

      9.1.5 ESTOPPEL CERTIFICATE.

            Upon not less than 10 days' prior notice by Tenant, to execute,
acknowledge and deliver to Tenant a statement in writing certifying that this
Lease is unmodified and in full force and effect and that except as stated
therein Landlord has no knowledge of any defenses, offsets or counterclaims
against its obligations under this Lease (or, if there have been any
modifications that the Lease is in full force and effect as modified and stating
the modifications and, if there are any defenses, offsets or counterclaims,
setting them forth in reasonable detail), the dates to which the Fixed Rent and
Additional Rent and other charges have been paid and a statement that, to the
best of Landlord's knowledge, Tenant is not in default hereunder (or if in
default, the nature of such default, in reasonable detail) and such other
matters reasonably required by Tenant or any prospective assignee of Tenant. Any
such statement delivered pursuant to this Section 9.1.5 may be relied upon by
any prospective assignee.

      9.1.6 LANDLORD'S TITLE.

            The Lot is currently a portion of a separate taxable parcel of Land
(the "Land") and Landlord is in the process of subdividing the Land so as to,
inter alia, establish the Lot as a separate taxable parcel and establish
easements which benefit and/or burden the Lot and the premises demised pursuant
to the Phase I Lease (collectively, the "Subdivision"). In connection therewith,
Landlord shall, on or before ninety (90) days after the date hereof, obtain,
file and record (i) a partial release of the Lot from Sun America Life Insurance
Company (the

                                       43
<PAGE>

"Mortgagee") of the Mortgage, Security Agreement, Fixture Filing, Financing
Statement and Assignment of Leases and Rents dated July 29, 1998 (the
"Mortgage") or (ii) a Subordination, Nondisturbance and Attornment Agreement
from the Mortgagee substantially in the form delivered to Tenant in connection
with the Phase I Lease. Landlord represents and warrants that except for the
Mortgage, currently there is no other mortgage on the Premises. Upon obtaining
such partial release, Landlord intends to convey the Lot and assign this Lease
to an entity owned and controlled by Landlord (the "Transfer"). Tenant agrees to
cooperate with Landlord in connection with such Subdivision and Transfer,
including without limitation, the execution, acknowledgment and delivery of an
instrument pursuant to which this Lease shall be subordinated to easements
reasonably established in connection with the Subdivision and Transfer and such
other documents as Landlord reasonably may request. In connection with the
Subdivision and Transfer, Landlord shall deliver to Tenant an easement plan (the
"Plan"), which Plan shall be attached hereto as Exhibit A and which Plan shall
show, inter alia, the Lot and an area or areas between the Building and the
Phase I Building within which Interconnections may be made.

      9.1.7 Utilities.

            Subject to Section 4.2.3 hereof, to bring (or cause to brought)
utilities for the Premises to the Utility Switching Points at Landlord's sole
cost and expense.

                                    ARTICLE X

                            MISCELLANEOUS PROVISIONS

10.1  NOTICES FROM ONE PARTY TO THE OTHER.

      All notices required or permitted hereunder shall be in writing and
addressed, if to the Tenant, at the Original Address of Tenant or such other
address as Tenant shall have last designated by notice in writing to Landlord
and, if to Landlord, at Landlord's Address or such other address as Landlord
shall have last designated by notice in writing to Tenant. Any notice shall be
deemed duly given if mailed to such address postage prepaid, registered or
certified mail, return receipt requested, when deposited with the U.S. Postal
Service, or if delivered by a recognized courier service (e.g. Federal Express)
when deposited with such courier service, or if delivered to such address by
hand, when so delivered.

10.2  QUIET ENJOYMENT.

      Landlord agrees that upon Tenant's paying the rent and performing and
observing the terms, covenants, conditions and provisions on its part to be
performed and observed, Tenant shall and may peaceably and quietly have, hold
and enjoy the Premises during the Term without any manner of hindrance or
molestation from Landlord or anyone claiming under Landlord, subject, however,
to the terms of this Lease.

                                       44
<PAGE>

10.3  EASEMENTS; CHANGES TO LOT LINES.

      Landlord reserves the right, from time to time, to grant easements
affecting the Premises or the Building or the Lot and to change or alter
existing boundaries of the Lot for purpose of developing and using the Lot so
long as such easements or such changes or alterations to existing boundaries of
the Lot do not materially interfere with Tenant's use of the Premises, and to
enter upon the Premises for purposes of constructing and maintaining any pipes,
wires and other facilities serving any portion of the Lot or of the Building,
subject to the terms of Section 5.1.7 hereof.

10.4  LEASE NOT TO BE RECORDED.

      Neither party shall record this Lease. Both parties shall execute and
deliver a notice of this Lease in such form, if any, as may be permitted by
applicable statute. If this Lease is terminated before the Term Expiration Date
the parties shall execute, deliver and record an instrument acknowledging such
fact and the actual date of termination of this Lease, and Tenant hereby
appoints Landlord its attorney-in-fact, coupled with an interest, with full
power of substitution to execute such instrument.

10.5  BIND AND INURE; LIMITATION OF LANDLORD'S 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. No owner of the Premises shall be liable
under this Lease except for breaches of Landlord's obligations occurring while
owner of the Premises. The obligations of Landlord shall be binding upon the
assets of Landlord which comprise the Premises but not upon other assets of
Landlord. No partner, trustee, stockholder, officer, director, employee or
beneficiary (or the partners, trustees, stockholders, officers, directors or
employees of any such beneficiary) of Landlord shall be personally liable under
this Lease and Tenant shall look solely to Landlord's interest in the Premises
in pursuit of its remedies upon an event of default hereunder, and the general
assets of the partners, trustees, stockholders, officers, employees or
beneficiaries (and the partners, trustees, stockholders, officers, directors or
employees of any such beneficiary) of Landlord shall not be subject to levy,
execution or other enforcement procedure for the satisfaction of the remedies of
Tenant; provided that the foregoing provisions of this sentence shall not
constitute a waiver of any obligation evidenced by this Lease and provided
further that the foregoing provisions of this sentence shall not limit the right
of Tenant to name Landlord or any individual partner or trustee thereof as part
defendant in any action or suit in connection with this Lease so long as no
personal money judgment shall be asked for or taken against any individual
partner, trustee, stockholder, officer, employee or beneficiary of Landlord.

10.6  ACTS OF GOD.

      In any case where either party hereto is required to do any act, delays
caused by or resulting from the occurrence of one or more Force Majeure Events
shall not be counted in determining the time during which work shall be
completed, whether such time be designated by

                                       45
<PAGE>

a fixed date, a fixed time or a "reasonable time", and such time shall be deemed
to be extended by the period of 'such delay.

10.7  LANDLORD'S DEFAULT.

      Landlord shall not be deemed to be in default in the performance of any of
its obligations hereunder unless it shall fail to perform such obligations and
such failure shall continue for a period of 30 days following receipt of notice
from Tenant or such additional time as is reasonably required to correct any
such default after notice has been given by Tenant to Landlord specifying the
nature of Landlord's alleged default. Landlord shall not be liable in any event
for incidental or consequential damages to Tenant by reason of any default by
Landlord hereunder, whether or not Landlord is notified that such damages may
occur. Except as expressly set forth in Section 3.2 and Section 6.2 hereof,
Tenant shall have no right to terminate this Lease for any default by Landlord
hereunder and no right, for any such default, to offset or counterclaim against
any rent due hereunder.

      Notwithstanding the foregoing, if any repairs to the Premises required by
this Lease or any maintenance, cleaning, or lighting of the common areas of the
Building or the Lot, are not performed by Landlord within thirty (30) days after
notice from Tenant (or such longer period as may be reasonably required in the
event that any such repair, maintenance, cleaning or lighting cannot be
completed within said thirty (30) day period), Tenant shall have the right to
perform such obligation of Landlord. If Tenant performs any such obligation of
Landlord, Landlord shall pay to Tenant the reasonable cost thereof within thirty
(30) days after notice from Tenant, provided, however, that in no event shall
Tenant have the right to offset or deduct the amount thereof against any payment
of rent due hereunder.

      If an emergency occurs where a repair is required to be done immediately
in order to avoid imminent danger to persons or material damage to the Premises,
Tenant shall have the right to self-help consistent with the immediately
preceding grammatical paragraph of this Section 10.7 after giving Landlord only
such notice as is reasonable under the circumstances, provided, however, that
formal notice shall be promptly given thereafter. However, the right of
self-help afforded to Tenant in this Section 10.7 shall be carefully and
judiciously exercised by Tenant, it being understood and agreed that except in
the case of an emergency, Landlord shall be given sufficient opportunity to take
the action required of Landlord to avoid such default, in order to avoid any
conflict with respect to whether or not self-help should have been availed of by
Tenant, or with respect to the reasonableness of the expenses incurred by
Tenant.

10.8  BROKERAGE

      Each party warrants and represents to the other party that it has had no
dealings with any broker or agent in connection with this Lease other than
Insignia/ESG ("Broker") and covenants to defend with counsel reasonably approved
by such other party, hold harmless and indemnify such other party from and
against any and all cost, expense or liability arising from any breach of the
foregoing warranty and representation.

                                       46
<PAGE>

10.9  APPLICABLE LAW AND CONSTRUCTION.

      This Lease shall be governed by and construed in accordance with the laws
of the state in which the Premises are located. If any term, covenant, condition
or provision of this Lease or the application thereof to any person or
circumstances shall be declared invalid, or unenforceable by the final ruling of
a court of competent jurisdiction having final review, the remaining terms,
covenants, conditions and provisions of this Lease and their application to
persons or circumstances shall not be affected thereby and shall continue to be
enforced and recognized as valid agreements of the parties, and in the place of
such invalid or unenforceable provision, there shall be substituted a like, but
valid and enforceable provision which comports to the findings of the aforesaid
court and most nearly accomplishes the original intention of the parties.

      There are no prior oral or written agreements, between Landlord and Tenant
affecting this Lease. This Lease may be amended, and the provisions hereof may
be waived or modified, only by instruments in writing executed by Landlord and
Tenant.

      The titles of the several Articles and Sections contained herein are for
convenience only and shall not be considered in construing this Lease.

      Unless repugnant to the context, the words "Landlord" and "Tenant"
appearing in this Lease shall be construed to mean those named above and their
respective heirs, executors, administrators, successors and assigns, and those
claiming through or under them respectively. If there be more than one tenant
the obligations imposed by this Lease upon Tenant shall be joint and several.

10.10 SUBMISSION NOT AN OFFER.

      The submission of a draft of this Lease or a summary of some or all of its
provisions does not constitute an offer to 'lease or demise the Premises, it
being understood and agreed that neither Landlord nor Tenant shall be legally
bound with respect to the leasing of the Premises unless and until this Lease
has been executed by both Landlord and Tenant and a fully executed copy
delivered to each of them.

10.11 SECURITY DEPOSIT.

      Tenant will provide within five (5) business days of the date hereof and
as a condition of this Lease and shall keep in effect throughout the Term, a
standby letter of credit in the Security Deposit Amount (as hereinafter defined)
as security for the performance of the obligations of Tenant hereunder in
accordance with the following requirements. As used herein, the term "Security
Deposit Amount" shall mean and refer to the sum of the Initial Estimated Annual
Additional Rent plus the Annual Fixed Rent Rate, as the same may be adjusted in
accordance with Section 2.3, Section 4.1(b) and Section 10.12 hereof. If Tenant
elects to have Landlord advance the Tenant Allowance, the Security Deposit
Amount shall thereupon be increased to $6,557,654 plus an amount equal to the
Initial Estimated Annual Additional Rent. On the effective date of any such
adjustment of the Annual Fixed Rent Rate, Tenant shall provide a

                                       47
<PAGE>

replacement letter of credit for the amount of the adjusted Security Deposit
Amount. Any letter of credit provided by Tenant shall be in form and substance
acceptable to Landlord and shall be drawn on BankBoston, N.A. (or another Boston
clearinghouse bank reasonably satisfactory to Landlord) for the Security Deposit
Amount in Landlord's favor. Tenant shall provide for replacements thereto to be
issued and delivered to Landlord at least 30 days prior to the expiration of the
then effective letter of credit, time being of the essence. The letter of credit
shall be payable to Landlord upon presentation at the issuing bank's offices in
Boston of a sight draft signed by Landlord stating that Landlord is entitled
thereto, and a certification of Landlord that the person signing the
certification is duly authorized to do so, that an uncured default exists under
the Lease, that the default has continued after the expiration of any applicable
cure period, and that the amount of the draft does not exceed the amount
reasonably required to cure such default (the "Default Certification"). Landlord
agrees that it shall present such sight draft and Default Certification only if
Tenant defaults in performance of its obligations hereunder and such default
shall have continued past any applicable notice and grace period. Any letter of
credit in effect during the last year of the Term shall expire no earlier than
30 days after the expiration of the Term of the Lease. The letter of credit
shall provide for partial draws and shall be assignable by Landlord. If Tenant
shall fail to perform any of its obligations under this Lease including a
failure timely to provide a replacement letter of credit, Landlord may, but
shall not be obligated to, apply the letter of credit to the extent necessary to
cure the default, and Tenant shall be obligated to reinstate the letter of
credit to the Security Deposit Amount then in effect. Tenant shall not have the
right to call upon Landlord to draw upon all or any part of the letter of credit
to cure any default or fulfill any obligation of Tenant, but such use shall be
solely in the discretion of Landlord. Provided that Landlord gives Tenant notice
of the name of such grantee or transferee, upon any conveyance by Landlord of
its interest under this Lease, the letter of credit may be delivered by Landlord
to Landlord's grantee or transferee. Upon any such delivery, Tenant hereby
releases Landlord herein named of any and all liability with respect to the
letter of credit, its application and return, and Tenant agrees to look solely
to such grantee or transferee. It is further understood that this provision
shall also apply to subsequent grantees and transferees. Notwithstanding the
foregoing, in lieu of a letter of credit, Tenant may deliver cash in the
Security Deposit Amount to Landlord, in which event applicable provisions of
this Section 10.11 shall apply to Landlord's use and delivery thereof. If Tenant
elects to deliver cash to Landlord in the Security Deposit Amount, Tenant,
subject to Landlord's prior written approval, may designate the investment
thereof, and interest earned thereon shall be disbursed to Tenant on a yearly
basis so long as no default has occurred and is then continuing.

10.12 OPTIONS TO EXTEND.

      (a) Tenant shall have two (2) options to extend the Term of this Lease
(the "Options to Extend") for successive periods of ten (10) years each (the
"Extension Periods"), subject to and on the terms set forth herein. Tenant may
only exercise the Options to Extend with respect to the entire Premises. If
Tenant shall desire to exercise any Option to Extend, it shall give Landlord a
notice (the "Inquiry Notice") of such desire not later than fifteen (15) months
prior to the expiration of the Initial Term of this Lease or the preceding
Extension Period, as the case may be. Thereafter, the Fair Market Rent (as
defined in Subsection (c) below) for the applicable Extension Period shall be
determined in accordance with Subsection (d) below. After the

                                       48
<PAGE>

applicable Fair Market Rent has been so determined, Tenant shall exercise each
Option to Extend by giving Landlord notice (the "Exercise Notice") of its
election to do so not later than twelve (12) months prior to the expiration of
the Initial Term of this Lease, or the preceding Extension Period, as the case
may be. If Tenant fails to timely give either the Inquiry Notice or the Exercise
Notice to Landlord with respect to any Option to Extend, Tenant shall be
conclusively deemed to have waived such Option to Extend hereunder.

      (b) Notwithstanding any contrary provision of this Lease, each Option to
Extend and any exercise by Tenant thereof shall be void and of no force or
effect unless on the dates Tenant gives Landlord its Inquiry Notice and Exercise
Notice for each Option to Extend and on the date of commencement of the each
Extension Period (i) this Lease is in full force and effect, (ii) there is no
Event, of Default of Tenant under this Lease, and (iii) Tenant has not assigned
or subleased (or agreed to assign or sublease) more than fifty percent (50%) of
the rentable floor area of the Premises.

      (c) All of the terms, provisions, covenants, and conditions of this Lease
shall continue to apply during each Extension Period, except that the Annual
Fixed Rent Rate during each Extension Period (the "Extension Rent") shall be
equal to the fair market rent for the Premises determined as of the date twelve
(12) months prior to expiration of the Initial Term or the preceding Extension
Period, as the case may be, in accordance with the procedure set forth in
Subsection (d) below (the "Fair Market Rent").

      (d) The Fair Market Rent for each Extension Period shall be determined as
follows: Within five (5) days after Tenant gives Landlord its Inquiry Notice
with respect to any Option to Extend, Landlord shall give Tenant notice of
Landlord's determination of the Fair Market Rent for the applicable Extension
Period. Within ten (10) days after Tenant receives such notice, Tenant shall
notify Landlord of its agreement with or objection to Landlord's determination
of the Fair Market Rent, whereupon the Fair Market Rent shall be determined by
arbitration conducted in the manner set forth below. If Tenant does not notify
Landlord within such ten (10) day period of Tenant's agreement with or objection
to Landlord's determination of the Fair Market Rent, then the Fair Market Rent
for the applicable Extension Period shall be deemed to be Landlord's
determination of the Fair Market Rent as set forth in the notice from Landlord
described in this subsection.

      (e) If Tenant notifies Landlord of Tenant's objection to Landlord's
determination of Fair Market Rent under the preceding subsection, such notice
shall also set forth a request for arbitration and Tenant's appointment of a
commercial real estate broker having at least ten (10) years experience in the
commercial leasing market in the City of Cambridge, Massachusetts (an
"Arbitrator"). Within five (5) days thereafter, Landlord shall by notice to
Tenant appoint a second Arbitrator. Each Arbitrator shall be advised to
determine the Fair Market Rent for the applicable Extension Period within thirty
(30) days after Landlord's appointment of the second Arbitrator. On or before
the expiration of such thirty (30) day period, the two Arbitrators shall confer
to compare their respective determinations of the Fair Market Rent. If the
difference between the amounts so determined by the two Arbitrators is less than
or equal to ten percent (10%) of the lower of said amounts then the final
determination of the Fair Market Rent shall be

                                       49
<PAGE>

equal to the average of said amounts. If such difference between said amounts is
greater than ten percent (10%), then the two arbitrators shall have ten (10)
days thereafter to appoint a third Arbitrator (the "Third Arbitrator"), who
shall be instructed to determine the Fair Market Rent for the applicable
Extension Period within ten (10) days after its appointment by selecting one of
the amounts determined by the other two Arbitrators. Each party shall bear the
cost of the Arbitrator selected by such party. The cost for the Third
Arbitrator, if any, shall be shared equally by Landlord and Tenant.

      (f) Regardless of the manner in which the Extension Rent is determined,
the Annual Fixed Rent for each Extension Period shall be subject to adjustment
as set forth in Section 4.1(b) hereof.

10.13 INTENTIONALLY OMITTED.

10.14 PARKING.

      Tenant shall be obligated to pay as Additional Rent monthly, the then fair
market value (estimated to be $150.00 per space per month in calendar year 2000
for Garage parking spaces) for each of the parking spaces to be leased to
Tenant. All parking spaces leased hereby may only be utilized by Tenant's
employees, visitors, sublessees or assignees, visiting or working at the
Premises. Tenant shall have the right to lease from Landlord such parking spaces
prior to the Commencement Date on the terms set forth herein. Landlord shall
have the right, from time to time, to relocate, on & temporary basis as may be
necessary to effect repairs and improvements to the Garage, parking spaces
located in the Garage to another location within 1000 feet of the Lot, provided
that in each instance such other location may be lawfully used for accessory
parking, and provided farther that the monthly rent to be paid by Tenant for
each temporarily relocated parking space shall be an amount equal to the fair
market value thereof but in no event more than the rent then being paid by
Tenant for a parking space in the Garage. If such relocation is required in
connection with repairs to the Garage, Landlord agrees to diligently pursue
completion of such repairs.

10.15 CONFIDENTIAL INFORMATION.

      Landlord hereby agrees that any and all knowledge, information, data,
materials, trade secrets, and other work product of a confidential nature
gained, obtained, derived, produced, generated or otherwise acquired by Landlord
with respect to Tenant's business (collectively "Confidential Information")
shall be kept confidential. Landlord shall use diligent efforts to ensure that
no Confidential Information is revealed, divulged, communicated, related, or
described to any person or entity without the written consent of Landlord,
except as may be required by applicable law.

10.16 SIGNAGE.

      Tenant shall be permitted, at its sole cost and expense, to install and
maintain signs on the exterior of the Building which read "Vertex" (or something
similar reasonably approved by

                                       50
<PAGE>

Landlord), provided that: (i) the size, location, quality, color and style of
such signs shall be subject to Landlord's approval, such approval not to be
unreasonably withheld or delayed, and (ii) such signs shall be subject to
limitations of applicable law, including, without limitation, the Cambridge
Zoning By-Law, as amended from time to time. Tenant shall secure all permits
necessary for the installation of such signs at its sole cost and expense. Upon
the expiration or sooner termination of the Term of this Lease, Tenant shall
remove such signs and repair any damage resulting therefrom at Tenant's sole
cost and expense.

      WITNESS the execution hereof under seal on the day and year first above
written.

               LANDLORD:      FORT WASHINGTON REALTY TRUST

                              /s/ David E. Clem
                              -----------------------------------------
                              David E. Clem, Trustee
                              and not individually

                              /s/ David M. Roby
                              ----------------------------------------
                              David M. Roby, Trustee
                              and not individually

               TENANT:        VERTEX PHARMACEUTICALS  INCORPORATED

                              By:  /s/ Joshua Boger
                                   -------------------------------------
                              Its: CHIEF EXECUTIVE OFFICER, PRESIDENT
                              hereunto duly authorized  17 September 1999

                              By:  /s/ Richard H. Aldrich
                                   -------------------------------------
                              Its: SENIOR VICE-PRESIDENT, CHIEF BUSINESS OFFICER
                              hereunto duly authorized  17 September 1999

                                       51<PAGE>
                                                                   Exhibit 10.18

                                      LEASE

                                 BY AND BETWEEN

                 FORT WASHINGTON LIMITED PARTNERSHIP, LANDLORD,

                                       AND

                       VERTEX PHARMACEUTICALS INCORPORATED

                                     TENANT

<PAGE>

                     DATE OF LEASE EXECUTION: March 3, 1995

                                    ARTICLE I

                                 REFERENCE DATA

1.1 SUBJECTS REFERRED TO: Each reference in this Lease to any of the following
subjects shall be construed to incorporate the data stated for that subject in
this Section 1.1:

ANNUAL FIXED RENT RATE:            The sum of :

                                   (i) Annual Fixed Rent Rate for the Phase I
                                   Space, (ii) Annual Fixed Rent Rate for the
                                   Phase II Space, and (iii) Annual Fixed Rent
                                   Rate for the Phase III Space, subject to
                                   adjustment in accordance with Article IV
                                   hereof.

ANNUAL FIXED RENT RATE FOR
PHASE I SPACE:                     Build-Out Rent Rate multiplied by 35,000
                                   r.s.f., subject to adjustment in accordance
                                   with Article IV hereof.

ANNUAL FIXED RENT RATE FOR PHASE
II SPACE:                          Prior to the Substantial Completion Date for
                                   the Phase II Space, in accordance with
                                   Section 3.2 hereof: Shell Rent Rate
                                   multiplied by 10,000 r.s.f.; and

                                   From and after the Substantial Completion
                                   Date for the Phase II Space, in accordance
                                   with Section 3.2 hereof: Build-Out Rent Rate
                                   multiplied by 10,000 r.s.f., subject to
                                   adjustment in accordance with Article IV
                                   hereof.

ANNUAL FIXED RENT RATE FOR PHASE
III SPACE:                         Shell Rent Rate multiplied by 14,213 r.s.f.,
                                   subject to adjustment in accordance with
                                   Article IV hereof.

                                      -1-
<PAGE>

APPROXIMATE TERM:                  10 years

BUILDING:                          The Building of which the Premises are a part
                                   known and numbered as 40 Erie Street,
                                   Cambridge, Massachusetts, containing
                                   approximately 100,417 r.s.f.

BUILDING ADDRESS:                  40 Erie Street Cambridge, Massachusetts

BUILDING DESIGN FLOOR AREA:        100,417 r.s.f.

BUILD-OUT RENT RATE:               $36.75 p.r.s.f.

INITIAL ESTIMATED ANNUAL
ADDITIONAL RENT:                   $313,828.90

LANDLORD:                          Fort Washington Limited Partnership, a
                                   Massachusetts limited partnership

LANDLORD'S ARCHITECT:              Dean Tucker Shaw, Inc.

LANDLORD'S & MANAGING AGENT'S
ADDRESS:                           c/o Hall Properties, Inc.
                                   One International place
                                   Suite 1410
                                   Boston, Massachusetts 02110

LANDLORD'S REPRESENTATIVE:         David Clem

LEASE YEAR:                        Each consecutive period of twelve (12)
                                   calendar months commencing on the
                                   Commencement Date if it occurs on the first
                                   day of a calendar month and otherwise
                                   commencing on the first day of the month
                                   immediately following the month in which the
                                   Commencement Date occurs, and each
                                   anniversary of such date, except that the
                                   first Lease Year shall also include the
                                   period from the Commencement Date until the
                                   first day of the following month in the event
                                   that the Commencement Date does not occur on
                                   the first day of a calendar month.

                                      -2-
<PAGE>

LOT:                               The land shown on Exhibit A-1 and more
                                   particularly described on Exhibit A-2
                                   attached hereto.

MANAGING AGENT:                    Hall Properties, Inc., or its designee

OPTION TO EXPAND:                  Option to Expand the Premises to include
                                   space on a second floor, if any, constructed
                                   on the Building, in accordance with Section
                                   10.13 hereof.

OPTIONS TO EXTEND:                 Two (2) Options to Extend the Term of this
                                   Lease for successive periods of five (5)
                                   years each, in accordance with Section 10.12
                                   hereof.

PERMITTED USES:                    General office, research and development,
                                   laboratory and light manufacturing.

PHASE I SPACE:                     The portion of the Premises consisting of
                                   approximately 35,000 rentable square feet
                                   ("r.s.f.") to be located within the Premises
                                   in accordance with Complete Plans therefor
                                   pursuant to Section 3.1 hereof.

PHASE II SPACE:                    The portion of the Premises consisting of
                                   approximately 10,000 r.s.f. to be located
                                   within the Premises in accordance with
                                   Complete Plans therefor pursuant to Section
                                   3.1 hereof.

PHASE III SPACE:                   The portion of the Premises consisting of
                                   approximately 14,213 r.s.f. to be located
                                   within the Premises in accordance with
                                   Complete Plans therefor pursuant to Section
                                   3.1 hereof.

PREMISES:                          Approximately 59,213 r.s.f. of space in the
                                   Building, as shown on Exhibit A, consisting
                                   of the Phase I Space, the Phase II Space and
                                   the Phase III space (each such Phase being

                                      -3-
<PAGE>

                                   sometimes hereinafter referred to as a "Phase
                                   of the Premises").

PREMISES DESIGN FLOOR AREA:        59,213 r.s.f.

PUBLIC LIABILITY INSURANCE
LIMITS :                           Bodily injury: $10,000,000

                                   Property Damage: $10,000,000

RIGHT OF FIRST REFUSAL:            Right of First Refusal with respect to
                                   additional space in the Building, in
                                   accordance with Section 10.14 hereof.

SCHEDULED SUBSTANTIAL COMPLETION
DATES:                             Phase I Space: January 1, 1996
                                   Phase II Space: January 1, 1997
                                   Phase III Space: January 1, 2001

SCHEDULED TERM COMMENCEMENT
DATE:                              January 1, 1996

SECURITY DEPOSIT:                  A letter of credit in the form of Exhibit L
                                   attached hereto in the initial amount of
                                   $2,200,000.00, subject to adjustment in
                                   accordance with Section 10.11 hereof).

SHELL RENT RATE:                   $18.00 p.r.s.f.

TENANT:                            Vertex Pharmaceuticals Incorporated.

TENANT'S ADDRESS (For Notice and   40 Allston Street
Billing):                          Cambridge, Massachusetts 02139-4211

TENANT ALLOWANCE:                  Phase I Space: $85.00 p.r.s.f.
                                   Phase II Space: $85.00 p.r.s.f.
                                   Phase III Space: $0.00 p.r.s.f.

TENANT'S DESIGN
COMPLETION DATE:                   Phase I Space: April 1, 1995
                                   Phase II Space: May 1, 1996
                                   Phase III Space: June 1, 2000

TENANT'S PROPORTIONATE
FRACTION:                          59.0%, subject to adjustment as provided in
                                   Section 2.3 hereof.

TENANT'S REPRESENTATIVE:           Alfred Vaz, Jr.

                                      -4-
<PAGE>

TERM EXPIRATION DATE:              December 31, 2005, subject to two (2) Options
                                   to Extend for successive periods of five (5)
                                   years each, in accordance with Section 10.12.

1.2  EXHIBITS.

      The Exhibits listed below in this section are incorporated in this Lease
by reference and are to be construed as a part of this Lease:

     EXHIBIT A.    Plan showing the Premises

     EXHIBIT A-1.  Plan showing the Lot

     EXHIBIT A-2.  Legal Description of Lot

     EXHIBIT B.    Intentionally Omitted

     EXHIBIT C.    Rules and Regulations

     EXHIBIT D.    Intentionally Omitted

     EXHIBIT E.    Plan Submission and Construction
                   Schedule

     EXHIBIT F.    Space Measurement Standards

     EXHIBIT G.    Outline Specifications for Base
                   Building
                   Improvements

     EXHIBIT H.    Schedule of Floor Load Limits

     EXHIBIT I.    Plan Showing Second Floor Space

     EXHIBIT J.    Landlord's Services

     EXHIBIT K.    Landlord's Title Insurance Commitment

     EXHIBIT L.    Letter of Credit

                          -5-
<PAGE>

1.3 TABLE OF CONTENTS

<TABLE>
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ARTICLE I - REFERENCE DATA......................................................       1

      Section  1.1     Subjects Referred To.....................................       1
      Section  1.2     Exhibits.................................................       5
      Section  1.3     Table of Contents........................................       6

ARTICLE II - PREMISES AND TERM..................................................       9

      Section 2.1      Premises.................................................       9
      Section 2.2      Term.....................................................      10
      Section 2.3      Modification of Certain Definitions;
                         Certificate Regarding Commencement
                         Date...................................................      10

ARTICLE III - IMPROVEMENTS......................................................      11

      Section  3.1     Initial Construction.....................................      11
      Section  3.2     Preparation of Premises for Occupancy                          15
      Section  3.3     General Provisions Applicable to
                         Construction...........................................      17
      Section  3.4     Representatives..........................................      19
      Section  3.5     Base Building Improvements...............................      19

ARTICLE IV - RENT...............................................................      20

      Section  4.1     The Fixed Rent...........................................      20
      Section  4.2     Additional Rent..........................................      22
               4.2.1   Real Estate Taxes........................................      22
               4.2.2   Insurance................................................      24
               4.2.2.1 Insurance Taken Out by Tenant............................      24
               4.2.2.2 Insurance Taken Out by Landlord .........................      25
               4.2.2.3 Tenant Reimbursement of Insurance
                         Taken Out by Landlord..................................      25
               4.2.2.4 Certain Requirements Applicable to
                         Insurance Policies.....................................      26
               4.2.2.5 Waiver of Subrogation....................................      26
               4.2.3   Utilities................................................      27
               4.2.4   Common Area Maintenance and Expenses                           28
               4.2.5   Payments on Account of Taxes,
                         Insurance and Utilities................................      31
               4.2.6   Equitable Adjustment of Additional Rent                        32
      Section  4.3     Late Payment of Rent.....................................      33
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ARTICLE V - TENANT'S ADDITIONAL COVENANTS.......................................       33

      Section 5.1      Affirmative Covenants....................................       33
              5.1.1    Perform Obligations......................................       33
              5.1.2    Occupancy and Use........................................       33
              5.1.3    Repair and Maintenance...................................       34
              5.1.4    Compliance with Law......................................       35
              5.1.5    Tenant's Work............................................       38
              5.1.6    Indemnity................................................       39
              5.1.7    Landlord's Right to Enter................................       39
              5.1.8    Personal Property at Tenant's Risk.......................       40
              5.1.9    Payment of Landlord's Cost of Enforcement................       40
              5.1.10   Yield Up.................................................       40
              5.1.11   Estoppel Certificate.....................................       41
              5.1.12   Landlord's Expenses Re Consents..........................       41
              5.1.13   Rules and Regulations....................................       41
              5.1.14   Loading..................................................       42
              5.1.15   Holdover.................................................       42

      Section 5.2      Negative Covenants.......................................       42
              5.2.1    Assignment and Subletting................................       42
              5.2.2    Nuisance.................................................       44
              5.2.3    Installation, Alterations or Additions...................       44

ARTICLE VI - CASUALTY OR TAKING.................................................       45

      Section 6.1      Termination..............................................       45
      Section 6.2      Restoration..............................................       45
      Section 6.3      Award....................................................       46

ARTICLE VII - DEFAULTS..........................................................       46

      Section 7.1      Events of Default........................................       46
      Section 7.2      Remedies.................................................       47
      Section 7.3      Remedies Cumulative......................................       49
      Section 7.4      Landlord's Right to Cure Defaults........................       49
      Section 7.5      Effect of Waivers of Default.............................       49
      Section 7.6      No Accord and Satisfaction...............................       50

ARTICLE VIII - MORTGAGES........................................................       50

      Section 8.1      Rights of Mortgage Holders...............................       50
      Section 8.2      Superiority of Lease; Option to Subordinate..............       51
      Section 8.3      Lease Amendments.........................................       52
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ARTICLE IX - LANDLORD'S ADDITIONAL COVENANTS....................................       52

      Section 9.1      Affirmative Covenants....................................       52

ARTICLE X - MISCELLANEOUS PROVISIONS............................................       54

      Section 10.1     Notices from One Party to the Other......................       54
      Section 10.2     Quiet Enjoyment..........................................       55
      Section 10.3     Easements; Changes to Lot Lines..........................       55
      Section 10.4     Leases Not To Be Recorded................................       55
      Section 10.5     Bind and Inure; Limitation of Landlord's Liability.......       55
      Section 10.6     Acts of God..............................................       56
      Section 10.7     Landlord's Default.......................................       56
      Section 10.8     Brokerage................................................       57
      Section 10.9     Applicable Law and Construction..........................       57
      Section 10.10    Submission Not an Offer..................................       58
      Section 10.11    Security Deposit.........................................       58
      Section 10.12    Options to Extend........................................       59
      Section 10.13    Right of First Refusal...................................       61
      Section 10.14    Option to Expand.........................................       62
      Section 10.15    Confidential Information.................................       63
      Section 10.16    Signage..................................................       63
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                                   ARTICLE II

                               PREMISES AND TERM

2.1 PREMISES.

      Landlord hereby leases and demises to Tenant and Tenant hereby leases from
Landlord, subject to and with the benefit of the terms, covenants, conditions
and provisions of this Lease, the Premises. Tenant shall have, as appurtenant to
the Premises, the right to use in common with others entitled thereto (i) the
common facilities included in the Building or on the Lot, (ii) the building
service fixtures and equipment serving the Premises, and (iii) the right to use
one hundred (100) parking spaces ("Tenant's Parking Spaces") on the Lot.

      Landlord reserves the right from time to time, without unreasonable
interference with Tenant's use, (a) to install, repair, replace, use, maintain
and relocate for service to the Premises and to other parts of the Building or
either, building service fixtures and equipment wherever located in the
Building, provided, however, that the Annual Fixed Rent, Additional Rent (as
defined in Section 4.2 hereof) and other charges payable hereunder by Tenant
shall be proportionally reduced in the event that any such installation or
relocation of service materially reduces the usable floor area of the Premises
(other than a temporary reduction to accommodate installation, repair,
replacement, maintenance and relocation of such service); and (b) to alter or
relocate any common facilities and/or Tenant's Parking Spaces on or off the Lot,
provided that in all events (1) substitutions are in compliance with applicable
zoning laws, (2) substitutions are substantially equivalent, (3) the total
number of parking spaces available to Tenant on or off the Lot is not decreased
and (4) any of Tenant's Parking Spaces relocated off the Lot shall be located
within three hundred (300) feet of the footprint of the Premises.

      Landlord and Tenant hereby acknowledge and agree that at all times the
Premises shall include the Phase I Space, the Phase II Space and the Phase III
Space, notwithstanding that the Leasehold Improvements (as defined in Section
3.1) for the Phase II Space and the Phase III Space shall be completed after the
Commencement Date in accordance with Article III hereof.

      The Premises shall be subject to expansion in accordance with the terms of
Section 10.13 and Section 10.14 hereof.

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2.2 TERM.

      To have and to hold for a period (the "Term") commencing on the earlier of
(a) the date which is the later of the Scheduled Term Commencement Date or the
Substantial Completion Date for the Phase I Space and (b) the date on which
Tenant occupies all or any part of the Premises (whichever of said dates is
appropriate being hereafter referred to as the "Commencement Date"), and
continuing until the Term Expiration Date, unless sooner terminated as provided
in Section 3.2 or in Article VII, and subject to extension in accordance with
the terms of Section 10.12 hereof. Tenant shall have the right to access the
Premises prior to the Commencement Date for purposes of installing equipment and
furnishings in accordance with and subject to the provisions of Section 3.2.

2.3 MODIFICATION OF CERTAIN DEFINITIONS; CERTIFICATE REGARDING COMMENCEMENT
DATE.

      Landlord and Tenant acknowledge that the actual Premises Design Floor
Area, the actual Building Design Floor Area and the actual floor area of each
respective Phases of the Premises may, upon completion of design of the Building
and each respective Phase of the Premises, be different than the Premises Design
Floor Area, the Building Design Floor Area and the floor area of each respective
Phase of the Premises as set forth in Article I hereof. Accordingly, after
completion of the design of the Building and after completion of the design of
each Phase of the Premises, Landlord will notify Tenant of the actual Premises
Design Floor Area, the actual Building Design Floor Area and the actual floor
area of each respective Phase of the Premises, each of which shall be measured
in accordance with the standards set forth on Exhibit F attached hereto, and
Landlord and Tenant will execute an amendment to this Lease modifying the
definitions of the Premises Design Floor Area, the Building Design Floor Area,
the Phase I Space, the Phase II Space, the Phase III Space, the Annual Fixed
Rent Rate, the Initial Estimated Annual Additional Rent and the Tenant's
Proportionate Fraction set forth in Article I hereof. Landlord and Tenant will
also execute, upon request of either, certificates acknowledging the
Commencement Date of this Lease as provided, for in Section 2.2 hereof, once
such commencement date has occurred (as described in Section 3.2 hereof) and the
Substantial Completion Date of each Phase of the Premises.

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                                   ARTICLE III

                                  IMPROVEMENTS

3.1 INITIAL CONSTRUCTION.

      (a) Plans. Landlord and Tenant shall proceed to develop and approve
construction drawings and specifications for each respective Phase of the
Premises (the "Complete Plans'") in accordance with the procedure set forth in
Exhibit E attached hereto. The Complete Plans for each Phase of the Premises
must be approved by Landlord not later than the applicable Tenant Design
Completion Date and, in order to meet that deadline, Tenant agrees to submit to
Landlord for its approval design development plans for the Phase I Space and the
Phase II Space on or before March 15, 1995.

      Landlord and Tenant shall initial the Complete Plans after the same have
been approved by Landlord and Tenant in accordance with said Exhibit E. Tenant's
interior furnishings, i.e., specification, coordination, supply and installation
of furniture, furnishings, telephones and movable equipment, will be the
responsibility of Tenant. Tenant's installation of furnishings shall be
coordinated with any work being performed by Landlord in such manner as to
maintain harmonious labor relations during the initial build-out of the Phase I
Space and the Phase II Space and not to damage the Building or Lot or interfere
with Building or Lot operations. Except for installation of furnishings and the
installation of telephone outlets (which must be performed by a telephone
company at the Tenant's direction and expense) all work described in the
Complete Plans for the Phase I Space and Phase II Space (the "Leasehold
Improvements") shall be performed by Siena Construction or another contractor
designated by Landlord and reasonably approved by Tenant ("Landlord's
Contractor").

      Landlord and Tenant agree that Landlord shall have no obligation to
construct leasehold improvements to the Phase III Space. Tenant agrees to
commence all required improvements to the Phase III Space in a substantial and
continuing manner on or before August 1, 2001 and to complete the same on or
before December 31, 2001, at Tenant's sole cost and expense in accordance with
Complete Plans approved by Landlord with a contractor selected by Tenant but
reasonably approved by Landlord. All improvements made by Tenant to the Phase
III Space shall become part of the Premises upon installation thereof by Tenant
and shall not be the subject of any leasing or installment sale contract whereby
title to or a lien on any of said improvements vests in any party other than
Landlord.

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      (b) Leasehold Improvement Costs. Landlord hereby agrees to furnish Tenant
with a copy of the contract for the Leasehold Improvements for the Phase I Space
and the Phase II Space, as the case may be, by and between Landlord and
Landlord's Contractor within ten (10) days after execution thereof. On or before
July 1, 1995 in the case of the Phase I Space and on or before July 1, 1996 in
the case of the Phase II Space, Landlord shall give Tenant a notice (the
"Proposed GMP Notice") setting forth for the applicable Phase, Landlord's
Contractor's guaranteed maximum price ("GMP") and estimated cost of the work,
inclusive of Landlord's Contractor's Fee (the "Estimated Cost of the Work").
Tenant shall thereafter have the right to approve or reject such proposed GMP by
notice given to Landlord. In the event the Tenant rejects the GMP the Tenant
shall, at its sole cost and expense, make such changes to the Complete Plans as
shall be required to produce a revised GMP acceptable to Tenant. Landlord shall
neither commence, nor be required to commence, any Leasehold Improvements for
the Phase I Space or the Phase II Space, as the case may be, until the parties
have agreed upon the GMP for the applicable Phase. In the event that Landlord
and Tenant have not agreed upon the GMP for the applicable Phase on or before
the date which is sixty (60) days after the Proposed GMP Notice for the Phase I
Space or the Phase II Space, as the case may be, and Tenant does not agree in
writing to compensate Landlord for the delay in constructing the Leasehold
Improvements as set forth in this paragraph, Landlord shall have the right to
terminate this Lease upon notice to Tenant. Without limitation of any other
provision of this Lease, in the event that (i) Landlord does not elect to so
terminate this Lease, (ii) the parties subsequently agree in writing to the GMP
for the Phase I Space or the Phase II Space, as the case may be, and (iii) the
Substantial Completion Date for the applicable Phase does not occur on or before
the applicable Scheduled Substantial Completion Date as a result of any delay in
agreeing upon the applicable GMP, then the Annual Fixed Rent and Additional Rent
and other charges due hereunder shall nevertheless begin to accrue (in the case
of delay with respect to the Phase I Space) or continue to accrue (in the case
of delay with respect to the Phase II Space) on such applicable Scheduled
Substantial Completion Data; provided, however, that the Annual Fixed Rent Rate
for the Phase I Space or the Phase II Space, as the case may be, shall be
calculated at the Shell Rent Rate until such time as the Substantial Completion
Date for such Phase shall occur.

      Landlord shall submit all requests for change orders for the Leasehold
Improvements for the Phase I Space and the Phase II Space to Tenant for its
review and approval. No change order for the Leasehold Improvements for the
Phase I Space or the Phase II Space shall be effective unless approved by Tenant
in writing, such approval not to be unreasonably withheld or delayed.
Notwithstanding the foregoing, Tenant's review and approval shall

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not be required for change orders for the Base Building Improvements (as
defined in Section 3.5 hereof).

      Tenant shall pay to Landlord as Additional Rent a sum equal to all costs
incurred by Landlord on account of the Leasehold Improvements (excluding,
however, the Tenant Allowance, but including in the costs so incurred the cost
to Landlord of Landlord's Contractor's overhead and profit equal to 10% of costs
of work not covered by the Tenant Allowance), hereinafter called "Tenant
Improvement Reimbursement to Landlord" or "TIR". Tenant shall pay to Landlord
Landlord's Estimated TIR (as hereinafter defined) applicable to each respective
Phase of the Premises as construction of the Leasehold Improvements for such
Phase progresses, less retainage of 10%, within ten (10) days after submission
by Landlord to Tenant of a statement on or about the first day of each month
showing construction, engineering and design costs incurred. As used herein the
term "Estimated TIR" shall mean and refer to the difference between the
Estimated Cost of the Work for each applicable Phase and the Tenant Allowance.
The portion of Estimated TIR invoiced on each monthly statement shall be equal
to the product of (a) a fraction, the numerator of which is Landlord's Estimated
TIR and the denominator of which is the Estimated Cost of the Work, multiplied
by (b) the total construction, engineering and design costs shown on each such
statement. Each monthly statement shall be accompanied by a certificate of
Landlord's Contractor that all payments then due to laborers, materialmen and
subcontractors, less required retainage, have been made, less the aggregate
amount of prior monthly progress payments made by Tenant. On the earlier of the
occupancy by Tenant or the Substantial Completion Date (as defined in Section
3.2 hereof) of each respective Phase of the Premises, Tenant shall pay to
Landlord a sum sufficient to increase the total payments to Landlord on account
of TIR to ninety-five percent (95%) of TIR for such Phase (it being understood
that actual TIR for each Phase may be more or less than the Estimated TIR for
such Phase). The remaining five percent (5%) retainage withheld by Tenant for
each Phase shall be paid by Tenant within ten (10) days after delivery to Tenant
of (i) a permanent Certificate of Occupancy from the City of Cambridge for such
Phase, and (ii) a certificate of Landlord's architect certifying that all Punch
List Items (as defined in Section 3.2 hereof) for such Phase have been
reasonably satisfied after consultation between Landlord's architect and Tenant.
Landlord shall withhold from Landlord's Contractor retainage in the amount of
five percent (5%) of the cost of the work until such time as Landlord's
architect shall have delivered said certificate to Tenant (provided, however,
that Landlord may release any retainage held with respect to any supplier or
subcontractor which is finally and fully completed the portion of the work for
which it was responsible prior to the delivery of said certificate.)

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      In addition to paying TIR as above provided, Tenant shall pay an amount
equal to all costs incurred by Landlord as a result of any change orders signed
by Tenant and Landlord affecting the Complete Plans, including the cost to
Landlord of Landlord's Contractor's overhead and profit equal to 10% of those
costs exclusive of overhead and profit, but only to the extent that such change
order costs for each respective Phase of the Premises, together with all other
costs incurred on account of the Leasehold Improvements for such Phase, exceed
the applicable Tenant Allowance. Amounts due and payable on account of such
change orders for each respective Phase of the Premises shall be included in the
monthly statements relating to TIR provided for above, and Tenant shall pay
therefor in accordance with each such statement within ten (10) days, and in all
events by the Substantial Completion Date of each such Phase of the Premises.
Notwithstanding the foregoing, Tenant shall not be liable for any costs incurred
by Landlord as a result of change order relating solely to Base Building
Improvements (as hereinafter defined).

      Landlord shall deliver to Tenant a copy of each monthly requisition from
Landlord's Contractor for the Phase I Space and the Phase II Space promptly
after Landlord's receipt of the same

      Landlord shall select the subcontractors for each portion of the Phase I
Space and the Phase II Space from bids obtained from at least three (3)
subcontractors for each trade, unless otherwise agreed to by Tenant. Tenant
shall have the right to participate with Landlord and Landlord's Contractor in
the development of a list of qualified subcontractors from whom bids may be
solicited and to review such bids with Landlord and Landlord's contractor.
Landlord hereby agrees that the subcontractors selected by Tenant and identified
in a notice given by Tenant to Landlord within thirty (30) days after the date
of this Lease shall be included on the list of qualified subcontractors. Without
limitation of the foregoing, Landlord shall not be required to select any
subcontractor requested by Tenant for any portion of the Phase I or Phase II
Space work, regardless of whether such subcontractor is the low bidder for such
work or whether Tenant agrees to pay the incremental cost of selecting such
subcontractor in the event such subcontractor's bid exceeds that of the
subcontractor selected by Landlord or Landlord's Contractor, provided that
Landlord or Landlord's Contractor reasonably believes that the subcontractor
selected by Tenant may be unable to perform its work in a manner which will be
timely and satisfactory to Landlord and Landlord's Contractor and which will
preserve harmonious labor relations during the initial build out of the Phase I
Space and the Phase II Space. The Scheduled Substantial Completion Dates for the
Phase I and Phase II Space shall be extended for any delay caused by Tenant's
participation in, and review of, the subcontract bids. Tenant shall not be
deemed to have caused delay

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as a result of its review of any subcontract bid unless Tenant fails to complete
such review within three (3) days after Landlord's submission of such bid to
Tenant.

3.2 PREPARATION OF PREMISES FOR OCCUPANCY.

      Landlord agrees to use reasonable efforts to have the Phase I Space and
the Phase II Space ready for occupancy on or before the respective Scheduled
Substantial Completion Date for each such. Phase, which shall, however, be
extended in the case of each such Phase of the Premises for a period equal to
that of any delays due to Acts of God, or by changes ordered in the work, or by
labor disputes, fire, unusual delays in deliveries, unavoidable casualties or
other causes beyond Landlord's reasonable control (collectively, "Force Majeure
Events") and by delays due to any act or neglect of Tenant, or of any employee,
agent, or separate contractor of Tenant, including, without limitation, any
delay in determining the GMP in accordance with Section 3.1(b)) hereof (which
delay shall be measured from the date of the applicable Proposed GMP Notice)
(collectively, "Tenant Delay"). Each such respective Phase of the Premises shall
be deemed ready for occupancy on the Substantial Completion Date (as hereinafter
defined) for such Phase of the Premises. As used herein, the term "Substantial
Completion Date" for any Phase of the Premises shall mean and refer to the date
on which: (i) the Leasehold Improvements for such Phase, as specified in the
Complete Plans for such Phase are ready for occupancy as certified by Landlord's
architect, with the exception of minor items which can be fully completed by
Landlord within thirty (30) days without material interference with Tenant and
other items which because of the season or weather or the nature of the item are
not practicable to do at the time, provided that none of said items is necessary
to make the applicable Phase of the Premises tenantable for the Permitted Uses
(collectively "Punch List Items"), (ii) a Certificate of Occupancy from the City
of Cambridge (or a Temporary Certificate of Occupancy permitting Tenant to
occupy the applicable Phase with conditions which can be satisfied without
material interference with Tenant's use and occupancy of such Phase) shall have
been obtained, (iii) the applicable Phase is broom clean and free of debris, and
(iv) all utilities required for the use of the Premises have been brought by
Landlord to the Utility Switching Points (as defined in Section 4.2.3 hereof);
provided, however, that if Landlord is unable to complete construction of the
Phase I Space or the Phase II Space in accordance with the foregoing on or
before the Scheduled Substantial Completion Date for such Phase due to delay in
Tenant's compliance with the provisions of Section 3.1 or the schedule set forth
in Exhibit E of this Lease, or delays caused by Tenant's participation in, and
review of, subcontract bidding (as set forth above in Section 3.1(b)), then such
Phase of the

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Premises shall be deemed ready for occupancy no later than such Scheduled
Substantial Completion Date.

      Landlord shall permit Tenant and Tenant's contractors access for
installing equipment and furnishings in the Premises prior to the Commencement
Date if it can be done without material interference with completion of the
Building or remaining portions of the Leasehold Improvements.

      Notwithstanding the foregoing, Tenant shall have the right to terminate
this Lease upon notice given to Landlord (i) on or after the date which is 150
days prior to the Scheduled Substantial Completion Date for the Phase I Space,
as extended due to the occurrence of one or more Force Majeure Events or the
occurrence of any Tenant Delay, and continuing until such time as Landlord shall
have commenced the work for the Phase I Space in a substantial and continuing
manner, in the event that Landlord has not so commenced such work for the Phase
I Space on or before the date which is 180 days prior to the Scheduled
Substantial Completion Date for such Phase, as so extended, (ii) on or before
the date which is 210 days after the Scheduled Substantial Completion Date for
the Phase I Space in the event that the Scheduled Substantial Completion Date
for the Phase I Space, as extended due to the occurrence of any Tenant Delay,
does not occur on or before the date which is 180 days after the Scheduled
Substantial Completion Date for the Phase I Space, as so extended for Tenant
Delay, and (iii) on or after the expiration of any period of 30 consecutive days
during which the work for the Phase I Space or the Phase II Space has been
abandoned by Landlord (i.e, such work has ceased to proceed in a substantial and
continuing manner), which right to terminate shall continue until such time as
Landlord shall have re-commenced such work.

      Without limitation of the foregoing, in the event that the Substantial
Completion Date for the Phase I Space or the Phase II Space does not occur on or
before the applicable Final Outside Date (as hereinafter defined) for such
Phase, Tenant shall either (i) terminate the Lease in accordance with the
previous paragraph hereof, or (ii) be entitled to a credit against the monthly
installment(s) of Annual Fixed Rent next due from Tenant in an amount equal to
the Applicable Penalty (as hereinafter defined). As used herein the term, "Final
Outside Date" as applied to the Phase I Space or the Phase II Space, as the case
may be, shall mean and refer to the date which is thirty (30) days after the
Scheduled Substantial Completion Date for such Phase, as extended for any delays
caused by or resulting from Force Majeure Events or from Tenant Delay
(including, without limitation, delays caused by change orders or any failure by
Tenant to comply with the provisions of this Article III). As used in connection
with any delay in completion of the Phase I Space, the term "Applicable

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Penalty" shall mean and refer to the product of (a) the Annual Fixed Rent Rate
multiplied by (b) the Premises Design Floor Area, multiplied by (c) the number
of days of delay from the Final Outside Date for the Phase I Space to the
Scheduled Substantial Completion Data for the Phase I Space, multiplied by (d)
2, divided by (e) 365. As used in connection with any delay in completion of the
Phase II Space, the term "Applicable Penalty" shall mean and refer to the
product of (a) the Annual Fixed Rent Rate for the Phase II Space, multiplied by
(b) the floor area of the Phase II Space multiplied by (c) the number of days of
delay from the Final Outside Date for the Phase II Space to the applicable
Substantial Completion Date for the Phase II Space, multiplied by (d) 2, divided
by (e) 365.

3.3 GENERAL PROVISIONS APPLICABLE TO CONSTRUCTION.

      All construction work required or permitted by this Lease, whether by
Landlord or by Tenant, shall be done in a good and workmanlike manner and in
compliance with all applicable laws and all lawful ordinances, regulations and
orders of any governmental authority or insurer of the Building. Either party
may inspect the work of the other at reasonable times and shall give notice of
observed defects. Landlord shall not be responsible for any loss, damage, or
injury resulting from the installation of any components, fixtures, or equipment
provided they were appropriately specified and installed in accordance with the
manufacturer's or supplier's instructions; provided, however, that Landlord
shall assign any and all contractor's, manufacturer's and supplier's warranties
with respect to all components, fixtures, or equipment, including, without
limitation, Landlord's Contractor's warranty, to Tenant for the Term of this
Lease, upon the expiration or sooner termination of which such warranties shall
automatically revert to Landlord. Landlord's obligations under Section 3.1 and
3.2 with respect to each Phase of the Premises shall be deemed to have been
performed on the Substantial Completion Data for such Phase except for items
which are incomplete or do not conform with the requirements of Sections 3.1 and
3.2 and as to which Tenant shall in either case have given written notice to
Landlord prior to such date. If Tenant does not provide such written notice
prior to the Substantial Completion Date for each applicable Phase of the
Premises, a certificate of completion by a licensed architect or registered
engineer shall be conclusive evidence that Landlord has performed all such
obligations except for items stated in such certificate to be incomplete or not
in conformity with such requirements.

      Tenant will not make any alterations or additions to the Premises (other
than the initial improvements to each Phase and the Second Floor Space made in
accordance with Complete Plans

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approved by Landlord and Tenant, and any other improvements consistent
therewith, provided the same are approved by Landlord, such approval not to be
unreasonably withheld or delayed) without Landlord's approval, which approval
shall not be unreasonably withheld or delayed provided that Landlord and Tenant
shall agree in writing whether Tenant will be required to, permitted to or
forbidden to, at Tenant's sole cost and expense, remove any such alteration or
addition upon the expiration or termination of this Lease. Landlord's approval
of any alteration or addition which is not a Minor Alteration (as defined below
in this Section 3.3) shall be deemed to have been given if Landlord fails to
notify Tenant of its objection thereto within 10 business days after Tenant's
request for such approval. In circumstances in which Tenant desires the right to
remove additions or alterations at the expiration or termination of this Lease,
Landlord shall reasonably agree, and such agreement shall not be unreasonably
withheld or delayed (and shall be deemed to have been given if Landlord fails to
notify Tenant of its objection thereto within 10 business days after Tenant's
request for such agreement), to permit such removal (i) where items installed by
Tenant are in the nature of equipment, but are so affixed to Building that such
items may be construed as fixtures or (ii) where additions or alterations of
Tenant include specific items that after removal from the Building will have in
the aggregate for each such alteration or addition a fair market value of
$25,000.00 or greater. Tenant's rights to remove additions or alterations
hereunder shall not apply to replacement of items included in initial build-out
that are replaced due to the fact that such items have worn out or become
substantially obsolete. In the event the Tenant is required to or permitted to
remove any such alteration or addition, as a condition to Landlord's approval of
such alteration or addition, Tenant shall agree in writing to readapt, repair
and restore the Premises to the condition the same were in prior to such
alteration or addition. Landlord will disapprove any alterations or additions
requested by Tenant which will delay completion of the Premises or the Building,
unless Tenant shall agree (a) to pay all costs associated with such alterations,
additions and delay, (b) to extend the applicable Scheduled Substantial
Completion for the period of such delay, and (c) that the Annual Fixed Rent and
Additional Rent shall nonetheless begin to accrue from the date such Phase would
have been completed but for such delay. All changes and additions shall be part
of the Building except such items as by writing at the time of approval the
parties agree either shall be removed by Tenant on termination of this Lease, or
shall be removed or left at Tenant's election.

      Notwithstanding the foregoing, the parties hereby agree that for any
non-structural alterations or additions to the Premises which do not involve
modifications to the Building operating systems and for which the cost may be
reasonably estimated to be

                                      -18-
<PAGE>

less than $25,000 (each a "Minor Alteration"): (i) Landlord's prior written
consent shall not be required unless such Minor Alteration requires a building
permit from the City of Cambridge, in which case Landlord's reasonable consent
shall be required, provided that such consent shall be deemed to have been given
if Landlord fails to notify Tenant of its objection to such Minor Alteration
within 2 days after Tenant's request for Landlord's consent with respect
thereto, and (ii) upon the expiration or termination of this Lease, Tenant shall
readapt, repair and restore the Premises to the condition the same were in prior
to such Minor Alteration, regardless of whether Landlord's consent was required
or obtained with respect thereto.

      The parties further agree that (a) any request for consent to any
alteration or addition (including, without limitation, any Minor Alteration)
shall be accompanied by drawings and specifications in reasonable detail given
the size and scope of the proposed alteration or addition, and (b) Tenant shall
furnish Landlord as-built drawings showing any and all alterations or
additions (including, without limitation, any and all Minor Alterations) made by
Tenant or any assignee, sublessee or licensee of Tenant within 30 days after
completion of the same.

3.4 REPRESENTATIVES.

      Each party authorizes the other to rely in connection with their
respective rights and obligations under this Article III upon approval and other
actions on the party's behalf by Landlord's Representative in the case of
Landlord and Tenant's Representative in the case of Tenant or by any person
hereafter designated in substitution or addition by notice to the party relying.

3.5 BASE BUILDING IMPROVEMENTS.

      Landlord hereby agrees to make certain base building improvements to the
Building in accordance with the outline specifications set forth on Exhibit G
attached hereto (the "Base Building Improvements"). Landlord agrees to invest at
least $45.00 p.r.s.f. for the Base Building Improvements, of which at least
$33.00 p.r.s.f. shall be expended for hard construction costs. Landlord shall
complete those Base Building Improvements required to complete the Leasehold
Improvements on or before the Scheduled Substantial Completion Date for the
Phase I Space. In order to provide Landlord with sufficient time to complete
those Base Building Improvements required for Tenant's construction of leasehold
improvements in the Phase III Space, Tenant agrees to provide Landlord with six
(6) months' notice before Tenant undertakes construction of such improvements to
the Phase III Space.

                                      -19-
<PAGE>

                                   ARTICLE IV

                                      RENT

4.1 FIXED RENT.

      (a) Monthly Installments; Definitions. Tenant covenants and agrees to pay
rent to Landlord, without any offset or reduction whatsoever (except as may be
made in accordance with the express provisions of this Lease), at the Original
Address of Landlord or at such other place or to such other person or entity as
Landlord may by notice to Tenant from time to time direct, at the Annual Fixed
Rent Rate set forth in Article I, in equal installments equal to l/12th of the
Annual Fixed Rent Rate in advance on the first day of each calendar month
included in the Term; and for any portion of a calendar month at the beginning
or end of the Term, at that rate payable in advance for such portion.

      Landlord and Tenant hereby agree that the Annual Fixed Rent Rate shall
consist of the sum of (i) the Annual Fixed Rent Rate for the Phase I Space, (ii)
the Annual Fixed Rent Rate for the Phase II Space, and (iii) the Annual Fixed
Rent Rate for the Phase III Space, as the same may be adjusted in accordance
with the provisions of this Article IV. Landlord and Tenant further agree that
(A) prior to the Substantial Completion Date for the Phase II Space, the Annual
Fixed Rent Rate for the Phase II Space shall be equal to the actual area of the
Phase II Space multiplied by the Shell Rent Rate, and (B) on and after the
Substantial Completion Date for the Phase II Space, the Annual Fixed Rent Rate
for the Phase II Space shall be equal to the actual area of the Phase II Space
multiplied by the Build-Out Rent Rate. In the event that the Substantial
Completion Date for the Phase II space occurs on any day other than the first
day of a Lease Year, the Annual Fixed Rent for the Phase II Space shall be
prorated for the remainder of such Lease Year at the adjusted rate for the
Phase II Space.

      (b) adjustment for CPI. On December 31, 2000 (the "Adjustment Date"), the
Shell Rent Rate and the Build-Out Rent Rate shall be increased by multiplying
each of said rates by the lesser of (i) a fraction, the numerator of which shall
be the Price Index (as hereinafter defined) most recently established prior to
the Adjustment Date, and the denominator of which shall be the Base Price Index
(as hereinafter defined), or (ii) one hundred four percent (104%) per year,
compounded annually over the first five (5) years of the Term of this Lease. As
used herein, the term "Price Index" shall mean and refer to the "Consumer Price
Index for Urban Wage Earners and Clerical Workers, for the Boston, Massachusetts
area, All Items (1982-84=100)" published by the Bureau of Labor Statistics of
the United States Department of

                                      -20-
<PAGE>

Labor or successor or substitute index appropriately adjusted, and the term
"Base Price Index" shall mean and refer to the Price Index most recently
established prior to the Commencement Date. In the event the Price Index (or a
successor or substitute index) shall not be published for the City of Boston, or
for the months indicated above, the corresponding index for the United States
City Average (and if this is not available, a reliable governmental or other
nonpartisan publication evaluating similar or equivalent information as used in
the Price Index) shall be used. In the event the Price Index ceases to use the
1982-84 average of 100 as the basis of calculation, or if a substantial change
is made in the terms or numbers of items contained in the Price Index, than the
Price Index shall be adjusted to the figure that would have been arrived at had
the manner of computing the Price Index in effect at the date of this Lease not
been changed.

      (c) Adjustment for Unused Tenant Allowance. In the event the cost of
constructing the Leasehold Improvements for the Phase I Space or the Phase II
Space is less than the Tenant Allowance for such Phase, the Annual Fixed Rent
Rate for such Phase set forth in Schedule 1.1 shall be adjusted in accordance
with the following formula:

Adjusted Annual  =  Shell Rent  +  Actual TI  x  Build Out
Fixed Rent Rate     Rate           ---------     Rent Rate -
for Phase I or                       Tenant      Shell Rent
Phase II Space                     Allowance     Rate
(as the case may be)

Where "Actual TI" equals the actual portion of the Tenant Allowance for the
Phase I Space or the Phase II Space, as the case may be, expended by Landlord in
connection with the construction of the Leasehold Improvements for such Phase.

      By way of example, if the actual cost of completing the Leasehold
Improvements for the Phase I Space is $76.50, the Annual Fixed Rent for the
Phase I Space shall be adjusted as follows:

Adjusted Annual Fixed Rent = $18.00 + $76.50 ($36.75 - $18.00)
                                      ------
for Phases I Space                    $85.00

                                    = $18.00 + 16.88

                                    = $34.88

      (d) Adjustment for Tenant Allowance Reimbursement. Provided that at the
time Tenant exercises its rights under this subsection there is then outstanding
no Event of Default, Tenant shall have

                                      -21-
<PAGE>

the option (the "Reimbursement Option") to reduce the Build-Out Rent Rate to the
Shell Rent Rate effective from and after the Reimbursement Date (as hereinafter
defined). Tenant shall exercise the Reimbursement Option by paying Landlord the
entire Tenant Allowance incurred by Landlord in completing the Leasehold
Improvements for the Phase I Space and the Phase II Space on any date (the
"Reimbursement Date") occurring on or after May 1, 1999, provided that Tenant
shall give Landlord not less than thirty (30) days' notice prior to making such
payment.

      Upon exercise of the Reimbursement Option and payment to Landlord of the
entire Tenant Allowance incurred by Landlord in completing the Leasehold
Improvements for the Phase I Space and the Phase II Space in accordance with the
foregoing, the Base Rent Rate shall equal the Shell Rent Rate for purposes of
calculating Fixed Rent due on or after the Reimbursement Date. Notwithstanding
any other provision of this Lease, Fixed Rent due for any period prior to the
Reimbursement Date shall not be reduced as a result of the exercise by Tenant of
the Reimbursement Option. In the event that the Reimbursement Date occurs on any
day other than the first day of a Lease Year, the Fixed Rent for the remainder
of such Lease Year shall be prorated to reflect the reduced Build-Out Rent Rate
only for the remainder of such Lease Year.

4.2 ADDITIONAL RENT.

      In order that the Fixed Rent shall be absolutely net to Landlord, Tenant
covenants and agrees to pay, as Additional Rent, without any offset or reduction
whatsoever, taxes, municipal or state betterment assessments, insurance costs,
utility charges and Annual Maintenance Charges with respect to the Premises as
provided in this Section 4.2 as follows:

      As used herein, the term "Estimated Annual Additional Rent" shall mean and
refer to Landlord's estimate of the total amount of Additional Rent which may be
due from Tenant for any particular Lease Year. Landlord shall furnish Tenant
with a statement as soon as reasonably practicable after the commencement of
each Lease Year setting forth the amount of Landlord's Estimated Annual
Additional Rent for such Lease Year. Landlord's good faith estimate of the
Estimated Annual Additional Rent for the first fiscal year of the Term is set
forth in Section 1.1 as the "Initial Estimated Annual Additional Rent".

      4.2.1 REAL ESTATE TAXES. Tenant shall pay directly to the Landlord; (i)
all taxes, assessments (special or otherwise), levies, fees, water and sewer
rents and charges, and all other government levies and charges, general and
special, ordinary and extraordinary, foreseen and unforeseen, (and Tenant's
Proportionate Fraction of any such taxes, assessments, levies, fees and charges
if they are assessed against the entire Building or Lot) which are, at any time
prior to or during the Term hereof, imposed

                                      -22-
<PAGE>

or levied upon or assessed against (A) the Premises or the Building or the Lot,
(B) any Fixed Rent, Additional Rent or other sum payable hereunder or (C) this
Lease, or the leasehold estate hereby created, or which arise in respect of the
operation, possession or use of the Premises or the Building or the Lot; (ii)
all gross receipts or similar taxes imposed or levied upon, assessed against or
measured by any Fixed Rent, Additional Rent or other sum payable hereunder;
(iii) all sales, value added, use and similar taxes at any time levied, assessed
or payable on account of the acquisition, leasing or use of the Premises (and
Tenant's Proportionate Fraction of any such taxes if they are levied, assessed
or payable on account of the acquisition, leasing or use of the entire Building
or Lot); and (iv) all charges for utilities furnished to the Premises (and
Tenant's Proportionate Fraction of all charges for utilities furnished to the
entire Building or Lot) which may become a lien on the Building or the Lot or
the Premises (collectively "taxes and assessments" or if singular "tax or
assessment"). For each tax or assessment period, or installment period thereof,
wholly included in the Term, all such payments shall be made by Tenant not less
than five (5) days prior to the last date on which the same may be paid without
interest or penalty. For any fraction of a tax or assessment period, or
installment period thereof, included in the Term at the beginning or end
thereof, Tenant shall pay to Landlord, within 20 days after receipt of invoice
therefor, the fraction of taxes and assessments so levied or assessed or
becoming payable which is allocable to such included period. At Landlord's
option, Tenant shall pay taxes and assessments in accordance with Section 4.2.5
hereof.

      In the event that Tenant, together with all of the other tenants in the
Building, requests that Landlord apply for any abatement of, or otherwise
contest, any tax or assessment, Landlord shall file such abatement or otherwise
contest such tax or assessment and shall diligently pursue the same to
completion, provided that (i) Landlord receives notice of such request from
Tenant and all other tenants in the Building at least 30 days prior to the last
day on which such abatement or contest may validly be made under applicable law,
and (ii) the expenses of such proceedings, including, without limitation, any
penalties, interest, late fees or charges, and attorneys' fees incurred as a
result thereof, shall be included in the Annual Maintenance Charge of the then
current fiscal year.

      Nothing contained in this Lease shall, however, require Tenant to pay any
income taxes, excess profits taxes, excise taxes, franchise taxes ("Excluded
Taxes"), estate, succession, inheritance or transfer taxes, 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

                                      -23-
<PAGE>

assessed on Landlord a capital levy or other tax on the gross rents received
with respect to the Building or the Lot, or all of them, 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) measured by or
based, in whole or in part, upon gross rents, then any and all of such taxes,
assessments, levies or charges, to the extent so measured or based ("Substitute
Taxes"), shall be payable by Tenant; provided, however, that (i) Tenant's
obligation with respect to the aforesaid Substitute Taxes shall be limited to
the amount thereof as computed at the rates that would be payable if the
Premises were the only property of Landlord, and (ii) only that portion of the
Substitute Taxes in excess of the Excluded Taxes shall be payable by Tenant.
Landlord shall furnish to Tenant a copy of any notice of any public, special or
betterment assessment received by Landlord concerning the Premises.

      4.2.2 INSURANCE.

            4.2.2.1 INSURANCE TAKEN OUT BY TENANT.

                        Tenant shall take out and maintain throughout the Term
the following insurance:

                        (a) Comprehensive liability insurance indemnifying
Landlord and Tenant against all claims and demands for (i) injury to or death of
any person or damage to or loss of property, on the Premises or adjoining walks,
streets or ways, or connected with the use, condition or occupancy of any
thereof unless caused by the negligence of Landlord or its servants or agents,
(ii) violation of this Lease, or (iii) any act, fault or omission, or other
misconduct of Tenant or its agents, contractors, licensees, sublessees or
invitees, in amounts which shall, at the beginning of the Term, be at least
equal to the limits set forth in Section 1.1, and, from time to time during the
Term, shall be for such higher limits, if any, as are customarily carried in the
area in which the Premises are located on property similar to the Premises and
used for similar purposes; and shall be written on the "Occurrence Basis" and
include Host Liquor liability insurance; and

                        (b) Worker's compensation insurance with statutory
limits covering all of the Tenant's employees working on the Premises.

                                      -24-
<PAGE>

            4.2.2.2 INSURANCE TAKEN OUT BY LANDLORD.

                        Landlord shall take out and maintain throughout the Term
the following insurance:

                        (a) Comprehensive liability insurance for the Building
and Lot of the same nature and type as described in Section 4.2.2.1(a) of this
Lease, and with the same policy limits; and

                        (b) All risk, fire and casualty insurance on a 100%
replacement cost basis, together with rental loss coverage and, if the Building
is located in a flood zone, flood coverage to the extent the same is available,
insuring the Building and its rental value; and

                        (c) Insurance against loss or damage from sprinklers and
from leakage or explosions or cracking of boilers, pipes carrying steam or
water, or both, pressure vessels or similar apparatus, in the so-called "broad
form", in such amounts as are customary and commercially reasonable for
buildings in the Cambridge, Massachusetts area which are of like kind and
quality to the Building and have laboratory uses, and insurance against such
other hazards and in such amounts as may from time to time be required by any
bank, insurance company or other lending institution holding a first mortgage on
the Building and Lot.

      Landlord shall have no obligation to insure Tenant's personal property or
chattels, including without limitation, Tenant's trade fixtures.

            4.2.2.3 TENANT REIMBURSEMENT OF INSURANCE TAKEN OUT BY LANDLORD.

                        Tenant shall from time to time reimburse Landlord within
thirty days of Landlord's invoice for Tenant's Proportionate Fraction of
Landlord's costs incurred in providing the insurance provided pursuant to
Section 4.2.2.2 of this Lease, equitably prorated in the case of blanket
policies to reflect the insurance coverage reasonably attributable to the
Premises, and provided further that Tenant shall reimburse Landlord for all of
Landlord's costs incurred in providing such insurance which is attributable to
any special endorsement or increase in premium resulting from the business or
operations of Tenant, and any special or extraordinary risks or hazards
resulting therefrom, including without limitation, any risks or hazards
associated with the generation, storage and disposal of medical waste. At
Landlord's option, Tenant shall reimburse Landlord for insurance costs in
accordance with Section 4.2.5 hereof.

                                      -25-
<PAGE>

            4.2.2.4 CERTAIN REQUIREMENTS APPLICABLE TO INSURANCE POLICIES.

                        Policies for insurance provided for under the provisions
of Sections 4.2.2.2(b) and 4.2.2.2(c) shall, in case of loss, be first payable
to the holders of any mortgages on the Building and Lot under a standard
mortgagee's clause, and shall be deposited with the holder of any mortgage or
with Landlord, as Landlord may elect. All policies for insurance required to be
obtained by either party under the provisions of Section 4.2.2 shall be obtained
from responsible companies qualified to do business in the state in which the
Premises are located and in good standing therein, which companies and the
amount of insurance allocated thereto shall be subject to Landlord's approval.
Each party agrees to furnish the other with certificates of all such insurance
which such party is obligated to obtain pursuant to Section 4.2.2 prior to the
beginning of the Term hereof and with renewal certificates at least 30 days
prior to the expiration of the policy they renew. In addition, Tenant agrees to
furnish Landlord with any policies of insurance which Tenant is obligated to
obtain hereunder, including any renewal policies, upon request of any of
Landlord's mortgagees (provided that Tenant may redact from such policies any
Confidential Information, as defined in Section, 10.15 hereof). Each such policy
required to be maintained by Tenant shall name Landlord and Landlord's Managing
Agent as additional insureds and shall be noncancellable with respect to the
interest of Landlord, Landlord's Managing Agent and such mortgagees without at
least 30 days' prior written notice thereto.

            4.2.2.5 WAIVER OF SUBROGATION.

                        All insurance which is carried by either party with
respect to the Premises or to furniture, furnishings, fixtures or equipment
therein or alterations or improvements thereto, whether or not required, shall
include provisions which either designate the other party as one of the insured
or deny to the insurer acquisition by subrogation of rights of recovery against
the other party to the extent such rights have been waived by the insured party
prior to occurrence of loss or injury, insofar as, and to the extent that such
provisions may be effective without making it impossible to obtain insurance
coverage from responsible companies qualified to do business in the state in
which the Premises are located (even though extra premium may result therefrom)
and without voiding the insurance coverage in force between the insurer and the
insured party. In the event that extra premium is payable by either party as a
result of this provision, the other party shall reimburse the party paying such
premium the amount of such extra premium. If at the request of one party, this
non-subrogation provision is waived, then the obligation of reimbursement shall
cease for such

                                      -26-
<PAGE>

period of time as such waiver shall be effective, but nothing contained in this
Section 4.2.2.5 shall derogate from or otherwise affect releases elsewhere
herein contained of either party for claims. Each party shall be entitled to
have duplicates or certificates of any policies containing such provisions. Each
party hereby waives all rights of recovery against the other for loss or injury
against which the waiving party is protected by insurance containing said
provisions, reserving, however, any rights with respect to any excess of loss or
injury over the amount recovered by such insurance. Tenant shall not acquire as
insured under any insurance carried on the Premises under the provisions of this
Section 4.2.2 any right to participate in the adjustment of less or to receive
insurance proceeds and agrees upon request promptly to endorse and deliver to
Landlord any checks or other instruments in payment of loss in which Tenant is
named as payee.

      4.2.3 UTILITIES.

            Tenant shall pay directly to the proper authorities charged with the
collection thereof all charges for water, sewer, gas, electricity, telephone and
other utilities or services used or consumed on the Premises, whether called
charge, tax, assessment, fee or otherwise, including, without limitation, water
and sewer use charges and taxes, if any, all such charges to be paid as the same
from time to time become due. Upon Tenant's request, Landlord shall install at
Tenant's expense, which expense may be applied against the applicable Tenant
Allowance, separate meters to measure Tenant's consumption of any utility
servicing the Premises. If Tenant is not charged directly by the respective
utility for any of such utilities or services, Tenant shall from time to time,
within 20 days of receipt of Landlord's invoice therefor, pay to Landlord
Tenant's Proportionate Fraction of the total of such charges for the Building
and Lot, provided that if said utilities or services are provided to less than
all of the rentable space in the Building, Tenant shall pay its share of said
charges based upon the actual consumption shown on any separate meter for such
utility or service, and, in the event there is no such separate meter, based
upon the area of the Premises relative to the area of the entire space to which
utilities are provided, or such greater amount required, in Landlord's
reasonable judgment, by Tenant's disproportionate use of utilities, and provided
further that, at Landlord's option, all such charges shall be payable by Tenant
in accordance with Section 4.2.5. It is understood and agreed that (i) Landlord
shall be responsible for bringing such utilities to a common switching point(s)
at the Building, which, in the case of electricity shall mean the switch gear
and not the transformer (collectively, the "Utility Switching Points") at
Landlord's cost and expense; (ii) Tenant shall pay for any and all costs to
connect such utilities from such Utility

                                      -27-
<PAGE>

Switching Points to the Premises; (iii) Landlord shall be under no obligation to
furnish any utilities to the Premises (beyond the foregoing responsibility to
bring such utilities to the Utility Switching Points); and (iv) subject to
Section 3.2 hereof, Landlord shall not be liable for any interruption or failure
in the supply of any such utilities to the Premises; provided, however, that in
the event such loss or failure is due to Landlord's negligence or willful
misconduct, Landlord shall be responsible for restoring the supply of such
utilities to the Premises but otherwise shall have no liability to Tenant.
Without limitation of the foregoing, in the event of a Casualty or Taking, if
Landlord's architect reasonably determines that utilities will not be repaired
or restored so as to be available at the Utility Switching Points within one
year after the occurrence of such Casualty or Taking, then Tenant shall have the
right to terminate this Lease by notice given within 30 days after the date of
such determination.

      4.2.4 COMMON AREA MAINTENANCE AND EXPENSES.

            Landlord shall use reasonable efforts to maintain the common areas
of the Lot and the interior and exterior of the Building in a clean and orderly
condition in accordance with Exhibit J, except to the extent repairs or
maintenance are required due to Tenant's negligence or willful misconduct (in
which case Tenant shall promptly effect such repairs or maintenance or, at
Landlord's option, Landlord may effect such repairs or maintenance and charge
the entire cost thereof to Tenant as Additional Rent). Notwithstanding the
foregoing, it is expressly understood and agreed that Landlord shall have no
liability or responsibility for the storage, containment or disposal of any
hazardous or medical waste generated, stored or contained by Tenant, Tenant
hereby agreeing to store, contain and dispose of any and all such hazardous or
medical waste at Tenant's sole cost and expense in accordance with the
provisions of Article V hereof. Tenant shall pay to Landlord as Additional Rent
the Annual Maintenance Charge computed and payable as follows:

            (1) The Annual Maintenance Charge shall be equal to the sum of the
            Annual Lot Maintenance Charge and the Annual Building Maintenance
            and Operation Charge as hereinafter defined.

                  (a) The Annual Lot Maintenance Charge shall be equal to the
            product obtained by multiplying (x) the costs incurred by Landlord
            during the fiscal year (as hereinafter defined) for which the Annual
            Maintenance Charge is being computed (the "Current Fiscal Year") in
            providing Lot maintenance, including without limitation landscaping,
            street lighting, security (if required, in

                                      -28-
<PAGE>

            Landlord's judgment), maintenance and snow plowing, maintenance of
            Lot signage, maintenance of utilities, management fees and
            amortization of equipment to the extent used for Lot maintenance, by
            (y) a fraction whose numerator is the Premises Design Floor Area, as
            built, and whose denominator is the gross floor area of the
            buildings located on the Lot at the end of the Current Fiscal Year.

                  (b) The Annual Building Maintenance and Operation Charge shall
            be equal to Tenant's Proportionate Fraction of the reasonable costs
            incurred by Landlord during the Current Fiscal Year in providing
            Building maintenance, including without limitation repair,
            maintenance and cleaning of common facilities in the Building, and
            maintenance and repairing of all heating, plumbing, electrical, air
            conditioning and mechanical fixtures and equipment serving the
            Building or the Premises), elevators, trash dumpster rental, trash
            removal, remediation, performance of such other tasks as Tenant
            shall request and Landlord shall agree to perform, management fees
            (exclusive of leasing and sale commissions, fees paid in connection
            with tenant improvement costs, and such other fees or commissions
            paid in connection with the leasing, re-leasing, extension or
            renewal of leases for the Building or the Lot) and amortization of
            equipment to the extent used for Building maintenance.

      Notwithstanding the foregoing, in the event that any capital repair,
improvement or replacement to the common areas and facilities of the Building
and the Lot has a useful life of over one year (as determined in accordance with
generally accepted accounting practices consistently applied), then only the
amortized cost of such repair, improvement or replacement over said useful life
shall be included in the Annual Lot Maintenance Charge or the Annual Building
Maintenance Charge, as applicable.

      Tenant shall make payments on account of the Annual Maintenance Charge
monthly in advance on the first day of each calendar month during the Term. At
the beginning of every fiscal year, Landlord shall deliver to Tenant its
reasonable estimate of the Annual Maintenance Charge (the "Estimated Annual
Maintenance Charge") for the said fiscal year which estimate may include a
reasonable contingency of up to 5%, and Tenant shall make

                                      -29-
<PAGE>

payments on account of the Annual Maintenance Charge monthly in advance on the
first day of each calendar month during the Term in the amount of one-twelfth of
the Estimated Annual Maintenance Charge. Landlord reserves the right to
reasonably re-estimate and modify the Estimated Annual Maintenance Charge by
notice to Tenant once annually on or about July 1 of each Lease Year (the
"Additional Rent Adjustment Date"), and Tenant's payments shall thereupon be
adjusted accordingly. Not later than sixty (60) days after the end of each
fiscal year during the Term and after Lease termination, Landlord shall render a
statement ("Landlord's Statement") in reasonable detail and according to usual
accounting practices certified by Landlord and showing for the preceding fiscal
year or fraction thereof, as the case may be, the actual Annual Maintenance
Charges for the said fiscal year or fraction thereof, and thereupon any balance
owed by Tenant or excess paid by Tenant under this Section shall be paid to
Landlord, or credited to Tenant, as the case may be, on the next rent payment
date. Landlord shall furnish Tenant with copies of all reasonable documentation
and records for the Annual Maintenance Charges for any fiscal year upon Tenant's
request for the same; provided, however, that Landlord shall not be required to
furnish such copies for any fiscal year if Tenant has not requested such copies
within two (2) years after the expiration of such fiscal year.

      For purposes of this Lease, the first "fiscal year" shall be the annual
period commencing on the Commencement Date and ending on December 31 of the year
in which the Commencement Date occurs; subsequently, the term "fiscal year"
shall mean each consecutive annual period thereafter, commencing on the day
following the end of the preceding fiscal year. Landlord shall have the right
from time to time to change the periods of accounting under this Section 4.2.4
to any annual period other than a fiscal year, and upon any such change all
items referred to in this Section shall be appropriately apportioned. In all
Landlord's Statements rendered under this Section, amounts for periods partially
within and partially without the accounting periods shall be appropriately
apportioned, and any items which are not determinable at the time of a
Landlord's Statement shall be included therein on the basis of Landlord's
estimate, and with respect thereto Landlord shall render promptly after
determination a supplemental Landlord's Statement, and appropriate adjustment
shall be made according thereto. All of Landlord's Statements shall be prepared
on an accrual basis of accounting.

      Notwithstanding any other provision of this Section 4.2.4, if the Term
expires or is terminated as of a date other than the last day of a fiscal year,
then for such fraction of a fiscal year at the end of the Term, Tenant's last
payment to Landlord under this

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Section 4.2.4 shall be made on the basis of Landlord's best estimate of the
items otherwise includable in Landlord's Statement and shall be made on or
before the later of (a) 10 days after Landlord delivers such estimate to Tenant
or (b) the last day of the Term. Landlord shall thereafter prepare a Landlord's
Statement showing the actual Annual Maintenance Charge for such fiscal year, as
hereinabove provided, and an appropriate payment or refund shall thereafter
promptly be made upon submission of such Landlord's Statement to Tenant.

      Notwithstanding the foregoing, Landlord hereby agrees that HVAC service
and all other utilities shall be available to Tenant 24 hours per day, seven (7)
days per week, subject to the provisions hereof with respect to loss or
interruption of utilities and other services.

      4.2.5 PAYMENTS ON ACCOUNT OF TAXES, INSURANCE AND UTILITIES.

      Tenant shall make payments on account of the Annual Tax, Insurance and
Utility Charge (as hereinafter defined) monthly in advance on the first day of
each calendar month during the Term, which payments shall initially be in the
amount of the sum of the Initial Tax Charge, the Initial Insurance Charge and
the Initial Utility Charge (the "Estimated Initial Tax, Insurance and Utility
Charges"), At the beginning of every fiscal year, Landlord shall deliver to
Tenant its reasonable estimate of the Annual Tax, Insurance and Utility Charge
("the Estimated Annual Tax, Insurance and Utility Charge") for said fiscal year,
and, in lieu of payments of one twelfth of the Estimated Initial Tax, Insurance
and Utility Charge, Tenant shall make payments on account of the Annual Tax,
Insurance and Utility Charge monthly in advance on the first day of each
calendar month during the Term in the amount of one-twelfth of the Estimated
Annual Tax, Insurance and Utility Charge. Landlord reserves the right to
reasonably re-estimate and modify the Estimated Annual Tax, Insurance and
Utility Charge by notice to Tenant once annually on the Additional Rent
Adjustment Date (as defined in Section 4.2.4 hereof), and Tenant's payments
shall thereupon be adjusted accordingly.

      Not later than sixty (60) days after the end of each fiscal year during
the Term and after Lease termination, Landlord shall render a statement in
reasonable detail and according to usual accounting practices certified by
Landlord and showing for the preceding fiscal year or fraction thereof, as the
case may be, the actual Annual Tax, Insurance and Utility Charge for the said
fiscal year or fraction thereof, and thereupon any balance owed by Tenant or
excess paid by Tenant under this Section shall be paid to Landlord, or credited
to Tenant, as the case may be, on the next rent payment date. As used herein,
the term "Annual Tax,

                                      -31-
<PAGE>

Insurance and Utility Charge" shall mean and refer to the amount of funds paid
by Tenant pursuant to Section 4.2.1, 4.2.2 and 4.2.3 for the fiscal year in
question for costs actually incurred by Landlord (without any mark-up for
Landlord's overhead or profit). All payments under this Section shall to the
extent thereof relieve Tenant of its obligations under said Sections 4.2.1,
4.2.2 and 4.2.3 hereof.

      Landlord shall have the right from time to time to change the periods of
accounting under this Section 4.2.5 to any annual period other than a fiscal
year, and upon any such change all items referred to in this Section shall be
appropriately apportioned. In all Landlord's annual statements rendered under
this Section, amounts for periods partially within and partially without the
accounting periods shall be appropriately apportioned, and any items which are
not determinable at the time of such a statement shall be included therein on
the basis of Landlord's estimate, and with respect thereto Landlord shall render
promptly after determination a supplemental statement, and an appropriate
adjustment shall be made according thereto. All of landlord's statements under
this Section shall be prepared on an accrual basis of accounting.

      Notwithstanding any other provision of this Section 4.2.5, if the Term
expires or is terminated as of a date other than the last day of a fiscal year,
then for such fraction of a fiscal year at the end of the Term, Tenant's last
payment to Landlord under this Section 4.2.5 shall be made on the basis of
Landlord's best estimate of the items otherwise includable in the annual
statement rendered by Landlord under this Section and shall be made on or before
the later of (a) 10 days after Landlord delivers such estimate to Tenant or (b)
the last day of the Term, with an appropriate payment or refund to be made upon
submission of Landlord's statement.

      4.2.6 EQUITABLE ADJUSTMENT OF ADDITIONAL RENT.

      Notwithstanding any other provision of this Lease to the contrary, if the
amount charged to Tenant as Additional Rent under Section 4.2 for any Operating
Expense (as hereinafter defined) is based upon the proportion which the Premises
Design Floor Area bears to the Building Design Floor Area or the Design Floor
Area of all of the buildings on the Lot, and such amount is more or less than
the amount reasonably allocable to Tenant based upon the value or use of the
Premises, then such amount may be equitably adjusted by Landlord to reflect such
disproportionality. As used herein, the term "Operating Expense" shall mean and
refer to taxes and assessments, utilities, insurance, the Annual Lot Maintenance
Charge and the Annual Building Maintenance and Operation Charge.

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<PAGE>

4.3   LATE PAYMENT OF RENT.

      If any installment of rent is paid after the date the same was due, it
shall bear interest from the due date at the prime commercial rate of The First
National Bank of Boston, as it may be adjusted from time to time, plus 4% per
annum, but in no event more than the highest rate of interest allowed by
applicable law. Any amounts due under this Section 4.3 shall be Additional Rent.
The foregoing provisions of this Section 4.3 shall not apply to the first two
installments of rent paid after the date the same were due during each twelve
consecutive month period during the Term.

                                    ARTICLE V

                          TENANT'S ADDITIONAL COVENANTS

5.1   AFFIRMATIVE COVENANTS.

      Tenant covenants at its expense at all times during the Term and for such
further time as Tenant occupies the Premises or any part thereof;

      5.1.1 PERFORM OBLIGATIONS.

            To perform promptly all of the obligations of Tenant set forth in
this Lease; and to pay when due the Fixed Rent and Additional Rent and all
charges, rates and other sums which by the terms of this Lease are to be paid
by Tenant.

      5.1.2 OCCUPANCY AND USE.

            Continuously from the Commencement Date, to use and occupy the
Premises only for the Permitted Uses, and from time to time to procure all
licenses and permits necessary therefor at Tenant's sole expense.

            Without limitation, Tenant shall strictly comply with all federal,
state, and municipal laws, ordinances, and regulations governing the use of
Tenant's laboratory, scientific experimentation and the generation, storage,
containment and disposal of medical waste. Tenant shall be solely responsible
for procuring and complying at all times with any and all necessary permits
directly relating or incident to: the conduct of its office and research
activities on the demised premises; its scientific experimentation,
transportation, storage, handling, use and disposal of any chemical or
radioactive or bacteriological or pathological substances or organisms or other
hazardous wastes or environmentally dangerous substances or materials or medical

                                      -33-
<PAGE>

waste. Within ten (10) days of a request by Landlord, which request shall be
made not more than once during each period of twelve (12) consecutive months
during the Term hereof, unless otherwise requested by any mortgagee of Landlord,
Tenant shall furnish Landlord with copies of all such permits which Tenant
possesses or has obtained together with a certificate certifying that such
permits are all of the permits which Tenant possesses or has obtained with
respect to the Premises. Tenant shall be entitled to redact any Confidential
Information from the copies of such permits and accompanying certificates of
Tenant. Tenant shall promptly give notice to Landlord of any warnings or
violations relative to the above received from any federal, state, or municipal
agency or by any court of law and shall promptly cure the conditions causing any
such violations. Tenant shall not be deemed to be in default of its obligations
under the preceding sentence to promptly cure any condition causing any such
violation in the event that, in lieu of such cure, Tenant shall contest the
validity of such violation by appellate or other proceedings permitted under
applicable law, provided that: (i) any such contest is made reasonably and in
good faith, (ii) Tenant makes provisions, including, without limitation, posting
bond(s) or giving other security, acceptable to Landlord to protect Landlord,
the Building and the Lot from any liability, costs, damages or expenses arising
in connection with such violation and failure to cure, (iii) Tenant shall agree
to indemnify, defend (with counsel reasonably acceptable to Landlord) and hold
Landlord harmless from and against any and all liability, costs, damages, or
expenses arising in connection with such condition and/or violation, (iv) Tenant
shall promptly cure any violation in the event that its appeal of such violation
is overruled or rejected, and (v) Tenant shall certify to Landlord's
satisfaction that Tenant's decision to delay such cure shall not result in any
actual or threatened bodily injury or property damage to Landlord, any tenant or
occupant of the Building or the Lot, or any other person or entity. Landlord
agrees that any Confidential Information gained or obtained by Landlord pursuant
to this Section 5.1.2 shall be kept confidential in accordance with Section
10.15 hereof.

      5.1.3 REPAIR AND MAINTENANCE.

            Except as otherwise provided in Article VI, to keep the Premises
including, without limitation, all fixtures and equipment now or hereafter on
the Premises, or exclusively serving the Premises, but excluding the exterior
(exclusive of glass and doors) and structural elements of the Building and the
grounds and parking lot, which Landlord shall maintain and repair unless such
repairs are required because of Tenant's willful misconduct or negligence, in
good order, condition and repair and at least as good order, condition and
repair as they are in on the Commencement Date or may be put in during the Term,
reasonable use and

                                      -34-
<PAGE>

wear only excepted; to keep in a safe, secure and sanitary condition all trash
and rubbish temporarily stored at the Premises; and to make all repairs and
replacements and to do all other work necessary for the foregoing purposes
whether the same may be ordinary or extraordinary, foreseen or unforeseen.
Tenant shall secure, pay for and keep in force contracts with appropriate and
reputable service companies providing for the regular maintenance of the heating
and air-conditioning systems and copies of such contracts shall be furnished to
Landlord; provided, however, that it is agreed that Tenant shall be responsible
only for the repair and maintenance of the heating and air-conditioning systems
and the components thereof located within or exclusively serving the Premises.
It is further agreed that the exception of reasonable use and wear shall not
apply so as to permit Tenant to keep the Premises in anything less than
suitable, tenantlike, and efficient and usable condition considering the nature
of the Premises and the use reasonably made thereof, or in less than good and
tenantlike repair.

      5.1.4 COMPLIANCE WITH LAW.

            To make all repairs, alterations, additions or replacements to the
Premises required by any law or ordinance or any order or regulation of any
public authority other than major capital repairs, alterations, additions or
replacements to the foundations and structural elements of the Building which
are not required because of Tenant's failure to comply with the Provisions of
Article 5.1.3 hereof; to keep the Premises equipped with all safety appliances
so required; to pay all municipal, county, or state taxes assessed against the
leasehold interest hereunder, or against personal property of any kind on or
about the Premises; and to comply with the orders and regulations of all
governmental authorities with respect to zoning, building, fire, health and
other codes, regulations, ordinances or laws applicable to the Premises.

            The Tenant shall not use, generate, manufacture, produce, handle,
store, release, discharge or dispose of in, on, under or about the Premises or
transport to or from the Premises, or allow its employees, agents, contractors,
invitees or any other person or entity to do so, any oil, hazardous or toxic
materials or hazardous or toxic wastes or medical waste (collectively,
"hazardous materials") except to the extent that the following conditions
regarding the use, generation, manufacture, production, handling, storing,
releasing, discharging, disposal or transport (individually or collectively, the
"Use") of hazardous materials shall be satisfied: (i) the Use shall be directly
related to the operation of Tenant's business as permitted herein, (ii) Tenant
shall first provide Landlord with the list of the types and quantities of such
proposed hazardous materials which Tenant is

                                      -35-
<PAGE>

required to furnish to the applicable governmental authorities for purposes of
compliance with the Resource Conservation and Recovery Act, as amended (42
U.S.C. Section 9601, et seq.) (the "RCRA List") (or, in the event that the RCRA
List ceases to be required to be filed under such law, a list containing the
same information required to be listed on the RCRA List as of the date hereof),
and shall update such list as necessary for continuing accuracy, and such other
information reasonably satisfactory to Landlord as Landlord may reasonably
require concerning such Use, and (iii) such Use shall be in strict compliance
(at Tenant's expense) with all applicable laws, regulations, licenses and
permits. Landlord hereby covenants and agrees that the information contained in
any list, or update thereof, referred to in the foregoing clause (ii) shall be
kept confidential in accordance with Section 10.15 hereof. Notwithstanding the
foregoing, Tenant hereby agrees to consult and coordinate with Landlord prior to
transporting any hazardous materials to or from the Premises whenever (i) such
transportation is not of the kind regularly made during the ordinary course of
business by a person or entity operating a laboratory facility for the Permitted
Uses or (ii) Tenant has reason to believe that such transportation may result in
a public demonstration, protest or other similar disturbance at the Building or
the Lot. If the transportation, generation, manufacture, production, handling,
release, storage, use or disposal of any hazardous materials anywhere on the
Premises in connection with the Tenant's use of the Premises results in (1)
contamination of the soil, surface or ground water or (2) loss or damage to
person(s) or property, then Tenant agrees to respond in accordance with the
following paragraph:

            Tenant agrees (i) to notify Landlord immediately of any
            contamination, claim of contamination, loss or damage, (ii) after
            consultation and approval by Landlord, to clean up the contamination
            in full compliance with all applicable statutes, regulations and
            standards, and (iii) to indemnify, defend (with counsel acceptable
            to Landlord) and hold Landlord harmless from and against any claims,
            suits, causes of action, costs and fees, including attorneys' fees,
            arising from or connected with any such contamination, claim of
            contamination, loss or damage. No consent or approval of Landlord
            shall in any way be construed as imposing upon Landlord any
            liability for the means, methods, or manner of removal, containment
            or other compliance with applicable law for and with respect to the
            foregoing.

            Tenant shall promptly notify Landlord upon Tenant's receipt of any
inquiry, notice, or threat to give notice by any

                                      -36-
<PAGE>

government authority or any other third party with respect to any hazardous
materials. Notwithstanding the foregoing, Tenant shall not be liable to Landlord
hereunder for any contamination, claim of contamination, loss or damage arising
in connection with hazardous materials to the extent the same is the result of
(A) hazardous materials existing in the Building and the Lot prior to Tenant's
use or occupancy of the Premises, (B) migration of hazardous materials from any
site onto the Lot not caused by Tenant, (C) the generation, manufacture,
production, handling, release, storage, use or disposal of any hazardous
materials at the Building or the Lot by Landlord, any other tenant or occupant,
or any so-called "mid-night dumpers" or (D) the Use (as defined above in this
Section) by any party other than Tenant of hazardous materials at the Building
or the Lot after the date upon which Tenant has completely vacated the same,
including removal of all of its property (to the extent permitted herein) and
hazardous materials. Tenant's indemnification obligations under this Section
shall survive the expiration or earlier termination of this lease.

            Prior to vacating the Premises at the expiration of the term hereof,
Tenant at its sole cost and expense shall provide Landlord with an environmental
audit by a qualified environmental engineering firm satisfactory to Landlord,
which audit shall include reasonable subsurface testing if requested by
Landlord. The aforesaid environmental audit shall affirmatively certify that the
Premises are free from any and all contaminants, pollutants, radioactive
materials, hazardous wastes or materials, medical waste, bacteriological agents
or organisms which would render the Premises in violation of M.G.L. c. 21E, the
Comprehensive Environmental Response, Compensation, and Liability Act of 1980,
as amended, 42 U.S.C. Section 9602 et seq., or any other applicable laws, rules,
regulations or orders, as they may be amended or supplemented by administrative
regulations, from time to time.

            Nothing herein contained shall be construed to limit or impair
Tenant's obligation to comply with any law, code, rule or regulation which
requires Tenant to notify any governmental authority or any other person
concerning the Use (as defined above in this Section) of hazardous materials by
Tenant at the Premises.

            Tenant agrees that, with respect to the Premises, it shall be
responsible for compliance with the Americans with Disabilities Act (42 U.S.C.
Section 12101 et seq.) and the regulations and Accessibility Guidelines for
Buildings and Facilities issued pursuant thereto (collectively, the "ADA
Requirements").

            Tenant covenants and agrees that its use of the Premises shall not
cause a discharge of more than its pro rata share on a

                                      -37-
<PAGE>

square foot basis of the design flow gallonage per day of sanitary
(non-industrial) sewage allowed under the sewage discharge permit(s) for the
Building. Discharges in excess of that amount, and any discharge of industrial
sewage, shall only be permitted if Tenant, at its sole expense, shall have
obtained all necessary permits and licenses therefor, including without
limitation permits from state and local authorities having jurisdiction thereof.

      5.1.5 TENANT'S WORK.

            To procure at Tenant's sole expense all necessary permits and
licenses before undertaking any work on the Premises; to do all such work in
compliance with the applicable provisions of Sections 3.3 and 5.2.3 hereof; to
do all such work in a good and workmanlike manner employing materials of good
quality and so as to conform with all applicable zoning, environmental,
building, fire, health, and other codes, regulations, ordinances and laws and
the ADA Requirements; to furnish to Landlord prior to the commencement of any
such work for the Phase III Space, or any other such work for which the cost may
reasonably be estimated to exceed $100,000, a bond or other security acceptable
to Landlord assuring that any work commenced or continued by Tenant will be
completed in accordance with specifications approved in advance in writing by
Landlord; to keep the Premises at all times free of liens for labor and
materials; to employ for such work one or more responsible contractors whose
labor will work without interference with other labor working on the Premises;
to require such contractors employed by Tenant to carry worker's compensation
insurance in accordance with statutory requirements and comprehensive public
liability insurance covering any general contractors on or about the Premises in
amounts that at least equal the limits set forth in Section 1.1 and to submit
certificates evidencing such coverage to Landlord prior to the commencement of
such work; and to save Landlord harmless and indemnified from all injury, loss,
claims or damage to any person or property occasioned by or growing out of such
work. Notwithstanding any other provision of this Section 5.1.5 or any ether
provision of this Lease to the contrary, Landlord shall not, subject to
applicable law, prohibit Tenant from using non-union labor to perform any work
at the Premises after the completion of the build-out of the Phase I Space and
the Phase II Space. Without limitation of the foregoing, Landlord shall not
require Tenant's employees to unionize.

                                      -38-
<PAGE>

      5.1.6 INDEMNITY.

            To defend, with counsel approved by Landlord, all actions against
Landlord, any partner, trustee, stockholder, officer, director, employee or
beneficiary of Landlord, holders of mortgages secured by the Premises or the
Building and Lot and any other party having an interest in the Premises
("Indemnified Parties") with respect to, and to pay, protect, indemnify and save
harmless, to the extent permitted by law, all Indemnified Parties from and
against, any and all liabilities, losses, damages, costs, expenses (including
reasonable attorneys' fees and expenses), causes of action, suits, claims,
demands or judgments of any nature arising from (i) injury to or death of any
person, or damage to or loss of property, on the Premises or on adjoining
sidewalks, streets or ways connected with the use or occupancy thereof by Tenant
or its agents, contractors, licensees, employees, sublessees or invitees, unless
and to the extent caused by the negligence of Landlord or its servants or
agents, (ii) violation of this Lease by Tenant or its agents, contractors,
licensees, employees, sublessees or invitees, or (iii) any act, fault, omission,
or other misconduct of Tenant or its agents, contractors, licensees, employees,
sublessees or invitees.

      5.1.7 LANDLORD'S RIGHT TO ENTER.

            To permit Landlord and its agents to enter into the Premises at
reasonable times and upon at least 24 hours advance notice (except in case of
emergency in which event no prior notice shall be required) to examine the
Premises, make such repairs and replacements as Landlord may elect, without
however, any obligation to do so except as may be otherwise expressly set forth
in this Lease, and show the Premises to prospective purchasers and lenders, and,
during the last twelve months of the Term, to show the Premises to prospective
tenants and to keep affixed in suitable places notices of availability of the
Premises. Landlord's right to enter the Premises in accordance with the
foregoing shall be subject to Landlord's obligations pursuant to Section 10.15
hereof. Notwithstanding the foregoing, Landlord agrees that in the event that
Landlord shows the Premises to any prospective purchaser or tenant, Landlord
shall: (i) provide at least three (3) days' notice to Tenant identifying the
prospective purchaser or tenant, (ii) only show the Premises to such purchaser
or tenant if Landlord believes in good faith that such person or entity is a
bona fide prospective purchaser or tenant, (iii) conduct such showing in
compliance with such reasonable requests and instructions as Tenant may make for
purposes of protecting Tenant's Confidential Information.

                                      -39-
<PAGE>

      5.1.8 PERSONAL PROPERTY AT TENANT'S RISK.

            All of the furnishings, fixtures, equipment, effects and property of
every kind, nature and description owned or leased by Tenant or by any person
claiming by, through or under Tenant which, during the continuance of this Lease
or any occupancy of the Premises by Tenant or anyone claiming under Tenant, may
be on the Premises, shall, as between the parties, be at the sole risk and
hazard of 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 to be borne by Landlord, except that
Landlord shall in no event be indemnified or held harmless or exonerated from
any liability to Tenant or to any other person, for any injury, loss, damage or
liability to the extent (i) such injury, loss, damage or liability is the result
of the negligence or willful misconduct of Landlord, its contractors, agents or
employees, or (ii) such indemnification, agreement to hold harmless or
exoneration is prohibited by law.

      5.1.9 PAYMENT OF LANDLORD'S COST OF ENFORCEMENT.

            To pay on demand Landlord's expenses, including reasonable
attorney's fees, incurred in enforcing any obligation of Tenant under this Lease
or in curing any default by Tenant under this Lease as provided in Section 7.4.

      5.1.10 YIELD UP.

            Subject to Section 3.3 hereof, at the expiration of the Term or
earlier termination of this Lease: to surrender all keys to the Premises; to
remove all of its trade fixtures and personal property in the Premises; to
remove such installations and improvements made by Tenant as Landlord may
request and all Tenant's signs wherever located; to repair all damage caused by
such removal; and to yield up the Premises (including all installations and
improvements made by Tenant except for trade fixtures and such of said
installations or improvements as Landlord shall request Tenant to remove),
broom-clean and in the same good order and repair in which Tenant is obliged to
keep and maintain the Premises by the provisions of this Lease. Subject to
Section 3.3 hereof, any property not so removed shall be deemed abandoned and
may be removed and disposed of by Landlord in such manner as Landlord shall
determine and Tenant shall pay Landlord the entire cost and expense incurred by
Landlord in effecting such removal and disposition and in making any incidental
repairs and replacements to the Premises and for use and occupancy during the
period after the expiration of the Term and prior to Tenant's performance of its
obligations under this Section 5.1.10. Tenant

                                      -40-
<PAGE>

shall further indemnify Landlord against all loss, cost and damage resulting
from Tenant's failure and delay in surrendering the Premises as above provided.

      5.1.11 ESTOPPEL CERTIFICATE.

            Upon not less than 10 days' prior notice by Landlord, to execute,
acknowledge and deliver to Landlord a statement in writing certifying that this
Lease is unmodified and in full force and effect and that except as stated
therein Tenant has no knowledge of any defenses, offsets or counterclaims
against its obligations to pay the Fixed Rent and Additional Rent and any other
charges and to perform its other covenants under this Lease (or, if there have
been any modifications that the Lease is in full force and effect as modified
and stating the modifications and, if there are any defenses, offsets or
counterclaims, setting them forth in reasonable detail), the dates to which the
Fixed Rent and Additional Rent and other charges have been paid and a statement
that, to the best of Tenant's knowledge, Landlord is not in default hereunder
(or if in default, the nature of such default, in reasonable detail) and such
other matters reasonably required by Landlord or any prospective purchaser or
mortgagee of the Premises. Any such statement delivered pursuant to this Section
5.1.11 may be relied upon by any prospective purchaser or mortgagee of the
Premises, or any prospective assignee of any such mortgage.

      5.1.12 LANDLORD'S EXPENSES RE CONSENTS.

            To reimburse Landlord promptly on demand for all reasonable legal
expenses incurred by Landlord in connection with all requests by Tenant for
consent or approval under this Lease. Notwithstanding the foregoing, Tenant
shall not be liable for any reasonable legal expenses incurred by Landlord for
the first two (2) such requests made by Tenant during each period of twelve (12)
consecutive calendar months during the Term.

      5.1.13 RULES AND REGULATIONS.

            To comply with the Rules and Regulations set forth in Exhibit C, as
the same may be amended from time to time by Landlord to provide for the
beneficial operation of the Building and/or Lot, provided that such amendments
do not materially interfere with Tenant's right of use and enjoyment of the
Premises pursuant to this Lease, it being understood that, subject to the
provisions of Section 10.2 hereof, Landlord shall not be liable to Tenant for
the failure of other tenants of the Building or the Lot to conform to such Rules
and Regulations.

                                      -41-
<PAGE>

      5.1.14 LOADING.

            Not to place Tenant's Property, as defined in Section 5.1.8, upon
the Premises so as to exceed the floor load limits set forth in Exhibit H
attached hereto and not to move any safe, vault or other heavy equipment in,
about or out of the Premises except in such manner and at such times as Landlord
shall in each instance approve; Tenant's business machines and mechanical
equipment which cause vibration or noise that may be transmitted to the Building
structure or to any other leased space in the Building shall be placed or
maintained by Tenant in settings of cork, rubber, spring, or other types of
vibration eliminators sufficient to reduce such vibration or noise to a level
reasonably acceptable to Landlord.

      5.1.15 HOLDOVER.

            To pay to Landlord (i) the greater of twice (a) the then fair market
rent as reasonably determined by Landlord or (b) the total of the Fixed Rent,
Additional Rent, and all other payments then payable hereunder, for each month
or portion thereof Tenant shall retain possession of the Premises or any part
thereof after the termination of this Lease, whether by lapse of time or
otherwise, and (ii) all damages sustained by Landlord on account thereof;
provided, however, that any payments made by Tenant under the foregoing clause
(i) in excess of the then fair market rent for the Premises as so reasonably
determined by Landlord shall be applied against any damages under the foregoing
clause (ii). The provisions of this subsection shall not operate as a waiver by
Landlord of the right of re-entry provided in this Lease.

5.2   NEGATIVE COVENANTS.

      Tenant covenants at all times during the Term and for such further time as
Tenant occupies the Premises or any part thereof:

      5.2.1 ASSIGNMENT AND SUBLETTING.

            Not without the prior written consent of Landlord to assign this
Lease, to make any sublease, or to permit occupancy of the Premises or any part
thereof by anyone other than Tenant, voluntarily or by operation of law, except
as hereinafter provided; as Additional Rent, to reimburse Landlord promptly for
reasonable legal and other expenses incurred by Landlord in connection with any
request by Tenant for consent to assignment or subletting (subject to the
provisions of Section 5.1.12 hereof); no assignment or subletting shall affect
the continuing primary liability of Tenant (which, following assignment, shall
be joint and several with the assignee); no consent to any of the foregoing in a
specific instance shall operate as a waiver in any subsequent

                                      -42-
<PAGE>

Instance. Landlord's consent to any proposed assignment or subletting is
required both as to the terms and conditions thereof and as to the consistency
of the proposed assignee's or subtenant's business with other uses and tenants
in the Building. In addition, as to any assignee (but not as to any sublessee)
Landlord's consent shall be required as to the reasonable creditworthiness of
the proposed assignee in view of market conditions then prevailing for leases
having terms and conditions comparable to this Lease. Landlord's consent to any
assignment or subletting by Tenant shall not be unreasonably withheld, provided
that Tenant is not then in default under this Lease and that such assignee or
subtenant pays therefor the greater of the Fixed Rent, Additional Rent, and all
other payments then payable hereunder, or the then fair market rent for the
Premises. If Tenant requests Landlord's consent to assign this Lease or to
sublet any portion of the Premises such that Tenant shall not occupy at least
20,000 r.s.f. of the Premises after the date of commencement of such sublease,
Landlord shall have the option, exercisable by written notice to Tenant given
within 10 days after receipt of such request, to terminate this Lease as of the
date of commencement the proposed sublease or assignment; provided, however,
that Tenant shall have the right to rescind any such request in the event
Landlord elects to so terminate this Lease by notice given to Landlord within
five (5) days after the date of such termination notice from Landlord, in which
event such termination notice shall be of no further force or effect;

            If, at any time during the Term of this Lease, Tenant is:

                  (i) a corporation or a trust (whether or not having shares of
beneficial interest) and there shall occur any change in the identity of any of
the persons then having power to participate in the election or appointment of
the directors, trustees or other persons exercising like functions and managing
the affairs of Tenant; or

                  (ii) a partnership or association or otherwise not a natural
person (and is not a corporation or a trust) and there shall occur any change in
the identity of any of the persons who then are members of such partnership or
association or who comprise Tenant;

Tenant shall so notify Landlord and Landlord may terminate this Lease by notice
to Tenant given within 90 days thereafter if, in Landlord's reasonable judgment,
the credit of Tenant is thereby impaired. This paragraph shall not apply if the
initial Tenant named herein is a corporation and the outstanding voting stock
thereof is listed on a recognized securities exchange.

                                     -43-
<PAGE>

      Notwithstanding the foregoing provisions of this Section 5.2.1, Tenant may
assign this Lease or sublet any portion of the Premises without Landlord's
consent to (i) any successor of Tenant resulting from a merger or consolidation
of Tenant and (ii) any Affiliate of Tenant (as hereinafter defined) whose net
worth is equal to or greater than the net worth of Tenant AS of the date hereof,
provided that Tenant provides Landlord at least thirty (30) days prior notice of
such assignment or subletting pursuant to either of the foregoing clauses (i) or
(ii). As used herein, the term "Affiliate of Tenant" shall mean and refer to any
entity controlled by, controlling or under common control with Tenant.

      In the event that any assignee or subtenant pays to Tenant any amounts in
excess of the Fixed Rent, Additional Rent, and all other payments then payable
hereunder, or pro rata portion thereof on a square footage basis for any portion
of the Premises (such excess being hereinafter referred to as "Sublease
Profits"), Tenant shall promptly pay fifty percent (50%) of said Sublease
Profits to Landlord as and when received by Tenant after deduction of Tenant's
Sublease Costs (as hereinafter defined). The term "Sublease Costs" shall mean
and refer to Tenant's reasonable legal, brokerage and construction costs and
expenses incurred in good faith in view of the size and expected term of any
applicable sublease or assignment. Sublease Costs shall be amortized over the
term of the applicable sublease or assignment. In the event that Tenant
exercises its Reimbursement Option pursuant to Section 4.1(d) hereof, Fixed Rent
shall nevertheless be calculated as though such Reimbursement Option had not
been exercised for purposes of calculating Sublease Profits under this Section
5.2.1.

      5.2.2 NUISANCE.

            Not to injure, deface or otherwise harm the Premises; nor commit,
any nuisance; nor permit the emission of any noise, vibration or odor which is
contrary to any law or ordinance; nor make, allow or suffer any waste; nor make
any use of the Premises which is improper, offensive or contrary to any law or
ordinance or which will invalidate any of Landlord's insurance.

      5.2.3 INSTALLATION, ALTERATIONS OR ADDITIONS.

            Subject to the provisions of Section 3.3 and Section 5.1.5 hereof,
not to make any installations, alterations, or additions in, to or on the
Premises (including, without limitation, buildings, lawns, planted areas, walks,
roadways, parking and loading areas, but expressly excluding the initial
improvements to each Phase and the Second Floor Space made in accordance with
Complete Plans approved by Landlord and Tenant, and any other improvements
consistent therewith, provided the same are approved by Landlord, such approval
not to be unreasonably

                                      -44-
<PAGE>

withheld or delayed) nor to permit the making of any apertures in the walls,
partitions, ceilings or floors without on each occasion obtaining the prior
written consent of Landlord and then only pursuant to plans and specifications
approved by Landlord in advance in each instance.

                                   ARTICLE VI

                               CASUALTY OR TAKING

6.1   TERMINATION.

      In case during the period which is thirty (30) months prior to the
expiration of the Term all or any substantial part of the Premises or of the
Building or of the Lot or any one or more of them shall be taken by any public
authority or for any public use, or shall be destroyed or damaged by fire or
casualty, or by the action of any public authority, or Landlord receives
compensable damage by reason of anything lawfully done in pursuance of public or
other authority, (hereinafter referred to as the "Casualty or Taking"), then
this Lease may be terminated at the election of Landlord. Such election, which
may be made notwithstanding the fact that Landlord's entire interest may have
been divested, shall be made by the giving of notice by Landlord to Tenant
within 30 days after the Casualty or Taking.

6.2   RESTORATION.

      If Landlord does not exercise said election (or is not entitled to
exercise said election in the case of a Casualty or Taking occurring more than
thirty (30) months prior to the expiration of the Term of this Lease) , this
Lease shall continue in force and a just proportion of the rent reserved,
according to the nature and extent of the damages sustained by the Premises, but
not in excess of an equitable proportion of the net proceeds of insurance
recovered by Landlord under the rental insurance coverage carried pursuant to
Section 4.2.2.2, shall be abated from the date of the Casualty or Taking until
the Premises, or What may remain thereof, shall be put by Landlord in proper
condition for use subject to zoning and building laws or ordinances then in
existence, which, unless Landlord has exercised its option to terminate pursuant
to Section 6.1, Landlord covenants to do with reasonable diligence at Landlord's
expense, provided that Landlord's obligations with respect to restoration shall
not require Landlord to expend more than the net proceeds of insurance recovered
or damages awarded for such Casualty or Taking. "Net proceeds of insurance
recovered or damages awarded" refers to the gross amount of such insurance or
damages less the reasonable expenses of Landlord in connection with the
collection of the

                                      -45-
<PAGE>

same, including without limitation, fees and expenses for legal and appraisal
services.

      Notwithstanding the foregoing, in the event that Landlord's architect
reasonably determines that the Premises will not be repaired or restored (to the
extent permitted by the net proceeds of insurance recovered or damages awarded
from such Casualty or Taking) within one year after the occurrence of such
Casualty or Taking then Tenant shall have the right to terminate this Lease by
notice given within thirty (30) days after the date of such determination.

6.3   AWARD.

      Irrespective of the form in which recovery may be had by law, all rights
to damages or compensation shall belong to Landlord in all cases except as set
forth below in this Section 6.3. Tenant hereby grants to Landlord all of
Tenant's rights to such damages and compensation and covenants to deliver such
further assignments thereof as Landlord may from time to time request. It is
agreed and understood, however, that Landlord does not reserve to itself, and
Tenant does not assign to Landlord, any damages payable for (i) movable trade
fixtures installed by Tenant or anybody claiming under Tenant, at its own cost
and expense or (ii) relocation expenses or damages for loss of business (in
excess of any such damages attributable to the value of this lease) recoverable
by Tenant from such authority in a separate action.

                                   ARTICLE VII

                                    DEFAULTS

7.1   EVENTS OF DEFAULT.

      (a) If Tenant shall default in the performance of any of its obligations
to pay the Fixed Rent or Additional Rent hereunder and if such default shall
continue for 10 days after notice from Landlord to Tenant (provided, however,
that Landlord shall not be required to provide such notice more than two (2)
times in any period of twelve (12) consecutive calendar months) or if within 30
days after notice from Landlord to Tenant specifying any other default or
defaults Tenant has not commenced diligently to correct the default or defaults
so specified or has not thereafter diligently pursued such correction to
completion, or (b) if any assignment for the benefit of creditors shall be made
by Tenant, or by any guarantor of Tenant, or (c) if Tenant's leasehold interest
shall be taken on execution or other process of law in any action against
Tenant, or (d) if a lien or other involuntary encumbrance is filed against
Tenant's leasehold interest, and is

                                      -46-
<PAGE>

not discharged within thirty (30) days thereafter, or (e) if a petition is filed
by Tenant or any guarantor of Tenant for liquidation, or for reorganization or
an arrangement or any other relief under any provision of the Bankruptcy Code as
then in force and effect, or (f) if an involuntary petition under any of the
provisions of said Bankruptcy Code is filed against Tenant or any guarantor of
Tenant and such involuntary petition is not dismissed within ninety (90) days
thereafter, or (g) if Tenant fails to maintain the insurance required under
Section 4.2.2.1 hereof, then, and in any of such cases, Landlord and the agents
and servants of Landlord lawfully may, in addition to and not in derogation of
any remedies for any preceding breach of covenant, immediately or at any time
thereafter and without demand or notice, at Landlord's election, do any one or
more of the following: (1) give Tenant written notice stating that the Lease is
terminated, effective upon the giving of such notice or upon a date stated in
such notice, as Landlord may elect, in which event the Lease shall be
irrevocably extinguished and terminated as stated in such notice without any
further action, or (2) with or without process of law, in a lawful manner and
without illegal force, enter and repossess the Premises as of Landlord's former
estate, and expel Tenant and those claiming through or under Tenant, and remove
its and their effects, without being guilty of trespass, in which event the
Lease shall be irrevocably extinguished and terminated at the time of such
entry, or (3) pursue any other rights or remedies permitted by law. Any such
termination of the Lease shall be without prejudice to any remedies which might
otherwise be used for arrears of rent or prior breach of covenant, and in the
event of such termination Tenant shall remain liable under this Lease as
hereinafter provided. Tenant hereby waives all statutory rights of redemption
and Landlord, without notice to Tenant, may store Tenant's effects, and those of
any person claiming through or under Tenant, at the expense and risk of Tenant,
and, if Landlord so elects, may sell such effects at public auction or private
sale and apply the net proceeds to the payment of all sums due to Landlord from
Tenant, if any, and pay over the balance, if any, to Tenant.

7.2   REMEDIES.

      In the event that this Lease is terminated under any of the provisions
contained in Section 7.1 or shall be otherwise terminated for breach of any
obligation of Tenant, Tenant covenants to pay forthwith to Landlord, as
compensation, the excess of the total rent reserved for the residue of the Term
over the fair market rental value of the Premises for said residue of the Term.
In calculating the rent reserved there shall be included, in addition to the
Fixed Rent and Additional Rent, the value of all other considerations agreed to
be paid or performed by Tenant during said residue. Tenant further covenants (as

                                      -47-
<PAGE>

additional and cumulative obligations) after any such termination to pay
punctually to Landlord all the sums and to perform all the obligations which
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.
In calculating the amounts to be paid by Tenant pursuant to the next preceding
sentence Tenant shall be credited with any amount paid to Landlord as
compensation as in this Section 7.2 provided and also with the net proceeds of
any rent obtained by Landlord by reletting the Premises, after deducting all of
the Landlord's reasonable 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, it
being agreed by Tenant that Landlord may (i) relet the Premises or any part or
parts thereof, for a term or terms which may at Landlord's option be equal to or
less than or exceed the period which would otherwise have constituted the
balance of the Term and may grant such concessions and free rent as Landlord in
its reasonable judgment considers advisable or necessary to relet the same and
(ii) make such alterations, repairs and decorations in the Premises as Landlord
in its reasonable judgment considers advisable or necessary to relet the same,
and no action of Landlord in accordance with the foregoing or failure to relet
or to collect rent under reletting shall operate or be construed to release or
reduce Tenant's liability as aforesaid.

      In lieu of any other damages or indemnity and in lieu of full recovery by
Landlord of all sums payable under all the foregoing provisions of this Section
7.2, Landlord may by notice to Tenant, at any time after this Lease is
terminated under any of the provisions contained in Section 7.1 or is otherwise
terminated for breach of any obligation of Tenant and before such full recovery,
elect to recover, and Tenant shall thereupon pay, as liquidated damages, an
amount equal to the aggregate of the Fixed Rent and Additional Rent accrued in
the 12 months ended next prior to such termination, plus the amount of rent of
any kind accrued and unpaid at the time of termination and less the amount of
any recovery by Landlord under the foregoing provisions of this Section 7.2 up
to the time of payment of such liquidated damages.

      Nothing contained in this Lease shall, however, limit or prejudice the
right of Landlord to prove for and obtain in proceedings for bankruptcy or
insolvency 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 than, equal to, or less than the amount of the loss or
damages referred to above.

                                      -48-
<PAGE>

7.3   REMEDIES CUMULATIVE.

      Any and all rights and remedies which either Landlord or Tenant may have
under this Lease, and at law and equity, shall be cumulative and shall not be
deemed inconsistent with each other, and any two or more of all such rights and
remedies may be exercised at the same time insofar as permitted by law.

7.4   LANDLORD'S RIGHT TO CURB DEFAULTS.

      Landlord may, but shall not be obligated to, cure, at any time, following
10 days' prior notice to Tenant, except in cases of emergency when no notice
shall be required, any default by Tenant under this Lease; and whenever Landlord
so elects, all costs and expenses incurred by Landlord, including reasonable
attorneys' fees, in curing a default shall be paid by Tenant to Landlord as
Additional Rent on demand, together with interest thereon at the rate provided
in Section 4.3 from the date of payment by Landlord to the date of payment by
Tenant.

7.5   EFFECT OF WAIVERS OP DEFAULT.

      Any consent or permission by Landlord or Tenant to any act or omission by
the other party which otherwise would be a breach of any covenant or condition
herein, or any waiver by Landlord or Tenant of the breach of any covenant or
condition herein by the other party, shall not in any way be held or construed
(unless expressly so declared) to operate so as to impair the continuing
obligation of any covenant or condition herein, or otherwise, except as to the
specific instance, operate to permit similar acts or omissions.

      The failure of Landlord or Tenant to seek redress for violation of, or to
insist upon the strict performance of, any covenant or condition of this Lease
by the other party 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 Landlord, or
the payment by Tenant, as the case may be, of rent with knowledge of the breach
of any covenant of this Lease shall not be deemed to have been a waiver of such
breach by Landlord or Tenant, as the case may be. No consent or waiver, express
or implied, by Landlord or Tenant, as the case may be, 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.

                                      -49-
<PAGE>

7.6   NO ACCORD AND SATISFACTION.

      No acceptance by Landlord of a lesser sum than the Fixed Rent, Additional
Rent or any other charge then due shall be deemed to be other than on account of
the earliest installment of such rent or charge due, unless Landlord elects by
notice to Tenant to credit such sum against the most recent installment due, nor
shall any endorsement or statement on any check or any letter accompanying any
check or payment as rent or other charge be deemed an accord and satisfaction,
and Landlord may accept such check or payment without prejudice to Landlord's
right to recover the balance of such installment or pursue any other remedy in
this Lease provided.

                                  ARTICLE VIII

                                    MORTGAGES

8.1   RIGHTS OF MORTGAGE HOLDERS.

      The word "mortgage" as used herein includes mortgages, deeds of trust or
other similar instruments evidencing other voluntary liens or encumbrances, and
modifications, consolidations, extensions, renewals, replacements and
substitutes thereof. The word "holder" shall mean a mortgagee, and any
subsequent holder or holders of a mortgage. Until the holder of a mortgage shall
enter and take possession of the Premises for the purpose of foreclosure, such
holder shall have only such rights of Landlord as are necessary to preserve the
integrity of this Lease as security. Upon entry and taking possession of the
Premises for the purpose of foreclosure, such holder shall have all the rights
of Landlord. Notwithstanding any other provision of this Lease to the contrary,
including without limitation Section 10.5, no such holder of a mortgage shall be
liable either as mortgagee or as assignee to perform, or be liable in damages
for failure to perform, any of the obligations of Landlord unless and until such
holder shall enter and take possession of the Premises for the purpose of
foreclosure, and such holder shall not in any event be liable to perform or for
failure to perform the obligations of Landlord under Section 3.1. Upon entry for
the purpose of foreclosure, such holder shall be liable to perform all of the
obligations of Landlord (except for the obligations under Article III), subject
to and with the benefit of the provisions of Section 10.5, provided that a
discontinuance of any foreclosure proceeding shall be deemed a conveyance under
said provisions to the owner of the equity of the Premises. Except as provided
in Section 4.1(d), no Fixed Rent, Additional Rent or any other charge shall be
paid more than 10 days prior to the due dates thereof and payments made in
violation of this provision shall (except to the extent that

                                      -50-
<PAGE>

such payments are actually received by a mortgagee in possession or in the
process of foreclosing its mortgage) be a nullity as against such mortgagee and
Tenant shall be liable for the amount of such payments to such mortgagee.

      The covenants and agreements contained in this Lease with respect to the
rights, powers and benefits of a holder of a mortgage (including, without
limitation, the covenants and agreements contained in this Section 8.1)
constitute a continuing offer to any person, corporation or other entity, which
by accepting a mortgage subject to this Lease, assumes the obligations herein
set forth with respect to such holder; such holder is hereby constituted a party
of this Lease as an obligee hereunder to the same extent as though its name were
written hereon as such; and such holder shall be entitled to enforce such
provisions in its own name. Tenant agrees on request of Landlord to execute and
deliver from time to time any agreement which may be necessary to implement the
provisions of this Section 8.1.

8.2   SUPERIORITY OP LEASE; OPTION TO SUBORDINATE.

      This Lease shall be superior to and shall not be subordinate to any future
mortgage or other voluntary lien or other encumbrance of which the Premises are
a part; provided, however, that Landlord shall have the option to subordinate
this Lease to any such mortgage of which the Premises are a part provided that
Landlord obtains from the holder of record of any existing or future mortgage an
agreement with Tenant by the terms of which such holder will agree (a) to
recognize the rights of Tenant under this Lease, (b) to perform Landlord's
obligations hereunder arising after the date of such holder's acquisition of
title as hereinafter described, expressly excluding, however, Landlord's
obligations under Article III of this Lease, and (c) to accept Tenant as tenant
of the Premises under the terms and conditions of this Lease in the event of
acquisition of title by such holder through foreclosure proceedings or
otherwise, provided that Tenant will agree to recognize the holder of such
mortgage as Landlord in such event, which agreement shall be made expressly to
bind and inure to the benefit of the successors and assigns of Tenant and of the
holder and upon anyone purchasing said Premises at any foreclosure sale. Tenant
and Landlord agree to execute and deliver any appropriate instruments necessary
to carry out the agreements contained in this Section 8.2. Any such mortgage to
which this Lease shall subordinate may contain such terms, provisions and
conditions as the holder deems usual or customary.

                                      -51-
<PAGE>

8.3   LEASE AMENDMENTS.

      Tenant agrees to make such changes in this Lease as may be reasonably
required by the holder of any mortgage of which the Premises are a part, or any
institution which may purchase all or a substantial part of Landlord's interest
in the Premises, provided that such changes may not increase the Fixed Rent or
other payments due hereunder or otherwise materially affect the obligations of
Tenant hereunder, and provided further that such changes do not (i) materially
interfere with Tenant's right of use and enjoyment of the Premises pursuant to
this Lease, (ii) limit, impair or delay Tenant's rights to sublease or assign
all or any portion of this Lease pursuant to Section 5.2.1 hereof, (iii) limit,
impair or delay Tenant's right to obtain a reduction or abatement of rent
pursuant to Section 3.2, Section 4.1(d) or Section 6.2, (iv) limit, impair or
delay Tenant's right to terminate this Lease pursuant to Section 3.2 or Section
6.2, or (v) otherwise unreasonably limit, impair or delay Tenant's rights
hereunder.

                                   ARTICLE IX

                         LANDLORD'S ADDITIONAL COVENANTS

9.1   AFFIRMATIVE COVENANTS.

      Landlord covenants at all times during the Term:

      9.1.1 PERFORM OBLIGATIONS.

            To perform promptly all of the obligations of Landlord set forth in
this Lease, including, without limitation, furnishing, through Landlord's
employees or independent contractors, the services (the cost of which is to be
included in the Annual Maintenance Charge) listed in Exhibit J and completing
construction of the Phase I Space and the Phase II Space in accordance with
Article III;

      9.1.2 REPAIRS.

            Except as otherwise provided in Article VI, to make such repairs
(the cost of which is to be included in the Annual Maintenance Charge) to the
roof, exterior walls, exterior windows and waterproofing, floor slabs, other
structural components and common areas and facilities of the Building as may be
necessary to keep them in good, serviceable condition. Without limitation of the
foregoing, Landlord shall be responsible for the maintenance and repair of the
heating and air-conditioning systems and the components thereof serving the
Building, except for such systems

                                      -52-
<PAGE>

and the components thereof as are located within the Premises or exclusively
serve the Premises (as provided in Section 5.1.3 hereof).

      9.1.3 COMPLIANCE WITH LAW.

            To make all repairs, alterations, additions or replacements to the
Building or the Lot (the costs of which are to be included in the Annual
Maintenance Charge) required by any law, ordinance or order or regulation of any
public authority including repairs, alterations, additions or replacements to
the foundations and structural elements of the Building, except as required
because of Tenant's failure to comply with the provisions of Section 5.1.3
hereof; to keep the Building equipped with all safety appliances so required
(the costs of which are to be included in the Annual Maintenance Charge);
subject to Section 4.2.1, to pay all municipal, county, or state taxes assessed
against the Building or the Lot, or against Landlord's personal property of any
kind on or about the Building or the Lot; and to comply with the orders and
regulations of all governmental authorities with respect to zoning, building,
fire, health and other codes, regulations, ordinances or laws applicable to the
Building or the Lot, including the ADA Requirements (as defined in Section 5.1.4
hereof) and any codes, regulations, ordinances or laws relating to hazardous
materials (as defined in Section 5.1.4), subject to, and without limitation of,
Tenant's obligations with respect to such codes, regulations, ordinances or
laws. The costs incurred by Landlord in connection with the foregoing compliance
obligations shall be included in the Annual Maintenance Charge. All of the
foregoing covenants and obligations are subject to, and without limitation of,
all of Tenant's obligations under this Lease, including, without limitation,
those set forth in Sections 4.2 and 5.1.4.

      9.1.4 INDEMNITY.

            To defend, with counsel reasonably approved by Tenant, all actions
against Tenant, any partner, trustee, stockholder, officer, director, employee
or beneficiary of Tenant ("Tenant's Indemnified Parties") with respect to, and
to pay, protect, indemnify and save harmless, to the extent permitted by law,
all Tenant's Indemnified Parties from and against any and all liabilities,
losses, damages, costs, expenses (including reasonable attorneys' fees and
expenses), causes of action, suits, claims, demands or judgments of any nature
to which any of Tenant's Indemnified Parties is subject arising from and to the
extent of any negligent or willful act, fault, omission, or other misconduct of
Landlord or its agents, contractors or employees.

                                      -53-
<PAGE>

      9.1.5 ESTOPPEL CERTIFICATE.

            Upon not less than 10 days' prior notice by Tenant, to execute,
acknowledge and deliver to Tenant a statement in writing certifying that this
Lease is unmodified and in full force and effect and that except as stated
therein Landlord has no knowledge of any defenses, offsets or counterclaims
against its obligations under this Lease (or, if there have been any
modifications that the Lease is in full force and effect as modified and stating
the modifications and, if there are any defenses, offsets or counterclaims,
setting them forth in reasonable detail), the dates to which the Fixed Rent and
Additional Rent and other charges have been paid and a statement that, to the
best of Landlord's knowledge, Tenant is not in default hereunder (or if in
default, the nature of such default, in reasonable detail) and such other
matters reasonably required by Tenant or any prospective assignee of Tenant. Any
such statement delivered pursuant to this Section 9.1.5 may be relied upon by
any prospective assignee.

      9.1.6 LANDLORD'S TITLE.

      Within ten (10) days after the full execution and delivery of this Lease,
to give Tenant a copy of Landlord's owner's policy of title insurance with
respect to the Lot, which policy shall evidence no monetary liens as of the date
of this Lease and no other liens or encumbrances except as set forth on the
commitment for title insurance attached hereto as Exhibit K.

      9.1.6 UTILITIES.

      Subject to Section 4.2.3 hereof, to bring (or cause to brought) utilities
for the Premises to the Utility Switching Gears at Landlord's sole cost and
expense.

                                    ARTICLE X

                            MISCELLANEOUS PROVISIONS

10.1  NOTICES FROM ONE PARTY TO THE OTHER.

      All notices required or permitted hereunder shall be in writing and
addressed, if to the Tenant, at the Original Address of Tenant or such other
address as Tenant shall have last designated by notice in writing to Landlord
and, if to Landlord, at Landlord's Address or such other address as Landlord
shall have last designated by notice in writing to Tenant. Any notice shall be
deemed duly given if mailed to such address postage prepaid, registered or
certified mail, return receipt requested, when deposited with the U.S. Postal
Service, or if delivered by a

                                      -54-
<PAGE>

recognized courier service (e.g. Federal Express) when deposited with such
courier service, or if delivered to such address by hand, when so delivered.

10.2  QUIET ENJOYMENT.

      Landlord agrees that upon Tenant's paying the rent and performing and
observing the terms, covenants, conditions and provisions on its part to be
performed and observed, Tenant shall and may peaceably and quietly have, hold
and enjoy the Premises during the Terra without any manner of hindrance or
molestation from Landlord or anyone claiming under Landlord, subject, however,
to the terms of this Lease.

10.3  EASEMENTS; CHANGES TO LOT LINES.

      Landlord reserves the right, from time to time, to grant easements
affecting the Premises or the Building or the Lot and to change or alter
existing boundaries of the Lot for purpose of developing and using the Lot so
long as such easements or such changes or alterations to existing boundaries of
the Lot do not materially interfere with Tenant's use of the Premises, and to
enter upon the Premises for purposes of constructing and maintaining any pipes,
wires and other facilities serving any portion of the Lot or of the Building,
subject to the terms of Section 5.1.7 hereof.

10.4  LEASE NOT TO BE RECORDED.

      Neither party shall record this Lease. Both parties shall execute and
deliver a notice of this Lease in such form, if any, as may be permitted by
applicable statute. If this Lease is terminated before the Term Expiration Date
the parties shall execute, deliver and record an instrument acknowledging such
fact and the actual date of termination of this Lease, and Tenant hereby
appoints Landlord its attorney-in-fact, coupled with an interest, with full
power of substitution to execute such instrument.

10.5  BIND AND INURE; LIMITATION OF LANDLORD'S 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. No owner of the Premises shall be liable
under this Lease except for breaches of Landlord's obligations occurring while
owner of the Premises. The obligations of Landlord shall be binding upon the
assets of Landlord which comprise the Premises but not upon other assets of
Landlord. No partner, trustee, stockholder, officer, director, employee or
beneficiary (or the partners, trustees,

                                      -55-
<PAGE>

stockholders, officers, directors or employees of any such beneficiary) of
Landlord shall be personally liable under this Lease and Tenant shall look
solely to Landlord's interest in the Premises in pursuit of its remedies upon an
event of default hereunder, and the general assets of the partners, trustees,
stockholders, officers, employees or beneficiaries (and the partners, trustees,
stockholders, officers, directors or employees of any such beneficiary) of
Landlord shall not be subject to levy, execution or other enforcement procedure
for the satisfaction of the remedies of Tenant; provided that the foregoing
provisions of this sentence shall not constitute a waiver of any obligation
evidenced by this Lease and provided further that the foregoing provisions of
this sentence shall not limit the right of Tenant to name Landlord or any
individual partner or trustee thereof as part defendant in any action or suit in
connection with this Lease so long as no personal money judgment shall be asked
for or taken against any individual partner, trustee, stockholder, officer,
employee or beneficiary of Landlord.

10.6  ACTS OF GOD.

      In any case where either party hereto is required to do any act, delays
caused by or resulting from the occurrence of one or more Forcs Majeure Events
shall not be counted in determining the time during which work shall be
completed, whether such time be designated by a fixed date, a fixed time or a
"reasonable time", and such time shall be deemed to be extended by the period of
such delay.

10.7  LANDLORD'S DEFAULT.

      Landlord shall not be deemed to be in default in the performance of any of
its obligations hereunder unless it shall fail to perform such obligations and
such failure shall continue for a period of 30 days following receipt of notice
from Tenant or such additional time as is reasonably required to correct any
such default after notice has been given by Tenant to Landlord specifying the
nature of Landlord's alleged default. Landlord shall not be liable in any event
for incidental or consequential damages to Tenant by reason of any default by
Landlord hereunder, whether or not Landlord is notified that such damages may
occur. Except as expressly set forth in Section 3.2 and Section 6.2 hereof,
Tenant shall have no right to terminate this Lease for any default by Landlord
hereunder and no right, for any such default, to offset or counterclaim against
any rent due hereunder.

      Notwithstanding the foregoing, if any repairs to the Premises or any
maintenance, cleaning, or lighting of the common areas of the Building or the
Lot, are not performed by Landlord or the Premises any time after the
Substantial Completion Date of each

                                      -56-
<PAGE>

respective Phase of the Space and within thirty (30) days after notice from
Tenant (or such longer period as may be reasonably required in the event that
any such repair, maintenance, cleaning or lighting cannot be completed within
said thirty (30) day period), Tenant shall have the right to perform such
obligation of Landlord. If Tenant performs any such obligation of Landlord,
Landlord shall pay to Tenant the reasonable cost thereof within thirty (30) days
after notice from Tenant, provided, however, that in no event shall Tenant have
the right to offset or deduct the amount thereof against any payment of rent due
hereunder.

      If an emergency occurs where a repair is required to be done immediately
in order to avoid imminent danger to persons or material damage to the Premises,
Tenant shall have the right to self-help consistent with the immediately
preceding grammatical paragraph of this Section 10.7 after giving Landlord only
such notice as is reasonable under the circumstances, provided, however, that
formal notice shall be promptly given thereafter. However, the right of
self-help afforded to Tenant in this Section 10.7 shall be carefully and
judiciously exercised by Tenant, it being understood and agreed that except in
the case of an emergency, Landlord shall be given sufficient opportunity to take
the action required of Landlord to avoid such default, in order to avoid any
conflict with respect to whether or not self-help should have been availed of by
Tenant, or with respect to the reasonableness of the expenses incurred by
Tenant.

10.8  BROKERAGE.

      Each party warrants and represents to the other party that it has had no
dealings with any broker or agent in connection with this Lease other than Lynch
Murphy Walsh & Partners ("Broker") and covenants to defend with counsel
reasonably approved by such other party, hold harmless and indemnify such other
party from and against any and all cost, expense or liability arising from any
breach of the foregoing warranty and representation.

10.9  APPLICABLE LAW AND CONSTRUCTION.

      This Lease shall be governed by and construed in accordance with the laws
of the state in which the Premises are located. If any term, covenant, condition
or provision of this Lease or the application thereof to any person or
circumstances shall be declared invalid, or unenforceable by the final ruling of
a court of competent jurisdiction having final review, the remaining terms,
covenants, conditions and provisions of this Lease and their application to
persons or circumstances shall not be affected thereby and shall continue to be
enforced and recognized as valid agreements of the parties, and in the place of
such invalid or unenforceable provision, there shall be substituted a

                                      -57-
<PAGE>

like, but valid and enforceable provision which comports to the findings of the
aforesaid court and most nearly accomplishes the original intention of the
parties.

      There are no prior oral or written agreements between Landlord and Tenant
affecting this Lease. This Lease may be amended, and the provisions hereof may
be waived or modified, only by instruments in writing executed by Landlord and
Tenant.

      The titles of the several Articles and Sections contained herein are for
convenience only and shall not be considered in construing this Lease.

      Unless repugnant to the context, the words "Landlord" and "Tenant"
appearing in this Lease shall be construed to mean those named above and their
respective heirs, executors, administrators, successors and assigns, and those
claiming through or under them respectively. If there be more than one tenant
the obligations imposed by this Lease upon Tenant shall be joint and several.

10.10 SUBMISSION NOT AN OFFER.

      The submission of a draft of this Lease or a summary of some or all of its
provisions does not constitute an offer to lease or demise the Premises, it
being understood and agreed that neither Landlord nor Tenant shall be legally
bound with respect to the leasing of the Premises unless and until this Lease
has been executed by both Landlord and Tenant and a fully executed copy
delivered to each of them.

10.11 SECURITY DEPOSIT.

Tenant will provide within five (5) business days of the date hereof and as a
condition of this Lease and shall keep in effect throughout the Term, a standby
letter of credit in the Security Deposit Amount (as hereinafter defined) as
security for the Performance of the obligations of Tenant hereunder in
accordance with the following requirements. As used herein, the term "Security
Deposit Amount" shall mean and refer to the sum of the $300,000 plus the Annual
Fixed Rent Rate, as the same may be adjusted in accordance with Section 4.1(b),
Section 4.1(c), Section 4.1(d), Section 10.12, Section 10.13 and Section 10.14
hereof. On the effective date of any such adjustment of the Annual Fixed Rent
Rate, Tenant shall provide a replacement letter of credit for the amount of the
adjusted Security Deposit Amount. Any letter of credit provided by Tenant shall
be in the form of Exhibit L attached hereto and shall be drawn on BayBank, N.A.
(or another Boston clearinghouse bank reasonably satisfactory to Landlord) for
the Security Deposit Amount in Landlord's favor. Tenant shall provide for
replacements thereto to be issued and

                                      -58-
<PAGE>

delivered to Landlord at least 30 days prior to the expiration of the then
effective letter of credit, time being of the essence. The letter of credit
shall be payable to Landlord upon presentation at the issuing bank's offices in
Boston of a sight draft signed by Landlord stating that Landlord is entitled
thereto, and a certification of Landlord in the form of Exhibit A attached to
said Exhibit L (the "Default Certification"). Landlord agrees that it shall
present such sight draft and Default Certification only if Tenant defaults in
performance of its obligations hereunder and such default shall have continued
past any applicable notice and grace period. Any letter of credit in effect
during the last year of the Term shall expire no earlier than 30 days after the
expiration of the Term of the Lease. The letter of credit shall provide for
partial draws and shall be assignable by Landlord. If Tenant shall fail to
perform any of its obligations under this Lease including a failure timely to
provide a replacement letter of credit, Landlord may, but shall not be obligated
to, apply the letter of credit to the extent necessary to cure the default, and
Tenant shall be obligated to reinstate the letter of credit to the Security
Deposit Amount then in effect. Tenant shall not have the right to call upon
Landlord to draw upon all or any part of the letter of credit to cure any
default or fulfill any obligation of Tenant, but such use shall be solely in the
discretion of Landlord. Provided that Landlord gives Tenant notice of the name
of such grantee or transferee, upon any conveyance by Landlord of its interest
under this Lease, the letter of credit may be delivered by Landlord to
Landlord's grantee or transferee. Upon any such delivery, Tenant hereby releases
Landlord herein named of any and all liability with respect to the letter of
credit, its application and return, and Tenant agrees to look solely to such
grantee or transferee. It is further understood that this provision shall also
apply to subsequent grantees and transferees.

10.12 OPTIONS TO EXTEND.

      (a) Tenant shall have two (2) options to extend the Term of this Lease
(the "Options to Extend") for successive periods of five (5) years each (the
"Extension Periods"), subject to and on the terms set forth herein. Tenant may
only exercise the Options to Extend with respect to the entire Premises. If
Tenant shall desire to exercise any Option to Extend, it shall give Landlord a
notice (the "Inquiry Notice") of such desire not later than fifteen (15) months
prior to the expiration of the Initial Term of this Lease or the preceding
Extension Period, as the case may be. Thereafter, the Fair Market Rent (as
defined in Subsection (c) below) for the applicable Extension Period shall be
determined in accordance with Subsection (d) below. After the applicable Fair
Market Rent has been so determined, Tenant shall exercise each Option to Extend
by giving Landlord notice (the "Exercise Notice")

                                      -59-
<PAGE>

of its election to do so not later than twelve (12) months prior to the
expiration of the Initial Term of this Lease, or the preceding Extension Period,
as the case may be. If Tenant fails to timely give either the Inquiry Notice or
the Exercise Notice to Landlord with respect to any Option to Extend, Tenant
shall be conclusively deemed to have waived such Option to Extend hereunder.

      (b) Notwithstanding any contrary provision of this Lease, each Option to
Extend and any exercise by Tenant thereof shall be void and of no force or
effect unless on the dates Tenant gives Landlord its Inquiry Notice and Exercise
Notice for each Option to Extend and on the date of commencement of the each
Extension Period (i) this Lease is in full force and effect, (ii) there is no
Event of Default of Tenant under this Lease, and (iii) Tenant has not assigned
or subleased (or agreed to assign or sublease) more than fifty percent (50%) of
the rentable floor area then comprising the Premises.

      (c) All of the terms, provisions, covenants, and conditions of this Lease
shall continue to apply during each Extension Period, except that the Annual
Fixed Rent Rate during each Extension Period (the "Extension Rent") shall be
equal to the fair market rent for the Premises determined as of the date twelve
(12) months prior to expiration of the Initial Term or the preceding Extension,
Period, as the case may be, in accordance with the procedure set forth in
Subsection (d) below (the "Fair Market Rent").

      (d) The Fair Market Rent for each Extension Period shall be determined as
follows: Within five (5) days after Tenant gives Landlord its Inquiry Notice
with respect to any Option to Extend, Landlord shall give Tenant notice of
Landlord's determination of the Fair Market Rent for the applicable Extension
Period. Within ten (10) days after Tenant receives such notice, Tenant shall
notify Landlord of its agreement with or objection to Landlord's determination
of the Fair Market Rent, whereupon the Fair Market Rent shall be determined by
arbitration conducted in the manner set forth below. If Tenant does not notify
Landlord within such ten (10) day period of Tenant's agreement with or objection
to Landlord's determination of the Fair Market Rent, then the Fair Market Rent
for the applicable Extension Period shall be deemed to fee Landlord's
determination of the Fair Market Rent as set forth in the notice from Landlord
described in this subsection.

      (e) If Tenant notifies Landlord of Tenant's objection to Landlord's
determination of Fair Market Rent under the preceding subsection, such notice
shall also set forth a request for arbitration and Tenant's appointment of a
commercial real estate broker having at least ten (10) years experience in the
commercial

                                     -60-
<PAGE>

leasing market in the City of Cambridge, Massachusetts (an "Arbitrator"). Within
five (5) days thereafter, Landlord shall by notice to Tenant appoint a second
Arbitrator. Each Arbitrator shall be advised to determine the Fair Market Rent
for the applicable Extension Period within thirty (30) days after Landlord's
appointment of the second Arbitrator. On or before the expiration of such thirty
(30) day period, the two Arbitrators shall confer to compare their respective
determinations of the Fair Market Rent. If the difference between the amounts so
determined by the two Arbitrators is less than or equal to ten percent (10%) of
the lower of said amounts then the final determination, of the Fair Market Rent
shall be equal to the average of said amounts. If such difference between said
amounts is greater than ten percent (10%), then the two arbitrators shall have
ten (10) days thereafter to appoint a third Arbitrator (the "Third Arbitrator"),
who shall be instructed to determine the Fair Market Rent for the applicable
Extension Period within ten (10) days after its appointment by selecting one of
the amounts determined by the other two Arbitrators. Each party shall bear the
cost of the Arbitrator selected by such party. The cost for the Third
Arbitrator, if any, shall be shared equally by Landlord and Tenant.

10.13 RIGHT OF FIRST REFUSAL.

      Provided that at the time Tenant exercises its rights under this Section:
(i) there is then outstanding no Event of Default of Tenant under this Lease,
(ii) Tenant has not assigned or subleased (or agreed to assign or sublease) more
than fifty percent (50%) of the rentable floor area then comprising the
Premises, and, (iii) at least two Lease Years have elapsed under the Term of
this Lease, then in the event that Landlord receives any written offer (an
"Offer") from any third party (a "Third Party Offeror") to lease any space in
the Building which may thereafter become available to lease ("Available Space"),
Landlord shall furnish Tenant with a notice (the "ROFR Notice") offering Tenant
the option to lease such Available Space on the terms and conditions specified
in the Offer (a "Right of First Refusal"). In the event Tenant elects to
exercise its Right of First Refusal with respect to any such Available Space,
Tenant shall send to Landlord notice of such election together with a certified
or bank check in the amount of one twelfth (1/12) of the annual fixed rent rate
for said Available Space (as set forth in the Offer) and said Available Space
shall thereafter be leased to Tenant on the terms and conditions set forth in
the Offer. In the event such notice and check are not sent to Landlord within
five (5) business days after the ROFR Notice with respect to any such Available
Space, or if after giving notice of election of its Right of First Refusal with
respect, to any such Available Space, Tenant shall fail to perform its
obligation to lease such Available Space, then

                                     -61-
<PAGE>

Landlord may retain the deposit delivered by Tenant and may lease said
Available Space to the Third Party Offerer, or any affiliate thereof, upon the
terms and conditions contained in the Offer.

      Any person dealing with such Available Space may without further inquiry
conclusively rely upon a representation in a certificate or notice of lease of
the Landlord or its successors in title to said Available Space as to whether or
not the provisions of this section have been satisfied.

10.14 OPTION TO EXPAND.

      (a) Tenant shall have an option to cause Landlord to construct an addition
to the Premises (the "Option to Expand") to include space on the second floor of
the Premises in substantially the location shown on Exhibit I attached hereto
(the "Second Floor Space"). If Tenant desires to exercise its Option to Expand,
Tenant shall give a notice to Landlord of its desire to do so. In the event of
such notice by Tenant, Landlord agrees to negotiate in good faith with Tenant
the terms of an amendment to this Lease expanding the Premises to include the
Second Floor Space, Tenant hereby agreeing that the hard and soft costs of
constructing the Second Floor Space must be recovered by Landlord over the term
of the lease of the Second Floor Space. In the event that Landlord and Tenant
shall fail to execute such an amendment to this Lease within 90 days after such
notice, then Tenant shall have the rights set forth in Subsection 10.14(b)
below.

      (b) In the event that Landlord and Tenant shall fail to reach an agreement
under the foregoing Subsection 10.14(a) pursuant to which Landlord shall
construct the Second Floor Space, then Tenant shall have the right to construct
said Second Floor Space upon terms and conditions reasonably acceptable to
Landlord, which terms and conditions shall include at least the following: (i)
Tenant shall give Landlord notice of its election to construct the Second Floor
Space at least 6 months prior to Tenant's anticipated date for commencing
construction thereof; (ii) all required improvements' for the Second Floor Space
(including, without limitation, any and all structural alterations or additions
to the Building or the Premises) shall be made at Tenant's sole cost and expense
pursuant to plans and specifications prepared by Tenant, reasonably approved by
Landlord, and approved by any mortgagee(s) of Landlord; (iii) the general
contractor for the Second Floor Space shall be subject to the reasonable
approval of Landlord and any mortgagee(s) of Landlord; (iv) all improvements
made to the Second Floor Space and to the Building and the Premises in
connection therewith shall become part of the Premises upon installation thereof
by Tenant; and (v) Tenant shall comply with all provisions of this Lease with
respect to such work, including, without limitation, Section 5.1.5

                                      -62-
<PAGE>

hereof. Upon completion of such Second Floor Space, Landlord and Tenant shall
execute an amendment to this Lease modifying the definition of the Premises
Design Floor Area, the Building Design Floor Area, the Premises Usable Floor
Area, the Annual Fixed Rent Rate, Tenant's Proportionate Fraction and such other
terms as may be modified or affected by the inclusion of the Second Floor Space
in the Premises. In the event that the Second Floor Space is constructed by
Tenant pursuant to this Subsection 10.14(b), the Annual Fixed Rent for the
Second Floor Space shall be equal to the product of the Shell Rent Rate then in
effect multiplied by the rentable floor area of the Second Floor Space.

      (c) Notwithstanding any contrary provision of this Lease, the Option to
Expand and Tenant's exercise thereof under either of the foregoing Subsections
(a) or (b) of this Section 10.14 shall be void and of no force or effect unless:
(i) on the date Tenant notifies Landlord that it is exercising the Option to
Expand, on the date, of execution of the lease amendments described above, and
on the date of occupancy of the Second Floor Space pursuant thereto (A) this
Lease is in full force and effect, (B) there is no Event of Default of Tenant
under this Lease, and (C) Tenant has not assigned or subleased (or agreed to
assign or sublease) more than fifty percent (50%) of the rentable floor area
then comprising the Premises; and (ii) Tenant granted permission to Landlord to
make the structural modifications to the Building and the Premises necessary for
the Second Floor.

      10.15 CONFIDENTIAL INFORMATION. Landlord hereby agrees that any and all
knowledge, information, data, materials, trade secrets, and other work product
of a confidential nature gained, obtained, derived, produced, generated or
otherwise acquired by Landlord with respect to Tenant's business (collectively
"Confidential Information") shall be kept confidential. Landlord shall use
diligent efforts to ensure that no Confidential Information is revealed,
divulged, communicated, related, or described to any person or entity without
the written consent of Landlord, except as may be required by applicable law.

      10.16. SIGNAGE. Tenant shall be permitted, at its sole cost and expense,
to install and maintain signs on the exterior of the Building which read
"Vertex" (or something similar reasonably approved by Landlord), provided that:
(i) the size, location, quality, color and style of such signs shall be subject
to Landlord's approval, such approval not to be unreasonably withheld or
delayed, and (ii) such signs shall be subject to limitations of applicable law,
including, without limitation, the Cambridge Zoning By-Law, as amended from time
to time. Tenant shall secure all permits necessary for the installation of such
signs at its sole cost and expense. Upon the expiration or sooner termination of
the Term of this Lease, Tenant shall remove such signs and

                                     -63-
<PAGE>

repair any damage resulting therefrom at Tenant's sole cost and expense.

      WITNESS the execution hereof under seal on the day and year first above
written.

                      LANDLORD:    FORT WASHINGTON LIMITED PARTNERSHIP

                                   By:  Fort Washington, Inc., its
                                        general partner

                                        By: /s/ David E. Clem
                                            --------------------------
                                        Its: President
                                        hereunto duly authorized

                      TENANT:      VERTEX PHARMACEUTICALS INCORPORATED

                                   By: /s/ Richard H. Aldrich
                                       -------------------------------
                                       Its: Senior Vice President
                                       hereunto duly authorized

                                       [ILLEGIBLE]

                                      -64-

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