Document:

<PAGE>
                                                                    Exhibit 10.9

                                      LEASE

                                 by and between

                               KENDALL SQUARE, LLC

                                    LANDLORD,

                                       and

                              GENZYME CORPORATION,

                                     TENANT

<PAGE>

                                    ARTICLE I

                                 REFERENCE DATA

1.1   SUBJECTS REFERRED TO

ANNUAL FIXED RENT RATE PER         Subject to Sections 4.1(b), 10.11 and 10.12
RENTABLE SQUARE FOOTAGE            hereof, an amount determined by (a)
OF PREMISES:                       dividing (i) the Project Cost by (ii) the
                                   Rentable Square Footage of Building and (b)
                                   multiplying the result thereof by twelve
                                   percent (12%).

APPROXIMATE TERM:                  Subject to Section 10.12 hereof, fifteen
                                   (15) years.

BUILDING:                          The building now known as Building D,
                                   Cambridge Research Park, Cambridge,
                                   Massachusetts, to be constructed by
                                   Landlord and containing approximately
                                   300,000 rentable square feet ("r.s.f."), of
                                   which approximately 15,000 r.s.f. shall be
                                   ground floor retail space (the "Retail
                                   Space").  Tenant shall have the right to
                                   name the Building "The Genzyme
                                   Building," "Genzyme Center" or a similar
                                   name, but in each case including the street
                                   address of the Building.

COMPLEX:                           An approximately ten acre parcel of land,
                                   including the Lot and all buildings thereon
                                   and improvements thereto hereafter
                                   constructed by Landlord or by an Affiliate
                                   of Landlord (as such term is defined below
                                   in this Section 1.1), in Cambridge,
                                   Massachusetts currently owned by Landlord
                                   and shown on a plan entitled "Master Plan"
                                   dated June 1, 1999, Scale 1"=50', a reduced
                                   copy of which is attached as Exhibit A
                                   hereto, but excluding any portion of the
                                   Complex which is not hereafter owned by
                                   Landlord or an Affiliate of Landlord.

EXPANSION SPACES:                  As determined and defined in Section 10.11
                                   hereof.

INITIAL ESTIMATED ANNUAL
ADDITIONAL RENT FOR BUILDING:      $2,467,500.00 based upon $10.50/r.s.f.

<PAGE>

INITIAL ESTIMATED ANNUAL
ADDITIONAL RENT FOR COMPLEX:       $352,500.00 based upon $1.50/r.s.f.

LANDLORD:                          Kendall Square, LLC, formerly known as
                                   Cambridge Research Park, LLC, a Delaware
                                   limited liability company.

LANDLORD'S ARCHITECT:              As determined pursuant to Section 3.1.1
                                   hereof.

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

LANDLORD'S CONTRACTOR:             As determined pursuant to Section 3.1.1
                                   hereof.

LANDLORD'S REPRESENTATIVE:         Each of David E. Clem and Robert L. Green

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.

LOT:                               The land on which the Building is to be
                                   constructed and shown on Exhibit A as
                                   being generally bounded by Atheneum
                                   Street, Kendall Street, Easement, and land of
                                   others, the legal description of which (and a
                                   plan for which) shall be prepared by
                                   Landlord prior to the commencement of
                                   construction of the Base Building
                                   Improvements by Landlord and attached
                                   hereto as Exhibit A-1 and which plan shall
                                   be attached hereto as Exhibit A-2.

MANAGING AGENT:                    To be determined by Landlord based upon a
                                   list of managing agents proposed from time
                                   to time by Landlord and approved by
                                   Tenant, such approval not to be unreasonably
                                   withheld, conditioned or delayed.

                                     - 2 -
<PAGE>

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 uses and accessory uses
                                   customarily incidental to general office
                                   uses.

PREMISES:                          Those portions of the Building demised to
                                   Tenant under this Lease from time to time,
                                   as such portions are determined pursuant to
                                   Section 3.1.1 hereof and subject to
                                   Sections 5.1.2 and 10.11 hereof.

PROJECT COST:                      As determined pursuant to Section 3.1.1
                                   hereof.

COMMERCIAL  LIABILITY
INSURANCE LIMITS:                  Bodily Injury:    $10,000,000
                                   Property Damage:  $10,000,000

RENTABLE SQUARE                    As determined pursuant to Section 2.3
FOOTAGE OF BUILDING:               hereof, but the Building is currently
                                   estimated to consist of 300,000 r.s.f. of
                                   space.

RENTABLE SQUARE                    As determined pursuant to Section 2.3
FOOTAGE OF PREMISES:               hereof, but the Premises are currently
                                   estimated to consist initially of
                                   approximately 235,000 r.s.f. of space in the
                                   Building to be constructed by Landlord on the
                                   Lot; and the Rentable Square Footage of
                                   Premises shall be increased at such time as
                                   each Expansion Space is demised to Tenant;
                                   and the Expansion Spaces are currently
                                   estimated to consist of approximately 50,000
                                   r.s.f. of space, in the aggregate, in the
                                   Building to be constructed by Landlord on the
                                   Lot, subject to Sections 5.1.2 and 10.11
                                   hereof.

SCHEDULED SUBSTANTIAL
COMPLETION DATE:                   September 1, 2002

SCHEDULED TERM
COMMENCEMENT DATE:                 December 1, 2002

                                     - 3 -
<PAGE>

TENANT:                            Genzyme Corporation, a Massachusetts
                                   corporation.

TENANT'S ADDRESS                   One Kendall Square
(For Notice and Billing):          Building 1400
                                   Cambridge, Massachusetts 02139

TENANT'S ARCHITECT:                To be determined by Tenant.

TENANT'S PROPORTIONATE
FRACTION FOR BUILDING:             As determined and as the same may be
                                   adjusted pursuant to Sections 2.3 and
                                   10.11 hereof.

TENANT'S PROPORTIONATE
FRACTION FOR COMPLEX:              As determined and as the same may be
                                   adjusted pursuant to Sections 2.3 and
                                   10.11 hereof.

TENANT'S CONSTRUCTION
REPRESENTATIVE:                    To be identified by Tenant by notice to
                                   Landlord.

TENANT'S LEASE REPRESENTATIVE:     Evan M. Lebson

TERM COMMENCEMENT DATE:            The earlier of (a) the date that is
                                   ninety-one (91) days after the Substantial
                                   Completion Date or (b) the date on which
                                   Tenant's personnel occupy all or any portion
                                   of the Premises for the conduct of any aspect
                                   of Tenant's business (as opposed to the
                                   conduct by Tenant of any of Tenant's Work or
                                   the installation in the Premises of fixtures,
                                   furnishings, equipment or personal property);
                                   provided, however, if Tenant only occupies a
                                   portion of the Premises for the conduct of
                                   its business prior to the date determined by
                                   the preceding clause (a), Tenant shall only
                                   be obligated to pay Rent calculated with
                                   respect to such portion of the Premises so
                                   occupied.

TERM EXPIRATION DATE:              The last day of the fifteenth Lease Year,
                                   subject to two (2) Options to Extend for
                                   successive periods of ten (10) years each, in
                                   accordance with Section 10.12.

                                     - 4 -
<PAGE>

The following capitalized terms shall have the respective meanings set forth
below or as referenced in this Lease:

ADA Requirements: 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.

Additional Rent Adjustment Date: As defined in Section 4.2.4

Additional Rent: All rent, charges and other sums, other than Fixed Rent, due
Landlord pursuant to this Lease.

Affiliate of Landlord: A person or entity controlled by, controlling or under
common control with Landlord.

Affiliate of Tenant: Any entity controlled by, controlling or under common
control with Tenant.

Annual Building Maintenance and Operation Charge: As defined in Section 4.2.4

Annual Complex Maintenance and Operation Charge: As defined in Section 4.2.4.

Annual Maintenance Charge: The sum of the Annual Building Maintenance and
Operation Charge, the Annual Complex Maintenance and Operation Charge and the
Nonstandard Charge.

Annual Tax, Insurance and Utility Charge: The sum of the Initial Tax Charge, the
Initial Insurance Charge and the Initial Utility Charge.

Appraiser: A commercial real estate broker having at least ten (10) years
experience in the commercial leasing market in the City of Cambridge,
Massachusetts.

Approved Contractor: As defined in Section 3.2.1.

Arbitrator: As defined in Section 10.13.

Architect's Contract: As defined in Section 3.1.1.

Availability Notice: As defined in Section 10.18

Base Building Improvements: As defined in Section 3.1.1.

Brokers: Trammel Crow Company and Insignia/ESG.

Building Common Areas: Interior and exterior common areas and facilities of the
Building and the Lot.

Casualty Restoration Completion Date: As defined in Section 6.1.

CDD: Community Development Department of the City of Cambridge, Massachusetts.

Change Orders: As defined in Section 3.1.2.

                                     - 5 -
<PAGE>

Chapter 91 Determination: As defined in Section 3.1.4.

Commencement Date: As defined and determined pursuant to Section 2.2.

Complex Common Areas: Interior and exterior common areas and facilities of the
Complex.

Confidential Information: 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.

Construction Contract: As defined in Section 3.1.1.

Construction Documents: As defined in Section 3.2.1.

Design Development Documents: As defined in Section 3.1.1.

Design Process Letter: Letter dated January 17, 2000 from David Clem of Lyme
Properties, LLC to Henri Termeer, President and CEO of Tenant.

Developer's Fee: As defined in Section 3.1.1.

Development Approvals: As defined in Section 3.1.4.

Early Occupancy Space: As defined in Section 10.11.

Environmental Agreement: The Environmental Agreement between Landlord and Tenant
in the form of Exhibit E hereto.

Environmental Remediation: As defined in Section 3.1.3.

Estimated Annual Additional Rent: Landlord's estimate of the total amount of
Additional Rent which may be due from Tenant for any particular Lease Year with
respect to the Building, Lot and Complex.

Estimated Annual Maintenance Charge: Landlord's reasonable estimate of the
Annual Maintenance Charge.

Estimated Annual Tax, Insurance and Utility Charge: Landlord's reasonable
estimate of the Annual Tax, Insurance and Utility Charge.

Event of Default: As defined in Section 7.1.

Excluded Taxes: Any income taxes, excess profits taxes, excise taxes, franchise
taxes, or any taxes or assessments with respect to the Garage and other
buildings leased or available for lease (and the parcels of land upon which such
buildings are situated), other than the Building, the Lot and any building or
portion of a building in the Complex which is not available for lease (and the
parcel(s) of land on which the same may be located), in the Complex.

Exercise Notice: As defined in Section 10.12.

                                     - 6 -
<PAGE>

Expansion Spaces: As defined in Section 10.11.

Extension Periods: As defined in Section 10.12.

Extension Rent: As defined in Section 10.12.

Fair Market Rent: As defined in Section 10.12.

Final Design Documents: As defined in Section 3.1.1.

Final Project Budget: All hard costs and soft costs incurred by Landlord in
connection with the construction of the Base Building Improvements as reflected
in a final project budget to be mutually approved by Landlord and Tenant, as
provided in Section 3.1.1.

First Expansion Space: As defined in Section 10.11.

Fixed Rent: As defined in Section 4.1.

Garage Owner: The owner of the Garage.

Garage Parking Spaces: Tenant's Parking Spaces and the Valet Parking Spaces.

Garage: The underground parking structure to be constructed and/or owned by
Landlord or an Affiliate of Landlord south of Kendall Street, as shown on
Exhibit A hereto.

Holder: As defined in Section 8.1.

Impositions: As defined in Section 4.2.4(1)(a).

Indemnified Parties: As defined in Section 5.1.6.

Initial Estimated Annual Additional Rent for Building: As defined in
Section 4.2.

Inquiry Notice: As defined in Section 10.12.

Labor Unrest: As defined in Section 3.2.1.

Landlord Milestone Date: As defined in Section 3.2.

Landlord's Statement: As defined in Section 4.2.4.

Landlord's Work: The construction of the Base Building Improvements in
accordance with the Final Design Documents as affected by Change Orders.

MCP: Massachusetts Contingency Plan, 310 CMR 40.0000 et seq., as amended.

MEPA Certificate: Certificate of the Secretary of Environmental Affairs on the
Final Environmental Impact Report dated April 15, 1999.

                                     - 7 -
<PAGE>

Minor Alteration: As defined in Section 3.2.1.

Mitigation Expenses: As defined in Section 3.1.1.

Mortgage: Mortgages, deeds of trust or other similar instruments evidencing
other voluntary liens or encumbrances, and modifications, consolidations,
extensions, renewals, replacements and substitutes thereof.

Nonstandard Charge: As defined in Section 4.2.4.

Nonstandard Costs: As defined in Section 4.2.4.

Option to Lease: The option to lease with respect to Building B executed by
Landlord and Tenant simultaneously with the execution hereof.

Options to Extend: As defined in Section 10.12.

Order of Conditions: Order of Conditions issued July 12, 1999 by the City of
Cambridge Conservation Commission.

Outside Completion Date: As defined in Section 3.2.

Parking Fee: As defined in Section 10.14.

Preliminary Design Concept: As defined in Section 3.1.1.

Progress Schedule: As set forth in Exhibit B hereto.

PTDM Approval: The PTDM Decision, the PTDM Letter and PTDM Plan.

PTDM Decision: PTDM Ordinance Final Decision issued April 20, 1999 by the CDD.

PTDM Letter: The Letter dated April 20, 1999 to Robert L. Green of Lyme
Properties from Susanne Rasmussen of the CDD attached to the PTDM Decision.

PTDM Plan: Landlord's Parking Transportation Demand Management Plan dated April
9, 1999.

PUD Approval: The PUD Permit and the Settlement Agreement.

PUD Permit: A Special Permit issued by the City of Cambridge Planning Board,
Case No. PB #141 filed April 7, 1999 (the "PUD Permit"), recorded with the
Middlesex South District Registry of Deeds (the "Registry") in Book 31137, Page
89.

Punch List Items: 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.

Relevant Market: As defined in Section 10.12(b).

                                     - 8 -
<PAGE>

Remediation Documents: The documents set forth on Exhibit E-1 hereto.

Rentable Square Feet, Rentable Square Footage or r.s.f.: The rentable square
footage of the space in question as measured in accordance with the standard set
forth in Section 2.3.

Response Action Outcome: As such term is defined in the MCP.

ROFR Notice: As defined in Section 10.18.

ROFR Space: As defined in Section 10.18.

ROFR: As defined in Section 10.18.

Schematic Design Documents: As defined in Section 3.1.1.

Settlement Agreement: A Settlement Agreement dated May 24, 1999 among Barbara
Broussard, Mary DeFreitas, the East Cambridge Planning Team and Landlord.

SNDA: Subordination, Non-disturbance and Attornment Agreement.

Subdivision: As defined in Section 9.1.6.

Sublease Costs: As defined in Section 5.2.1.

Sublease Profits: As defined in Section 5.2.1.

Subsequent Approvals: As defined in Section 3.1.4.

Substantial Completion Date: As defined in Section 3.2.

Substitute Taxes: As defined in Section 4.2.1.

Tenant Delay: As defined in Section 3.1.1.

Tenant's Property: 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.

Tenant's Proportionate Fraction for Building: As defined in Section 2.3

Tenant's Proportionate Fraction for Complex: As defined in Section 2.3.

Tenant's Work: As defined in Section 3.2.1.

TI Factor: As defined in Section 10.11.

Title Exceptions: Item Nos. 2 through 12, inclusive set forth in Schedule B,
Part 1 of the Title Policy.

                                     - 9 -
<PAGE>

Title Policy: Owner's Policy No. 136-00-336684 dated August 19, 1998, issued by
Lawyer's Title Insurance Corporation.

TMA: The Charles River Transportation Management Association or any other
transportation management association of which Landlord is a member.

Transfer: As defined in Section 9.1.6.

Utilities: As defined in Section 4.2.3.

Utility Service Provider or Utility Service Providers: As defined in
Section 4.2.3.

Utility Services: As defined in Section 4.2.3.

Valet Parking Fee: As defined in Section 10.14.

Valet Parking Spaces: As defined in Section 10.14.

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 Complex
EXHIBIT A-1                       Legal Description
EXHIBIT A-2                       Plan showing Lot
EXHIBIT A-3                       Confirmation of Commencement Date and
                                    Rentable Square Footage
EXHIBIT A-4                       Confirmation of Location of Expansion
                                     Spaces
EXHIBIT B                         Progress Schedule
EXHIBIT B-1                       Project Budget Form
EXHIBIT B-2                       Annual Maintenance Charge Categories
EXHIBIT C                         Rules and Regulations
EXHIBIT D                         SNDA Form
EXHIBIT E                         Environmental Agreement
EXHIBIT E-1                       Remediation Documents

1.3   TABLE OF CONTENTS

<TABLE>
<S>                                                                                        <C>
ARTICLE I..............................................................................     1

   1.1  SUBJECTS REFERRED TO...........................................................     1

   1.2  EXHIBITS.......................................................................    10

   1.3  TABLE OF CONTENTS..............................................................    10
</TABLE>

                                     - 10 -
<PAGE>

<TABLE>
<S>                                                                                        <C>
ARTICLE II.............................................................................     1

   2.1  PREMISES.......................................................................     1

   2.2  TERM...........................................................................     2

   2.3  MODIFICATION OF CERTAIN DEFINITIONS; CERTIFICATE REGARDING.....................     2

ARTICLE III............................................................................     3

   3.1  INITIAL DESIGN AND CONSTRUCTION................................................     3

        3.1.1  Architect Selection Process, Development of Design Documents,
               Landlord's Contractor and Project Cost..................................     3

        3.1.2  Change Orders...........................................................    10

        3.1.3  Environmental Remediation...............................................    12

        3.1.4  Development Approvals and Title Exceptions..............................    12

   3.2  PREPARATION OF PREMISES FOR PERFORMANCE OF TENANT'S WORK.......................    14

        3.2.1. Performance of Tenant's Work............................................    16

   3.3  GENERAL PROVISIONS APPLICABLE TO CONSTRUCTION..................................    21

   3.4  REPRESENTATIVES................................................................    21

   3.5  LANDLORD INDEMNITY AND CORRECTION OF LANDLORD'S WORK...........................    21

ARTICLE IV.............................................................................    22

   4.1  FIXED RENT.....................................................................    22

   4.2  ADDITIONAL RENT................................................................    22

        4.2.1  Real Estate Taxes.......................................................    23

        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........    26

               4.2.2.4  Certain Requirements Applicable to Insurance Policies..........    26

               4.2.2.5  Waiver of Subrogation..........................................    26

        4.2.3  Utilities for Premises..................................................    27

        4.2.4  Common Area Maintenance and Expenses....................................    28

        4.2.5  Payments on Account of Taxes, Insurance and Utilities...................    32

   4.3  LATE PAYMENT OF RENT...........................................................    33

ARTICLE V..............................................................................    34

   5.1  AFFIRMATIVE COVENANTS..........................................................    34
</TABLE>

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<PAGE>

<TABLE>
<S>                                                                                        <C>
        5.1.1  Perform Obligations.....................................................    34

        5.1.2  Occupancy and Use.......................................................    34

        5.1.3  Repair and Maintenance..................................................    35

        5.1.4  Compliance with Law.....................................................    35

        5.1.5  Tenant's Work...........................................................    37

        5.1.6  Indemnity...............................................................    37

        5.1.7  Landlord's Right to Enter...............................................    38

        5.1.8  Personal Property at Tenant's Risk......................................    38

        5.1.9  Payment of Landlord's Cost of Enforcement...............................    39

        5.1.10 Yield Up................................................................    39

        5.1.11 Estoppel Certificate....................................................    39

        5.1.12 Landlord's Expenses Re: Consents........................................    40

        5.1.13 Rules and Regulations...................................................    40

        5.1.14 Loading.................................................................    40

        5.1.15 Holdover................................................................    40

   5.2  NEGATIVE COVENANTS.............................................................    40

        5.2.1  Assignment and Subletting...............................................    41

        5.2.2  Nuisance................................................................    42

        5.2.3  Installation, Alterations or Additions..................................    42

ARTICLE VI.............................................................................    43

6.1     DAMAGE BY FIRE.................................................................    43

   6.2  CONDEMNATION...................................................................    46

   6.3  AWARD..........................................................................    47

ARTICLE VII............................................................................    47

   7.1  EVENTS OF DEFAULT..............................................................    47

   7.2  REMEDIES.......................................................................    48

   7.3  REMEDIES CUMULATIVE............................................................    49

   7.4  LANDLORD'S RIGHT TO CURE DEFAULTS..............................................    49

   7.5  EFFECT OF WAIVERS OF DEFAULT...................................................    49

   7.6  NO ACCORD AND SATISFACTION.....................................................    50

ARTICLE VIII...........................................................................    50

   8.1  RIGHTS OF MORTGAGE HOLDERS.....................................................    50

   8.2  SUPERIORITY OF LEASE; OPTION TO SUBORDINATE....................................    51
</TABLE>

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<PAGE>

<TABLE>
<S>                                                                                        <C>
   8.3  LEASE AMENDMENTS...............................................................    51

ARTICLE IX.............................................................................    52

   9.1  AFFIRMATIVE COVENANTS..........................................................    52

        9.1.1  Perform Obligations.....................................................    52

        9.1.2  Repairs.................................................................    52

        9.1.3  Compliance with Law.....................................................    52

        9.1.4  Indemnity...............................................................    53

        9.1.5  Estoppel Certificate....................................................    53

        9.1.6  Subdivision.............................................................    53

ARTICLE X..............................................................................    54

   10.1    NOTICES FROM ONE PARTY TO THE OTHER.........................................    54

   10.2    QUIET ENJOYMENT.............................................................    54

   10.3    EASEMENTS; CHANGES TO LOT LINES.............................................    54

   10.4    LEASE NOT TO BE RECORDED....................................................    54

   10.5    BIND AND INURE; LIMITATION OF LANDLORD'S LIABILITY..........................    55

   10.6    ACTS OF GOD.................................................................    55

   10.7    LANDLORD'S DEFAULT..........................................................    55

   10.8    BROKERAGE...................................................................    56

   10.9    APPLICABLE LAW AND CONSTRUCTION.............................................    56

   10.10   SUBMISSION NOT AN OFFER.....................................................    57

   10.11   EXPANSION OF PREMISES.......................................................    57

   10.12   OPTIONS TO EXTEND...........................................................    59

   10.13   ARBITRATION.................................................................    61

   10.14   PARKING.....................................................................    62

   10.15   CONFIDENTIAL INFORMATION....................................................    63

   10.16   SIGNAGE.....................................................................    63

   10.17   BUILDING B LEASE............................................................    64

   10.18   RIGHT OF FIRST REFUSAL......................................................    64

   10.19   RETAIL TENANTS..............................................................    65

   10.20   ACCESS......................................................................    65

   10.21   COOPERATION.................................................................    65
</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 areas and common facilities, if any, included in the Building, on
the Lot or in the Complex, (ii) the building service fixtures and equipment
serving the Premises, and (iii) subject to Section 10.14 hereof, (a) the right
to use that number of nonreserved parking spaces determined by dividing the
Rentable Square Footage (as the same shall be determined pursuant to Section 2.3
hereof and as the same may be increased pursuant to Section 10.11 hereof) by 500
("Tenant's Parking Spaces") in the underground parking garage located south of
Kendall Street as shown on Exhibit A hereto (the "Garage") and (b) the right to
use that number of nonreserved Valet Parking Spaces, as such term is defined and
such number is determined pursuant to Section 10.14 hereof. To the extent that
the Premises includes all of the rentable square footage on a particular floor
of the Building, Tenant shall have exclusive use of the common areas on such
floors, but Landlord shall have the rights set forth in the next paragraph
hereof.

      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, including the perimeter walls of the Premises, on the roof of the
Building, in mechanical penthouses and in any space in or adjacent to the
Premises used for shafts, stacks, pipes, conduits, wires and appurtenant
fixtures, ducts, electric or other utilities, telecommunications equipment or
other Building facilities, as well as the right of access (which right of access
shall be at reasonable times and upon reasonable notice, except in the case of
emergency) through the Premises for the purpose of operation, maintenance and
repair, 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); notwithstanding the
foregoing provisions of this clause (a), to the extent that the Premises include
all of the rentable square footage on a particular floor of the Building,
Landlord's right to install, repair, replace, use, maintain and relocate such
building service fixtures and equipment on such floor of the Building shall be
limited to placing or installing such building service fixtures and equipment in
shafts, pipes, stacks, conduits, chases and risers located within the central
core common area of such floor or in such other locations on such floor as may
be set forth in the Final Design Documents; (b) to construct, alter or relocate
any Building Common Areas and/or Complex Common Areas (provided that, except for
any construction, alteration or relocation of Complex Common Areas required or
permitted by the Development Approvals or Subsequent Approvals, no such
construction, alteration or relocation of any such Complex Common Areas shall
substantially or materially increase any payments due from Tenant under this
Lease and any such construction, alteration or relocation shall be

<PAGE>

substantially similar in quality and utility to Complex Common Areas being
altered or relocated or substantially similar to common areas of first-class,
mixed use projects in the Relevant Market); and (c) after construction of the
Building to construct, alter or relocate any Building Common Areas , subject to
approval by Tenant, such approval not to be unreasonably withheld, delayed or
conditioned.

2.2   TERM.

      To have and to hold for a period (the "Term") commencing on the Term
Commencement Date (as such term is defined in Section 1.1 hereof) (the
"Commencement Date") and continuing until the Term Expiration Date, unless
sooner terminated as provided herein, including Section 3.2 and Article VII
hereof, and subject to extension in accordance with the terms of Section 10.12
hereof.

2.3   MODIFICATION OF CERTAIN DEFINITIONS; CERTIFICATE REGARDING COMMENCEMENT
      DATE.

      Landlord and Tenant acknowledge that the actual rentable square footage of
the Premises, the Expansion Spaces (as such term is defined in Section 10.11
hereof) and the Building may, upon completion of construction of the Base
Building Improvements (as hereinafter defined), be different than the estimates
as set forth in Article I hereof. Accordingly, after completion of construction
of the Base Building Improvements, Landlord will notify Tenant of the Rentable
Square Footage of the Premises, the Expansion Spaces and the Building, all of
which shall be measured by Landlord's Architect in accordance with the ANSI/BOMA
Z65.1-1996 Standard Method for Measuring Building Rentable Area, approved June
7, 1996 for a single tenant building. Landlord's Architect's measurement shall
be subject to review and approval by Tenant's Architect for conformity to the
foregoing standard. If necessary, Landlord and Tenant will execute an amendment
to this Lease modifying the definitions of Rentable Square Footage of Premises,
Rentable Square Footage of Building, the location in the Building of the
Premises and the Expansion Spaces, Tenant's Proportionate Fraction for Building,
Tenant's Proportionate Fraction for Complex, Initial Estimated Annual Additional
Rent for Building, Initial Estimated Annual Additional Rent for Complex, and
such other terms and provisions, if any, of this Lease as may be necessary to
reflect such actual measurements. Landlord and Tenant will also execute, upon
request of either, a certificate, substantially in the form of Exhibit A-3
hereto, acknowledging, inter alia, 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), and the Rentable Square Footage of the
Building, the Premises and the Expansion Spaces. After such measurement by
Landlord's Architect, the Premises, the Expansion Spaces and the Building shall
not be subject to remeasurement without the consent of Landlord and Tenant.
Anything in this Lease to the contrary notwithstanding, the Premises, as
initially demised, shall include all of the Rentable Square Footage in the
Building other than the Expansion Spaces and the Retail Space. "Tenant's
Proportionate Fraction for Building" shall be the ratio, expressed as a
percentage, of the Rentable Square Footage of Premises, including the Expansion
Spaces and any ROFR Space (from and after the dates on which each of the
Expansion Spaces or any ROFR Space is demised to Tenant), to the Rentable Square
Footage of Building. "Tenant's Proportionate Fraction for Complex" shall be the
ratio, expressed as a percentage, of the Rentable Square Footage of the
Premises, including the Expansion Spaces and

                                      -2-
<PAGE>

any ROFR Space, when applicable, to the Rentable Square Footage of all buildings
(other than any garages in the Complex and any building or portion thereof which
is not available for lease such as an information kiosk), including the
Building, which Landlord is permitted to develop in the Complex pursuant to the
PUD Approval. Landlord shall have the right to estimate on a good faith basis,
from time to time, the Rentable Square Footage of all such buildings prior to
the construction thereof. From time to time after construction of each building
in the Complex, Landlord shall notify Tenant of the Rentable Square Footage of
such buildings and shall provide to Tenant, upon Tenant's request, such
information as Tenant may reasonably request with respect to the calculation
thereof. Within thirty (30) days after notice of such Rentable Square Footage,
Tenant shall pay to Landlord or Landlord shall credit to Additional Rent next
due from Tenant, as applicable, any underpayment owed or overpayment made, as
applicable, by Tenant based upon the estimated Rentable Square Footage of such
buildings, on account of Tenant's Proportionate Fraction for Complex of taxes
and assessments and Tenant's Proportionate Fraction for Complex of the Annual
Complex Maintenance and Operation Charge and the actual Rentable Square Footage
of such buildings.

                                   ARTICLE III

                           BASE BUILDING IMPROVEMENTS

3.1   INITIAL DESIGN AND CONSTRUCTION.

      3.1.1 ARCHITECT SELECTION PROCESS, DEVELOPMENT OF DESIGN DOCUMENTS,
LANDLORD'S CONTRACTOR AND PROJECT COST.

      Landlord has initiated a design competition for the Building as more fully
set forth in a letter dated January 17, 2000 from David Clem of Lyme Properties,
LLC to Henri Termeer, President and CEO of Tenant (the "Design Process Letter"),
which is incorporated herein by reference thereto. Landlord and Tenant agree to
cooperate in connection with the design competition as set forth in the Design
Process Letter. Upon completion of such design competition, Landlord and Tenant
shall mutually select Landlord's Architect (for purposes of this Lease the term
"Landlord's Design Team" shall be deemed to refer not only to Landlord's
Architect but also to the principal engineers, architects and other design
consultants employed by Landlord or Landlord's Architect for the design and
construction of the Building) and Landlord's Design Team (to the extent then
identified). Landlord and Tenant shall also mutually establish and agree upon
(i) a preliminary design concept for the Building (the "Preliminary Design
Concept"), which shall be based upon Tenant's program, outline building
specifications, schedule requirements, and (ii) a preliminary hard and soft cost
budget for the Base Building Improvements (the "preliminary Project Cost
budget"). Such selection of Tenant's Architect and Landlord's Design Team (to
the extent then identified), the establishment and agreement upon the
Preliminary Design Concept and the establishment and agreement upon such
preliminary Project Cost budget shall be made by the respective dates for each
set forth in the progress schedule attached as Exhibit B hereto (the "Progress
Schedule"). Landlord and Tenant acknowledge and agree that the Progress Schedule
represents a good faith estimate of the respective dates upon which the items
set forth in the Progress Schedule are to occur but that the Progress Schedule
is subject to finalization by Landlord and Tenant upon selection of Landlord's
Architect and Landlord's Contractor. Accordingly, references in this Lease to
the Progress

                                      -3-
<PAGE>

Schedule shall mean the Progress Schedule attached hereto as Exhibit B as the
same may be finalized. The Preliminary Design Concept shall describe the general
scope and quality of the Building and shall identify those portions of the
Building which will constitute the Premises initially leased to Tenant and the
Expansion Spaces. If despite the diligent and good faith efforts of Landlord and
Tenant the parties are unable to mutually select Landlord's Architect, to
establish a mutually acceptable Preliminary Design Concept and/or a mutually
acceptable preliminary Project Cost budget by the applicable dates set forth in
the Progress Schedule, either Landlord or Tenant may terminate this Lease upon
thirty (30) days' prior written notice to the other party. During such thirty
(30) day period Landlord and Tenant shall use diligent and good faith efforts to
select such Architect, establish such Preliminary Design Concept and/or
preliminary Project Cost budget, as applicable, failing which this Lease shall
terminate on the date set forth in such notice. In the event of such
termination, all third party, out-of-pocket costs incurred by Landlord in
connection with the design competition and design process solely for the
Building shall be shared equally by Landlord and Tenant.

      Landlord and Tenant also agree to cooperate in good faith in connection
with the design of the Building and the Base Building Improvements and the
development of a mutually acceptable Final Project Budget (as defined below).
Neither Landlord nor Tenant shall be required to approve any Design Documents
unless the Building includes at least 285,000 r.s.f. of office space and at
least 15,000 r.s.f. of ground floor retail space and each Expansion Space
includes approximately 25,000 r.s.f. of contiguous space on a single floor of
the Building. The size, layout, design and base building fit-up of the Building
and Base Building Improvements and the Final Project Budget shall be mutually
agreed upon by Landlord and Tenant. Landlord and Tenant acknowledge and agree
that during design development process contemplated hereby, Tenant, with
Landlord's approval, which approval shall not be unreasonably withheld,
conditioned or delayed, shall have the right to approve and/or design base
building electrical, mechanical, HVAC and other systems and design
specifications (such as, by way of example, entranceway and lobby design,
internal stair location, elevator size and finish, location of electrical and
telecom rooms and equipment, location of mechanical shaftways and equipment,
structural bay spaces, window mullions spacing and sill depth and height, floor
to floor heights and the like), provided that the specifications for such base
building elements are equal or better than the quality of the specifications
therefor in a newly constructed, first-class office tower in Cambridge.

      Each party acknowledges and agrees that it is in its best interest for the
Substantial Completion Date to occur on or before the Scheduled Substantial
Completion Date and that the Final Project Budget be substantially consistent
with the preliminary Project Cost budget and the updates thereto. Accordingly,
each party hereby agrees that it shall keep the other party informed of the
progress and status of its efforts to achieve the various milestones and other
scheduled completion dates set forth on the Progress Schedule; and that if
either party concludes that a milestone or other scheduled completion date will
not be met, such party shall use reasonable efforts to notify the other party of
the occurrence of any act, event, condition, omission or circumstance which
could have a substantial or material impact on: (i) the ability to achieve the
various milestone and other scheduled completion dates set forth on the Progress
Schedule; (ii) the cost or financial viability of the construction of the
Building or Complex; or (iii) Tenant's use and occupancy of the Premises.

                                      -4-
<PAGE>

      After Landlord's Architect and the Landlord's Design Team have been
selected and Landlord and Tenant have mutually established the Preliminary
Design Concept, Landlord shall enter into a contract (the "Architect's
Contract") with Landlord's Architect. The Architect's Contract shall be subject
to Tenant's prior approval, which approval shall not be unreasonably withheld,
conditioned or delayed. The Architect's Contract shall include provisions
requiring Landlord's Architect to submit all Design Documents (as hereinafter
defined) for each stage in the design process simultaneously to both Landlord
and Tenant sufficiently prior to the applicable Design Document completion date
for such design stage set forth in the Progress Schedule, in order to provide
Landlord and Tenant with reasonably sufficient time (in any event not less than
ten (10) business days for initial review of the applicable Design Document)
following receipt of such submission to review, comment on and approve such
submission prior to such completion date, to be responsible for the adequacy,
accuracy and completeness of the Final Design Documents approved by Landlord and
Tenant, and to cause the Final Design Documents to comply with all applicable
laws, regulations, building codes, building design standards, the Development
Approvals, Subsequent Approvals, agreed upon floor loading limits, and with
respect to all materials, equipment and special designs, processes or products,
that the same do not infringe any patent or other proprietary rights of others.
Contemporaneously with the selection of Landlord's Architect, Landlord shall
retain a construction manager or similar consultant (the "Construction Cost
Estimator"), reasonably satisfactory to the Tenant, to work with and provide
pre-construction services for Landlord, Tenant and Landlord's Architect to
develop, and provide value engineering services to control, a preliminary
Project Cost budget and updates thereto during the Schematic Design Documents
phase and the Design Development Documents phase. The parties acknowledge that
Landlord may, subject to Tenant's reasonable approval, elect, at any time, to
hire such Construction Cost Estimator as Construction Manager/General Contractor
for the Building. Subject to the limitations hereafter set forth with respect to
Tenant's right to disapprove bids with respect to the Construction Contract,
each of Landlord and Tenant shall have the right to approve such preliminary
Project Cost budget and updates thereto, which approvals shall not be
unreasonably withheld, conditioned or delayed.

      Landlord shall cause Landlord's Architect to prepare schematic
architectural plans, structural and engineering plans, elevations and building
sections, and site plans for the Building based upon the Preliminary Design
Concept ("Schematic Design Documents"), which shall mean a conceptual design of
the Base Building Improvements illustrating the scale and relationship of the
components of the Base Building Improvements and calculating the gross floor
area and Rentable Square Footage of the Building. Additionally, the Construction
Cost Estimator shall prepare an updated budget for the Base Building
Improvements for approval by Landlord and Tenant. The Schematic Design Documents
shall show walkways and plazas for the Building and Lot, major landscape
features, hardscaping, scale and relationship of other major components of the
Building and Lot, pedestrian and vehicular (including service) access and flow
through the Lot, lighting, survey information such as existing elevations,
benchmarks and utilities, and any known construction limits. The Schematic
Design Documents shall also include models, color renderings and outline
specifications indicating all basic materials and systems of the proposed Base
Building Improvements. The Schematic Design Documents and all subsequent Design
Documents shall be prepared and distributed by Landlord's Architect in both hard
copy and electronic format.

                                      -5-
<PAGE>

      During the development of the Schematic Design Documents, Landlord and
Tenant shall meet with representatives of the CDD (as defined in Section 3.1.4
hereof) and their respective consultants to review and refine the design of the
Building and the Lot. Thereafter, Landlord shall seek approval of the Schematic
Design Documents by the Planning Board (as defined in Section 3.1.4 hereof), and
Tenant shall cooperate with Landlord in connection therewith. If despite the
good faith efforts of Landlord and Tenant the Planning Board rejects the design
reflected in the Schematic Design Documents or insists upon major changes to the
Schematic Design Documents which are unacceptable to either Landlord or Tenant
and if as a result thereof Landlord is unable to obtain approval of the
Schematic Design Documents by the Planning Board by the date set forth in the
Progress Schedule, as the same may be changed by any Tenant Delay, either
Landlord or Tenant may terminate this Lease upon thirty (30) days prior written
notice. If Planning Board approval of the Schematic Design Documents is not
obtained by the end of such thirty-day period, this Lease shall thereupon
terminate. In the event of such termination, all third party, out-of-pocket
costs incurred by Landlord in connection with the design competition and design
process solely for the Building and Lot shall be shared equally by Landlord and
Tenant. Planning Board approval of the Schematic Design Documents shall be a
Subsequent Approval (as defined in Section 3.1.4 hereof). Landlord and Tenant
agree to cooperate in connection with any conditions imposed by the Planning
Board in connection with the Planning Board's approval of the Schematic Design
Documents, such as further review of the Design Documents by the CDD.

      At such time as the Schematic Design Documents have been approved by
Landlord and Tenant (and by the Planning Board, if Landlord elects to have the
Schematic Design Documents approved by the Planning Board), Landlord shall cause
(i) Landlord's Architect to prepare further details and development of the
Schematic Design Documents (the "Design Development Documents") for the Building
and the Base Building Improvements, which shall mean plans, sections and
elevations, typical construction details, and equipment layouts showing the
scope, relationships, forms, size and appearance of the Base Building
Improvements and calculating the gross floor area and the Rentable Square
Footage of the Building and (ii) the Construction Cost Estimator, based upon
such Design Development Documents, to prepare an updated budget for the Base
Building Improvements, each for approval by Landlord and Tenant. Design
Development Documents shall include architectural, mechanical and electrical
drawings and details of the Building, exterior materials to be incorporated in
the Building, walkways and other plaza areas on the Lot, a site and landscape
plan for the Lot showing all site development and landscape detail for lighting,
paving, landscaping, utilities, grading, drainage, access and service areas. The
Design Development Documents shall also include outline specifications
indicating all basic materials and mechanical and electrical systems of the
proposed Base Building Improvements. As with the Schematic Development
Documents, the Design Development Documents shall be prepared and distributed in
both hard copy and electronic format.

      At such time as the Design Development Documents have been approved by
Landlord and Tenant, Landlord shall cause (i) Landlord's Architect to prepare
detailed construction drawings and specifications ("Final Design Documents")
which shall set forth in detail the requirements for construction of Landlord's
Work (including all architectural, mechanical, electrical and structural
drawings and detailed specifications), shall be fully coordinated with one
another and with field conditions as they exist on the Lot, shall show all work
necessary to complete Landlord's Work, including all cutting, fitting, and
patching and all connections to the

                                      -6-
<PAGE>

mechanical and electrical systems and components of the Base Building
Improvements, and shall include calculations of the gross floor area and the
Rentable Square Footage of the Base Building Improvements and (ii) the
Construction Cost Estimator, based upon such Final Design Documents, to prepare
an updated budget for the Base Building Improvements, each for approval by
Landlord and Tenant. The Final Design Documents shall be used for the
construction of the Base Building Improvements and shall be based upon the
Design Development Documents and other information which is relevant to the
design and construction of the Base Building Improvements. The Final Design
Documents shall be prepared and stamped by Landlord's Architect. The Preliminary
Design Concept, the Schematic Design Documents and the Final Design Documents
(collectively, the "Design Documents") shall not include any of Tenant's Work
(as hereinafter defined). As used herein, the term "Base Building Improvements"
shall mean the items of work and materials to be performed and supplied by
Landlord in accordance with the Final Design Documents as affected by Change
Orders (as such term is hereafter defined) and the term "Landlord's Work" shall
mean the construction of the Base Building Improvements in accordance with the
Final Design Documents as affected by Change Orders.

      The preliminary and final Schematic Design Documents, the preliminary and
final Design Development Documents, and the Final Design Documents shall be
submitted to both Landlord and Tenant for review and comment in accordance with
the dates and requirements included in the Progress Schedule, which shall be
incorporated, to the extent applicable, in the Architect's Contract. In each
instance both parties shall have until the respective completion dates set forth
in the Progress Schedule to review each submission and to notify the other party
of approval or disapproval. Tenant may disapprove any new items shown on a
submission which are not in compliance with (i) the Preliminary Design Concept
with regard to the Schematic Design Documents, (ii) the Schematic Design
Documents with regard to the Design Development Documents, or (iii) the Design
Development Documents with regard to the Final Design Documents, specifying and
detailing in each case such objections. To the extent that any such submission
is consistent with prior submissions approved by Tenant, such submission shall
be approved by Tenant, provided however, that Tenant shall have the right, with
Landlord's approval, to make modifications as set forth in the next sentence
hereof. Tenant, with the approval of Landlord, which approval shall not be
unreasonably withheld, conditioned or delayed, shall have the right to modify
the design of the Base Building Improvements at any time during the design
process contemplated hereby, provided that if as a result of any such design
change (a) any particular Design Document submission is not approved by the
applicable completion date set forth on the Progress Schedule, or (b) any such
design change delays the Substantial Completion Date, such lack of approval or
design change shall constitute a Tenant Delay for purposes of Section 3.2. In
the event Tenant shall fail to object to any Design Documents submissions by the
applicable completion date for such submission set forth in the Progress
Schedule, provided that Tenant had received such submission not less than five
(5) business days prior to the applicable completion date, such failure shall
constitute a Tenant Delay for purposes of Section 3.2. If Tenant objects to any
such submission because such submission is inconsistent with the prior
submission approved by Tenant, Landlord shall cause Landlord's Architect to
modify such submission to respond to Tenant's objection and submit such modified
submission to Tenant within five (5) business days after receipt of Tenant's
objections for approval by Tenant in the same manner as with regard to the prior
submission. Rejection of new items included in any submission that are
inconsistent with the approvals given

                                      -7-
<PAGE>

for prior design phases shall not give rise to a Tenant Delay so long as the
initial rejection was delivered within the dates established in the Progress
Schedule, as modified. Landlord and Tenant agree to cooperate with one another
diligently and in good faith so as to complete the review and approval of all
Design Document submission by the applicable completion dates set forth in the
Progress Schedule.

      Upon approval of the Final Design Documents, Landlord shall solicit bids
from general contractors or construction managers mutually identified by
Landlord and Tenant by the date set forth in the Progress Schedule. Landlord and
Tenant shall mutually determine whether such bids shall be submitted on the
basis of a guaranteed maximum or fixed price contract basis. Upon receipt of
bids from such general contractors or construction managers, Landlord and Tenant
shall mutually select Landlord's Contractor from among such bidders and
thereupon Landlord shall enter into a contract (the "Construction Contract")
with Landlord's Contractor for construction of the Base Building Improvements.
The parties acknowledge that if Landlord and Tenant have agreed to hire the
Construction Cost Estimator to serve as Construction Manager/General Contractor
for the Building, instead of soliciting bids from general contractors or
construction managers as described above, such Construction Manager shall
instead be required to solicit bids from all subcontractors, and in such case
Landlord and Tenant shall be entitled to approve the subcontractors selected by
the Construction Manager and require Landlord's Contractor to enter into such
subcontracts. The Construction Contract shall be subject to Tenant's prior
approval, which approval shall not be unreasonably withheld, conditioned or
delayed. If, after such bids are received, none of such bids is acceptable to
Tenant and/or Tenant seeks any change in previously approved Design Documents,
and as a result thereof Tenant fails or refuses to approve the Final Project
Budget (as hereinafter defined) by the date set forth in the Project Schedule,
such failure or refusal shall constitute a Tenant Delay. Additionally, Tenant
shall have no right to withhold approval of any general contractor bid or any
subcontractor bid if the total amount of all such bids does not exceed by more
than fifteen percent (15%) the estimated amount thereof set forth in the
preliminary budget for Project Cost or the updated budget for Project Cost, as
the case may be, then most recently approved by Landlord and Tenant.

      As used in this Lease, the term "Project Cost" shall mean (i) all hard
costs and soft costs incurred by Landlord in connection with the construction of
the Base Building Improvements as reflected in a final project budget (the
"Final Project Budget") to be mutually approved by Landlord and Tenant based
upon, inter alia, all amounts payable by Landlord pursuant to the Architect's
Contract and the Construction Contract, and including all liabilities and
expenses of all architectural and engineering services relating to Landlord's
Work, appropriate amounts for the other line item categories set forth on the
Project Budget form attached hereto as Exhibit B-1 (the "Project Budget Form"),
(ii) the net amount of all Change Orders (as hereinafter defined), (iii) a
pro-rata portion of all "Mitigation Expenses" (as such term is hereafter
defined) incurred by Landlord or any Affiliate of Landlord pursuant to the
Development Approvals and the Subsequent Approvals (as such terms are defined in
Section 3.1.4 hereof), which pro-rata portion shall be equal to the ratio of the
gross floor area of the Building (as such gross floor area is determined
pursuant to the Cambridge Zoning Ordinance) to 1,313,000 square feet (being the
maximum amount of gross floor area, determined pursuant to the Cambridge Zoning
Ordinance, permitted to be developed in the Complex pursuant to the PUD Approval
(as such term is defined in Section 3.1.4 hereof) except as hereinafter set
forth and (iv) taxes and assessments for

                                      -8-
<PAGE>

the Building and Lot, utility and insurance costs for the Building and Lot and
all other operating and maintenance costs for the Building and Lot, in each case
incurred by Landlord for the period from the Substantial Completion Date through
the Term Commencement Date. The term "Mitigation Expenses" shall mean all
capital expenses for housing linkage payments and public open space linkage
payments required pursuant to the PUD Approval with respect to the Building,
costs of traffic mitigation required by the Development Approvals and Subsequent
Approvals and infrastructure improvements for the Complex required by the
Development Approvals and Subsequent Approvals. Tenant's pro-rata share of
capital expenses for housing linkage payments and public open space linkage
payments required pursuant to the PUD Approval with respect to the Building
shall be equal to the ratio of the Rentable Square Footage of Premises to the
Rentable Square Footage of Building. For purposes of determining Project Cost,
the line item amount for Land and Environmental Remediation shall be fixed at
$60.00 per r.s.f. regardless of the actual cost of such line item; Project Cost
shall not include any amount for construction of the Garage (as hereinafter
defined) or for the leasing of (or the making of any tenant improvements to) the
Expansion Spaces to tenants; the developer's fee shall be fixed at five percent
(5%) of Project Cost (exclusive, however, of the line item amounts for Land and
Environmental Remediation, Development Fee/Supervision (the "Developer's Fee")
and Interest on Landlord's Equity (as such term is hereinafter defined)); the
broker's fee shall not exceed $7.50 per r.s.f. and no amount shall be included
in Project Cost for general or administrative expenses of Landlord or for
improvements to the Retail Space in excess of the cost of Base Building
Improvements related thereto, and no amount shall be included in Project Cost
for any interest, charges or fees paid or payable by Landlord with respect to
any construction loan financing for the Project. Project Cost shall include an
amount determined by multiplying all components of the Project Cost (exclusive,
however, of the line item for the Developer's Fee) by twelve percent (12%) per
annum for the period commencing as of the date the cost of each such component
is incurred through and including the Term Commencement Date (as so determined,
"Interest on Landlord's Equity"), it being understood and agreed by Landlord and
Tenant that Interest on Landlord's Equity with respect to any component of
Project Cost that Landlord has acquired, purchased or incurred prior to the date
of execution of this Lease, such as the cost of the Land & Environmental
component of the Project Cost, shall be deemed to begin to accrue, not as of the
date that the cost was actually incurred, but rather as of the execution date of
this Lease. Landlord shall account for all Project Costs on a so-called "open
book" basis. Within ninety (90) days after the Substantial Completion Date,
Landlord shall deliver to Tenant a full accounting of the Project Cost incurred
as of the Substantial Completion Date; Tenant, from the date hereof through the
date which is twelve (12) months after receipt of such full accounting of
Project Cost, may review all of Landlord's books and records relating to the
incurrence and payment of the Project Cost in order to verify and confirm the
accuracy thereof. If Landlord incurs any cost or expense properly includable in
Project Cost after the Substantial Completion Date (including, without
limitation, any Mitigation Expenses) which is not included in the full
accounting, Landlord shall notify Tenant thereof and Landlord shall provide
Tenant with such documentation with respect thereto as Tenant reasonably may
request. Landlord and Tenant acknowledge and agree that the incurrence of
additional costs and expenses by Landlord after the Substantial Completion Date
which are properly includable in Project Cost will result in an increase in the
Annual Fixed Rent Rate, which increase shall be effective as of the date of
incurrence thereof.

                                      -9-
<PAGE>

      Tenant shall have the right (but not the obligation) to offer to finance
up to twenty percent (20%) of the Project Cost as hereinafter provided ("Tenant
Financing"). If Tenant shall desire to offer Tenant Financing, Tenant shall give
Landlord written notice ("Financing Notice") of such offer not later than thirty
(30) days after Landlord and Tenant have agreed upon the first preliminary
Project Budget; such Financing Notice shall specify the amount (the "Tenant
Financing Amount") and other financial terms of such Tenant Financing described
below. Delivery by Tenant of a Financing Notice shall constitute a
representation by Tenant that it has the capacity to provide such Tenant
Financing.

      If Tenant offers to provide Tenant Financing and all of the terms and
conditions thereof, including without limitation, all documents evidencing and
securing Tenant Financing, are acceptable to Landlord and its construction
and/or permanent lenders, in their respective sole discretion, Tenant shall make
a loan to Landlord for an amount equal to the Tenant Financing Amount. The
Financing Notice shall specify the interest rate per annum payable on such
Tenant Financing loan and the repayment schedule (interest only until maturity;
or payments of principal and interest based upon the amortization period
specified by Tenant in the Financing Notice), which interest rate and repayment
schedule shall be subject to Landlord acceptance or rejection. Such Tenant
Financing loan shall be due and payable not later than the scheduled expiration
date of the initial Term of the Lease, and shall be secured by a mortgage on the
Lot and Building, which mortgage shall be subject and subordinate to the lien of
any mortgage granted to any lender now or hereafter providing Landlord with
construction and/or permanent financing ("Landlord's Project Financing"). The
amount by which the Project Cost exceeds the Tenant Financing Amount is herein
referred to as the "Landlord Project Cost Portion". Tenant agrees that upon any
uncured Event of Default hereunder, Landlord shall be entitled to offset against
any payments due under the Tenant Financing loan any amount owed by Tenant to
Landlord pursuant to this Lease. In addition, Tenant agrees that it will enter
into any intercreditor agreement (which may include typical subordinate lender
standstill agreements) requested by any lender providing Landlord's Project
Financing. Tenant shall disburse the proceeds of such Tenant Financing loan to
Landlord on the Commencement Date, and Landlord shall apply such proceeds to pay
down Landlord's Project Financing by the Tenant Financing Amount.

      If Tenant provides Tenant Financing as aforesaid, the definition of Annual
Fixed Rent Rate set forth in Section 1.1 shall be deemed to be revised to read
as follows:

      Subject to Sections 4.1(b), 10.11 and 10.12 thereof, an amount equal to
(a) the sum of (i) the product of (x) the Landlord Project Cost Portion
multiplied by (y) twelve percent (12%) plus (ii) the product of (x) the Tenant
Financing Amount multiplied by (y) the Tenant Financing interest rate per annum
payable on the Tenant Financing (or debt service constant if Tenant elects to
have the Tenant Financing amortized), divided by (b) the Rentable Square Footage
of the Building.

      3.1.2 CHANGE ORDERS.

      Tenant and Landlord recognize that it may be necessary or advisable to
make certain changes to the Final Design Documents or the Base Building
Improvements from time to time. Any changes shall be made in accordance with
this Lease.

                                      -10-
<PAGE>

            (a) In the event that Tenant shall request or authorize any Change
Order (as hereinafter defined) or additional services from Landlord with respect
to the Base Building Improvements for the purpose of upgrading or changing the
scope, manner of performance, or quality of the Base Building Improvements,
including any change of design, subcontractor, supplier or materials made at the
request of Tenant, such Change Order when implemented by Landlord may result in
a change in the Progress Schedule and/or the Project Cost. As used herein, the
term "Change Order" shall mean (A) a written order from Landlord which Landlord
determines is necessary to carry out the approved design and construction of the
Base Building Improvements, whether due to any mistake or omission in, or
clarification of, the Design Documents, or (B) a written order to Landlord from
Tenant requesting or authorizing a Change in Landlord's Work or an adjustment of
the Project Cost or the Progress Schedule. Landlord shall provide Tenant with
copies of all proposed and final Change Orders. In addition: subject to the last
sentence of this Subparagraph (a), (i) with respect to additive Change Orders,
the Project Cost shall be increased by an amount equal to the change in the cost
of Landlord's Work (as determined pursuant to the Construction Contract and/or
the Architect's Contract) attributable to such Change Order, and (ii) with
respect to deductive Change Orders, the Project Cost shall be reduced by the
change in the cost of Landlord's Work (as determined pursuant to the
Construction Contract and/or the Architect's Contract) attributable to such
Change Order. Additive or deductive Change Orders shall result in an increase or
decrease, respectively, in the Developer's Fee (but not in any change of the
percentage thereof), and Landlord shall be reimbursed by Tenant, either directly
or through an increase or decrease, as applicable, in the Project Cost, for the
actual cost of overhead and profit, if any, which Landlord is required to pay to
Landlord's Contractor on account of any additive Change Order as set forth in
the Construction Contract approved by Tenant and Landlord. Anything herein to
the contrary notwithstanding, Tenant's approval of a Change Order shall be
required for each of the following, which approval shall not be unreasonably
withheld, conditioned or delayed:

      (1) A change in the rentable square footage of the Premises by more than
      four percent (4%) in the aggregate;

      (2) A change in the rentable square footage of any floor included within
      the Premises by more than five percent (5%) in the aggregate;

      (3) Any material change in the elevator systems or mechanical or
      electrical systems serving the Premises with the result that such systems
      would be significantly different than those shown on the Final Design
      Documents;

      (4) Any material change in standard structural floor load capacities from
      those shown on the Final Design Documents;

      (5) Any material change in the quality of materials and fixtures used in
      the lobby of the Building, passenger elevator cabs serving the Premises,
      or washrooms serving the Premises with the result that the same would be
      significantly different from to those shown on the Final Design Documents;

      (6) Any change in the exterior design or appearance of the Building; or

                                      -11-
<PAGE>

      (7) Any individual Change Order which increases the Project Cost by more
      than $500,000.00.

            (b) If Tenant requests or requires any Change Order for the Base
Building Improvements, 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, which approval shall not be unreasonably withheld,
delayed or conditioned. No Change Order for the Base Building Improvements
requested by Tenant shall be effective unless approved by Landlord's
Representative and Tenant's Construction Representative in writing, such
approval not to be unreasonably withheld, delayed or conditioned. Tenant may
withdraw any Change Order requested by Tenant at any time prior to Tenant's
Construction Representative having given its approval to such Change Order.
Promptly, but not later than ninety (90) days after the Term Commencement Date,
Landlord shall deliver a set of as built plans and operating and maintenance
manuals for the Base Building Improvements to Tenant.

      3.1.3 ENVIRONMENTAL REMEDIATION.

      Landlord shall be responsible for undertaking and completing environmental
remediation of the Lot ("Environmental Remediation") pursuant to an
Environmental Agreement (the "Environmental Agreement") between Landlord and
Tenant in the form of Exhibit E hereto. Landlord has delivered to Tenant the
documents set forth on Exhibit E-1 hereto (the "Remediation Documents") and
Landlord will provide Tenant with such additional information pertaining to
Environmental Remediation and hazardous substances present on other portions of
the Lot as Tenant reasonably may request, including correspondence with
regulatory authorities and other governmental bodies with respect to the
Environmental Remediation. Landlord will provide Tenant with notice of any
circumstances relating to the Environmental Remediation which could delay the
Substantial Completion Date, as affected by any Tenant Delay, by more than
thirty (30) days.

      3.1.4 DEVELOPMENT APPROVALS AND TITLE EXCEPTIONS.

      Reference is hereby made to (i) a Special Permit issued by the City of
Cambridge Planning Board (the "Planning Board"), Case No. PB #141 filed April 7,
1999 (the "PUD Permit"), recorded with the Middlesex South District Registry of
Deeds (the "Registry") in Book 31137, Page 89 as affected by a Settlement
Agreement dated May 24, 1999 among Barbara Broussard, Mary DeFreitas, the East
Cambridge Planning Team and Landlord (the "Settlement Agreement") (the PUD
Permit and the Settlement Agreement are herein collectively referred to herein
as the "PUD Approval"), (ii) PTDM Ordinance Final Decision issued April 20, 1999
by the Community Development Department ("CDD") of the City of Cambridge (the
"PTDM Decision"), the Letter dated April 20, 1999 to Robert L. Green of Lyme
Properties from Susanne Rasmussen of the CDD (the "PTDM Letter") attached to the
PTDM Decision, and Landlord's Parking Transportation Demand Management Plan
dated April 9, 1999 (the "PTDM Plan") (the PTDM Decision, the PTDM Letter and
PTDM Plan are collectively referred to herein as the "PTDM Approval"), (iii) a
Certificate of the Secretary of Environmental Affairs on the Final Environmental
Impact Report dated April 15, 1999 (the "MEPA Certificate"), (iv) an Order of
Conditions issued July 12, 1999 by the City of Cambridge Conservation Commission
(the "Order

                                      -12-
<PAGE>

of Conditions") and (v) a Determination for Issuance of a Waterways License
Amendment dated December 8, 1999, issued by the Executive Office of
Environmental Affairs of the Commonwealth of Massachusetts (the "Chapter 91
Determination"), (collectively, the "Development Approvals"). Reference is also
hereby made to Item Nos. 2 through 12, inclusive (the "Title Exceptions") set
forth in Schedule B, Part 1 of Owner's Policy No. 136-00-336684 dated August 19,
1998, issued by Lawyer's Title Insurance Corporation (the "Title Policy").
Tenant acknowledges that Landlord has delivered to Tenant copies of the
Development Approvals, the Title Exceptions and the Title Policy. This Lease,
and the development, construction and operation of the Building, the Lot and the
Complex, shall be subject to the Development Approvals, the license to be issued
pursuant to the Chapter 91 Determination, the Title Exceptions and all other
determinations, approvals, decisions and actions of governmental authorities
having jurisdiction of the Complex hereafter issued pursuant to or contemplated
by the Development Approvals and delivered to Tenant ("Subsequent Approvals").
Each of Landlord and Tenant, in the exercise of their respective rights and the
performance of their respective obligations pursuant to this Lease, shall
observe and comply with all requirements of the Development Approvals and
Subsequent Approvals. Landlord, however, shall not enter into any Subsequent
Approval which will materially and adversely decrease the amount of parking
spaces available at the Complex.

      Without limiting the generality of the foregoing, to the extent required
for compliance with the PUD Permit and the PTDM Approval, as the same may be
affected by Subsequent Approvals (but only for so long as the same remain in
force and effect from time to time), Tenant shall comply with the following: (a)
Tenant shall comply with the obligations of the PTDM Approval applicable to
tenants, employers and/or employees in the Complex, (b) Tenant shall cooperate
with Landlord in the implementation of the Additional Recommendations set forth
in the PTDM Letter if the mode split goal of the PTDM Approval is not achieved,
(c) Tenant shall cooperate with Landlord and/or the Charles River Transportation
Management Association or any other transportation management association of
which Landlord is a member (a "TMA") in implementing the PTDM Approval and TMA
programs (and Tenant is hereby encouraged to participate in all TMA programs),
(d) Tenant shall establish a guaranteed ride home program for Tenant's employees
as provided in the PUD Approval and the PTDM Approval, as the same may be
affected by Subsequent Approvals, (e) Tenant shall, either directly or through a
program established by Landlord or a TMA for tenants of the Complex, complete
surveys pursuant to Section III D of the PTDM Plan, (f) as required by the PUD
Approval and PTDM Approval, as the same may be affected by Subsequent Approvals,
Tenant shall contract with the MBTA and subsidize transit passes and commuter
rail passes in an amount not less than 60% of the cost thereof or such higher
percentage as Landlord may require if mode split commitments are not achieved,
(g) as provided in the PTDM Approval, as the same may be affected by Subsequent
Approvals, Tenant shall cooperate with Landlord in connection with surveys
concerning attitudes of employees and customers of Tenant in order to refine and
develop transportation demand management programs, (h) Tenant shall provide
Tenant's employees with information provided to Tenant by or on behalf of
Landlord pursuant to the PTDM Plan, including information on the advantages and
benefits of telecommuting, flexible time, compressed work week programs, the
materials to be provided pursuant to Section VII A of the PTDM Plan and other
programs recommended by the City of Cambridge, (i) as provided in the PTDM
Approval, as the same may be affected by Subsequent Approvals, Tenant is hereby
encouraged to use a commuter choice program which allows qualifying employees
the option of receiving the cash value of a

                                      -13-
<PAGE>

Tenant's Parking Space, (j) Tenant shall participate in and pay a reasonable
share (which shall not be less than Tenant's Proportionate Fraction for Complex)
of the cost of any TMA shuttle bus service serving the Complex, as contemplated
by the PTDM Approvals, and (k) Tenant shall cooperate with Landlord with respect
to all other aspects of the PTDM Approval.

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

      During the course of performance of Landlord's Work, Landlord and Tenant
shall meet on not less than a monthly basis to review the progress of Landlord's
Work. Landlord agrees to use reasonable and diligent efforts to have the
Premises ready for the performance of Tenant's Work on or before the Scheduled
Substantial Completion Date which Substantial Completion Date shall, however, be
extended for a period equal to that of (a) any delays due to Acts of God, labor
disputes or unrest, riots, fire, unusual delays in deliveries, casualties or
other causes beyond Landlord's reasonable control (collectively, "Force Majeure
Events") and (b) any delays due to (i) any Change Order, (ii) any act, omission
or neglect of Tenant, or of any employee, agent, or separate contractor of
Tenant, (iii) the concurrent performance of the Base Building Improvements and
Tenant's Work, (iv) Tenant's failure to furnish information or approve any of
the Design Documents, the Architect's Contract, the Construction Contract or the
Project Cost by the applicable dates set forth in the Progress Schedule or to
respond to any request by Landlord for information or approval within the time
period required by the Progress Schedule, or if no time period is required by
the Progress Schedule, within five (5) business days of Landlord's request, or
(v) any breach or default by Tenant in the performance of Tenant's obligations
pursuant to this Lease, even if such breach or default is cured (collectively,
"Tenant Delay"). In the event of any such Force Majeure Event, Landlord shall
use reasonable efforts to eliminate the cause of such delays or to secure
alternate supplies. If any one or more Force Majeure Events shall delay the
Substantial Completion Date by more than one hundred eighty (180) days, either
Landlord or Tenant may terminate this Lease upon not less than fifteen (15) days
prior written notice to the other. A Tenant Delay shall not affect the
Substantial Completion Date unless Landlord has given Tenant notice thereof and
Tenant fails to cure the same within two (2) business days, provided, however,
that Landlord shall not be required to provide Tenant with any notice or
opportunity to cure with respect to any date set forth in the Progress Schedule
by which Tenant must take any action, and any failure or refusal by Tenant to
take such action by any such date set forth in the Progress Schedule, which
failure or refusal in fact results in a delay, shall constitute a Tenant Delay.
As used herein, the term "Substantial Completion Date" shall mean and refer to
the date on which: (i) the Base Building Improvements have been substantially
completed in accordance with the Final Design Documents, as affected by Change
Orders, without material deviation therefrom as certified by Landlord's
Architect and confirmed by Tenant's Architect, which confirmation shall not be
unreasonably withheld, delayed or conditioned, 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 are broom clean and free of debris except to the extent, if
any, resulting from Tenant's Work, and (iv) all utilities required for the

                                      -14-
<PAGE>

use of the Premises have been brought by Landlord to the location(s) shown on
the Final Design Documents, as the same may be affected by Change Orders;
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
solely for purposes of determining the Commencement Date under Section 2.2 and
other applicable provisions of this Lease, 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 confirm 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 Work shall be deemed
to have been performed as of the date on which the Base Building Improvements
are substantially complete (as certified by Landlord's Architect and confirmed
by Tenant's Architect or such independent architect) except for latent defects,
Punch List Items and items which do not conform with the requirements of the
Final Design Documents, as affected by Change Orders, 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 date on which the Base Building Improvements are substantially complete,
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 latent defects, Punch List Items and items stated in such
certificate to be incomplete or not in conformity with such requirements.
Landlord shall use reasonable and diligent efforts to complete Punch List Items
within thirty (30) days after the Substantial Completion Date and in a manner
which will not unreasonably interfere with the performance of Tenant's Work (as
defined in Section 3.2.1 hereof). Landlord represents that Landlord has the
capacity to effect financial arrangements to enable Landlord to complete
Landlord's Work, and to otherwise enable Landlord to fulfill all of its
obligations under this Lease. After the commencement of construction of
Landlord's Work, Landlord shall furnish to Tenant, at Tenant's request, a copy
of the portions of Landlord's financing commitment for construction of the
Building confirming the availability of construction financing therefor. Tenant
agrees to observe any reasonable limitations on dissemination thereof to others
that Landlord may impose.

      Notwithstanding the foregoing, (A) if Landlord has not commenced the
Environmental Remediation of the Lot by the date set forth in the Progress
Schedule, (B) if Landlord has not filed a Response Action Outcome Statement (as
such term is defined in the Massachusetts Contingency Plan, 310 CMR 40.0000 et
seq., as amended ("MCP")) for the Lot by the date set forth in the Progress
Schedule, (C) if a building permit for the Base Building Improvements has not
been issued by the date set forth in the Progress Schedule, (D) if construction
of the Base Building Improvements has not commenced by the date set forth in the
Progress Schedule (each of the foregoing being referred to as a "Landlord
Milestone Date"), Tenant shall have the right to terminate this Lease as
hereinafter set forth. Upon request of Tenant, Landlord shall provide such
evidence as Tenant reasonably may request to evidence achievement of each
Landlord Milestone Date. Each of the Landlord Milestone Dates shall be extended
by one business day for each day of Tenant Delay occurring before a respective
Landlord Milestone Date. If

                                      -15-
<PAGE>

Landlord is in default with respect to any Landlord Milestone Date, as extended
as aforesaid by any Tenant Delay, Tenant shall notify Landlord within five (5)
business days thereof. If Landlord does not cure the default or alleged default
within thirty (30) business days after receipt of such notice, Tenant, upon
written notice, may thereupon terminate this Lease.

      If the Garage and the streets and roadways of the Complex necessary for
access to the Building and Garage are not substantially complete by the date of
substantial completion of Tenant's Work to be performed in connection with
Tenant's initial occupancy of the Premises then Tenant, as Tenant's sole remedy,
shall receive a credit against the initial and succeeding payments of the
Parking Fee due hereunder, until such credit is fully exhausted, equal to two
day's Parking Fee (for all of Tenant's Garage Parking Spaces or for such number
of Tenant's Garage Parking Spaces as may be unavailable, as applicable) for each
day from substantial completion of Tenant's Work until the Garage and such
street and roadway work are substantially complete.

      In the event Landlord's Work shall not be substantially completed by the
Scheduled Substantial Completion Date specified in Section 1.1 (as and to the
extent such Scheduled Substantial Completion Date may be extended by Tenant
Delays and delays caused by Force Majeure Events), then after the Term does
commence, Tenant, as Tenant's sole remedy, shall receive a credit against the
initial and succeeding payments of Annual Fixed Rent due hereunder, until such
credit is fully exhausted, equal to one day's Annual Fixed Rent for each day
from the Scheduled Substantial Completion Date (as so extended) until the date
that Landlord's Work is substantially completed.

      In the event Landlord's Work shall not be substantially completed within
one hundred eighty (180) days after the Scheduled Substantial Completion Date
(as and to the extent such Scheduled Substantial Completion Date may be extended
by Tenant Delays and delays caused by Force Majeure Events) (such 180th day is
referred to herein as the "Outside Completion Date"), Tenant may at any time
thereafter, but prior to Landlord's Work being substantially completed, notify
Landlord of Tenant's election to terminate this Lease. Tenant shall give
Landlord no less than thirty (30) days' prior notice of Tenant's intention to
terminate this Lease before notifying Landlord of Tenant's election to terminate
this Lease; it being understood and agreed, however, that Tenant may give notice
of such intention as early as thirty (30) days prior to the Outside Completion
Date.

      3.2.1. PERFORMANCE OF TENANT'S WORK.

      Except for the Base Building Improvements to be performed by Landlord in
accordance with the Design Documents, all of Tenant's initial interior
improvements, fixtures, finishes, furnishings, furniture, telephones, movable
equipment and signs (collectively, "Tenant's Work"), shall be performed and
provided at the sole cost and expense of Tenant. Tenant's performance of
Tenant's Work shall be coordinated with any work being performed by Landlord
and/or by any Affiliate of Landlord in the Building, on the Lot or in the
Complex in such manner as to maintain harmonious labor relations during the
performance of Landlord's Work (which Landlord expects will be performed by
union contractors) and thereafter and not to damage the Building or Lot or
interfere with Building or Lot operations or with any work being performed by or
on behalf of Landlord or any Affiliate of Landlord in the Complex. All work
described in

                                      -16-
<PAGE>

Tenant's Work shall be performed by an Approved Contractor. For purposes of this
Lease, an "Approved Contractor" shall mean a contractor or mechanic identified
by Tenant in writing, who has been approved by Landlord in writing (such
approval not to be unreasonably withheld, delayed or conditioned). Contractors
may be approved in one of two ways. First, Tenant may submit to Landlord in
writing from time to time a list (or a revised list) of contractors that Tenant
anticipates using from time to time to perform Tenant's Work. If Landlord fails
to object to any of the contractors identified on such list within ten (10) days
after receipt of such list from Tenant, all contractors identified on such list
shall be deemed "Approved Contractors". Second, Tenant may submit to Landlord
from time to time requests for Landlord to approve specific contractors (not
already on the list of Approved Contractors) to perform Tenant's Work. Landlord
shall have the right, upon written notice to Tenant to withdraw its approval of
any previously Approved Contractors at any time for any cause as determined in
Landlord's reasonable judgment. A contractor's failure to provide or maintain
adequate insurance levels shall be a reasonable basis for Landlord to withhold
or withdraw approval unless Tenant notifies Landlord in writing that such
contractor shall be covered by insurance then being maintained by Tenant and if
Tenant provides documentary evidence that said Contractor is covered and of the
amount of coverage. Additionally, Landlord may withhold or withdraw approval of
any contractor proposed by Tenant or previously approved by Landlord to perform
any Tenant's Work if Landlord determines, in Landlord's sole but reasonable
discretion, that any contractor proposed or previously approved by Landlord by
Tenant for the performance of any Tenant's Work does not have a sufficient
bonding capacity or may cause picketing, labor unrest, strikes, protests or
similar activities (collectively "Labor Unrest"). If after approval of an
Approved Contractor by Landlord, any Labor Unrest shall occur or be threatened
on account of any Tenant's Work, Landlord shall have the right to require
cessation of Tenant's Work until resolution of such Labor Unrest. Except as set
forth in Sections 3.2.1 and 5.1.10 hereof, all initial Tenant's Work which may
constitute a fixture, part of the real estate, a part of a Building system or
Building utilities shall become a part of the Premises and upon termination of
this Lease shall be considered to be the property of the Landlord.

      Unless Tenant agrees to readapt the Premises for a typical general office
layout prior to the expiration or termination of the Lease, Tenant shall not
effect any Tenant's Work (or any alterations or additions to the Premises after
performance of the initial Tenant's Work) that might require any unusual expense
to reuse the Premises for any general office use.

      Tenant's Work shall be performed in accordance with complete, consistent,
final construction drawings and specifications ("Construction Documents")
approved by Landlord in writing, which approval shall not be unreasonably
withheld, conditioned or delayed. The Construction Documents shall be prepared
and stamped by Tenant's Architect. Landlord reserves the right to reject, in
whole or in part, any or all of the Construction Documents which in its
reasonable opinion fail to comply with Sections 3.2.1, 3.3 and 5.1.5 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. Upon completion of Tenant's Work to prepare the Premises for
Tenant's initial occupancy, Tenant

                                      -17-
<PAGE>

shall provide Landlord with a set of as-built plans, and operating and
maintenance manuals therefor.

      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. 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 indemnify and hold Landlord
harmless if any Tenant's Work described in the Construction Documents (a) fails
to comply with all applicable laws, regulations, building codes, building design
standards, the Development Approvals and Subsequent Approvals, (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 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, 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 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.

      If Tenant enters into a contract with Landlord's Contractor for the
performance of Tenant's Work to prepare the Premises for Tenant's initial
occupancy, Landlord agrees that Tenant shall have a license to enter the
Premises at such time or times prior to the Term Commencement Date as Landlord,
in its sole but reasonable discretion, may permit to enable Tenant to carry out
Tenant's Work. Upon any such entry by Tenant prior to the Term Commencement
Date, Tenant shall be subject to, and perform all of its obligations under this
Lease except for the obligation to pay Rent, which obligation shall not arise
until the Term Commencement Date occurs.

      An Approved Contractor may be used by Tenant until Landlord notifies
Tenant that an Approved Contractor is no longer approved due to such Approved
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. When any Tenant's Work is in progress, Tenant shall cause to be maintained
insurance as may be required by Landlord covering any additional hazards due to
such Tenant Work, for the benefit of Landlord. It shall be a condition of
Landlord's approval of any Tenant's Work that certificates of such insurance
issued by a responsible insurance company qualified to do business in
Massachusetts and having a Best's Insurance Rating of A- or better,

                                      -18-
<PAGE>

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 Landlord for up to $25,000 of the reasonable,
out-of-pocket, third party costs of reviewing proposed Construction Documents
and Tenant's Work and inspecting installation of the same. At all times while
performing Tenant's Work, Tenant shall require each Approved Contractor
performing Tenant's Work to comply with all applicable laws, regulations,
permits and policies relating to such work. In performing Tenant's Work, each
Approved Contractor shall comply with Landlord's requirements set forth in
Section 3.2.1, 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 Approved Contractor shall not interfere with or disrupt
Landlord's Contractor. Each Approved Contractor shall in all events work on the
Premises without causing labor disharmony, coordination difficulties, or delay
or impairment of any guaranties, warranties or obligations of any contractors of
Landlord and without causing unreasonable interference with the rights of other
tenants in the Building. If any Approved Contractor uses any Building services
or facilities prior to the Commencement Date, 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). Tenant shall include a provision in each contract with
each Approved Contractor whereby such Approved Contractor shall, by entry into
the Building or onto the Lot or Complex, agree 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 or on the Lot or
Complex, to the same extent as Tenant has so agreed in this Lease, which
indemnities of Tenant and Approved Contractor shall be joint and several.

      Subject to Tenant's right to contest amounts due to any Approved
Contractor, Tenant shall pay on or prior to date when any such payment is due
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 to the furnishing of labor and materials) is
filed against the Premises, the Building, the Lot or the Complex or any part
thereof (regardless of whether Tenant contests the same) 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. Tenant shall prepare and complete not less than seventy-five percent (75%)
of the Rentable Square Footage of Premises for Tenant's occupancy within nine
(9) months after the Substantial Completion Date. If Tenant does not prepare and
complete the entire Premises for Tenant's occupancy within nine (9) months after
the Substantial Completion Date, Tenant shall deposit with Landlord on or before
the end of such nine-month period a completion bond, letter of credit or similar
security in the amount of Landlord's reasonable estimate of the cost to so
complete the Premises. The form and substance of such bond, letter of credit or
similar security shall be subject to Landlord's approval, which approval shall
not be unreasonably withheld, conditioned or delayed.

      After the performance of Tenant's Work to prepare the Premises for
Tenant's initial occupancy, (i) all Tenant's Work shall be subject to all
provisions of this Lease relating to Tenant's Work to prepare the Premises for
Tenant's initial occupancy, except as expressly set

                                      -19-
<PAGE>

forth herein, and (ii) Landlord and Tenant shall agree in writing, at the time
Landlord approves any alteration or addition, whether Tenant will be required
to, permitted to or forbidden to, at Tenant's sole cost and expense, remove any
such alteration or addition and/or to readapt, repair or restore the Premises to
substantially the condition the same were in prior to 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 hereinafter
defined in this Section 3.2) 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, conditioned 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 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. 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. 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 or left 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 less than $100,000 (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 five (5) days after Tenant's request
for Landlord's consent with respect thereto, and (ii) if Landlord's consent was
not obtained therefor, upon the expiration or termination of this Lease, Tenant
shall readapt, repair and restore the affected portion of the Premises to
substantially the condition the same were in prior to such Minor Alteration.
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
$100,000 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 to
prepare the Premises for Tenant's initial occupancy (a) any request for
Landlord's 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.

                                      -20-
<PAGE>

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 at the sole risk of the party performing
such work, in a good and workmanlike manner employing new materials of good
quality, and in compliance with all Development Approvals, Subsequent Approvals,
applicable laws, codes, ordinances, regulations and orders of any governmental
authority or insurer of the Building, including 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"). 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 the Base
Building Improvements, including, without limitation, Landlord's Contractor's
warranty as set forth in the Construction Contract, to Tenant for the Term of
this Lease, upon the expiration or sooner termination of which such warranties
shall automatically revert to Landlord.

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 Lease Representative in the case of Tenant lease matters
and Tenant's Construction Representative, in the case of Tenant design and
construction matters, or by any person hereafter designated in substitution or
addition by notice to the party relying.

3.5   LANDLORD INDEMNITY AND CORRECTION OF LANDLORD'S WORK.

      Landlord agrees to indemnify and hold Tenant harmless if any of Landlord's
Work described in the Final Design Documents (a) fails to comply with all
applicable laws, regulations, building codes, building design standards, the
Development Approvals and Subsequent Approvals and (b) with respect to all
materials, equipment and special designs, processes or products, infringes any
patent or other proprietary rights of others. If within one year after the
Substantial Completion Date (i) any item of Base Building Improvements does not
conform with the Final Design Documents or (ii) there is any latent defect or
any other 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 such nonconformity or defect to be
corrected without cost or expense to Tenant.

                                      -21-
<PAGE>

                                   ARTICLE IV

                                      RENT

4.1   FIXED RENT.

      (a) Monthly Installments; Definitions. Commencing on the Commencement
Date, Tenant covenants and agrees to pay Fixed Rent (as hereinafter defined) for
the Premises to Landlord by wire transfer as Landlord may from time to time
direct in writing, without any offset or reduction whatsoever (except as may be
made in accordance with the express provisions of this Lease), or in the absence
of wire transfer instructions from Landlord, 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, in the amount of (x) the Annual Fixed
Rent Rate set forth in Article I multiplied by (y) the Rentable Square Footage
of the Premises, and subject to adjustment as set forth in Sections 4.1(b),
5.1.2, 10.11 and 10.12 hereof (collectively "Fixed Rent" or "Annual Fixed
Rent"), in equal installments equal to 1/12th of the Fixed Rent 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 of Annual Fixed Rent. On the tenth anniversary of the
Commencement Date, each of the Annual Fixed Rent for the Premises (as initially
demised to Tenant) determined pursuant to Section 1.1 hereof, and the Annual
Fixed Rent for the Expansion Spaces as determined pursuant to Section 10.11
hereof, shall be increased by an amount determined by multiplying the Annual
Fixed Rent for the first Lease Year by twenty-one and nine tenths percent
(21.9%). For example, if for the first Lease Year the Annual Fixed Rent for the
Premises (as initially demised to Tenant) is $5,877,350.00, such increase shall
equal $1,287,139.65 and if the Annual Fixed Rent (including the TI Factor) for
both the Expansion Spaces as determined pursuant to Section 10.11(c) is
$1,520,500.00, such increase shall equal $332,989.50. The respective Annual
Fixed Rent for the Premises (as initially demised to Tenant) and the Expansion
Spaces, increased as aforesaid, shall be effective from the tenth anniversary of
the Commencement Date through the end of the fifteenth Lease Year.

      (c) Landlord shall send advance written notice to Tenant on a monthly
basis of the amount of Fixed Rent and Additional Rent due pursuant to this
Lease; such advance written notice shall be given not later than five (5)
business days prior to the first day of each calendar month.

4.2   ADDITIONAL RENT.

      As used herein, the term "Additional Rent" shall mean all rent, charges
and other sums, other than Fixed Rent, due Landlord pursuant to this Lease. All
regularly recurring items of Additional Rent, such as the Annual Maintenance
Charge, shall be paid by Tenant to Landlord by wire transfer as Landlord may
from time to time direct in writing, or in the absence of wire transfer
instructions from Landlord, at the Original Address of Landlord. Nonrecurring
items of Additional Rent shall be paid by Tenant to Landlord by check or wire
transfer as Tenant may from time to time elect. In order that the Fixed Rent
shall be absolutely net to Landlord,

                                      -22-
<PAGE>

commencing on the Commencement Date, Tenant covenants and agrees to pay, as
Additional Rent, without any offset or reduction whatsoever except as expressly
set forth in this Lease, taxes, municipal or state betterment assessments,
insurance costs, utility charges and the Annual Maintenance Charge 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 with respect to the Building, Lot
and Complex. 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" (as such term is defined in Section 4.2.4 hereof) of the Term is set forth
in Section 1.1 as the "Initial Estimated Annual Additional Rent for Building"
and "Initial Estimated Annual Additional Rent for Complex".

      4.2.1 REAL ESTATE TAXES. Tenant shall pay directly to the Landlord: (i)
Tenant's Proportionate Fraction for Building and Tenant's Proportionate Fraction
for Complex, respectively, of 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, which are, at any time during the Term hereof, imposed or levied
upon or assessed against the Premises, the Building, the Lot or the Complex, and
(ii) the full amount of any tax or assessment imposed or levied upon or against
(A) any Fixed Rent, Additional Rent or other sum payable hereunder, (B) this
Lease, or the leasehold estate hereby created, or which arise in respect of the
operation, possession or use of the Premises; (C) 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; and (D) 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
for Building and Tenant's Proportionate Fraction for Complex, respectively, of
any such taxes if they are levied, assessed or payable on account of the
acquisition, leasing or use of the entire Building Lot or the Complex) which may
become a lien on the Building, the Lot, the Premises or the Complex
(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 more than twenty (20)
days after receipt of an invoice therefor. 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 twenty 20 days
after receipt of an invoice therefor, Tenant's Proportionate Fraction for
Building and Tenant's Proportionate Fraction for Complex, as applicable, 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. Subject to Tenant's
payment to Landlord of taxes and assessments as and when required by this
Section 4.2.1, Landlord agrees to pay such tax and assessments to the proper
authorities prior to delinquency and to provide Tenant with evidence of such
payment upon request therefor. Anything herein to the contrary notwithstanding,
if and to the extent that the Lot is not a separately assessed parcel, Landlord
shall make reasonable allocation of any taxes and assessments between the Lot
and the Building and the Complex of which the Lot is a part.

                                      -23-
<PAGE>

      Tenant may apply for any abatement of, or otherwise contest, any tax or
assessment, provided that 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 paid by Tenant. Landlord and Tenant shall
discuss and may mutually agree upon any other tax initiatives available for the
Lot or Building.

      Nothing contained in this Lease shall, however, require Tenant to pay any
income taxes, excess profits taxes, excise taxes, franchise taxes, or any taxes
or assessments with respect to the Garage and other buildings leased or
available for lease (and the parcels of land upon which such buildings are
situated), other than the Building, the Lot and any building or portion of a
building in the Complex which is not designed and available for lease to third
parties (and the parcel(s) of land on which the same may be located) in the
Complex ("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, the Lot, or the Complex 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"), Tenant's Proportionate Fraction for Building and Tenant's Proportionate
Fraction for Complex, respectively, of 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 Building and Lot and buildings not
available for lease (and the parcel(s) of land on which the same may be located)
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 Building and Lot.

      4.2.2 INSURANCE.

            4.2.2.1 INSURANCE TAKEN OUT BY TENANT.

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

            (a)   Comprehensive general 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, in the Building or
on the Lot 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, Building and Lot
are located for property similar to the Premises, Building and Lot and used for
similar purposes, provided, however, Landlord shall not require an increase in
such limits more frequently than once every

                                      -24-
<PAGE>

three years, and in no event shall such limits exceed the limits required for
buildings in Cambridge similar to the Building, and shall be written on the
"Occurrence Basis"; and

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

All insurance required to be carried by Tenant pursuant to this Lease may be
provided under one or more "blanket" insurance policies covering other locations
and facilities operated by Tenant or any Affiliate of Tenant, provided that such
blanket policies otherwise comply with the provisions of this Section. In
addition, Tenant may satisfy the $10,000,000 per occurrence liability insurance
coverage, as the same may be increased pursuant to Section 4.2.2.1(a) hereof,
with excess liability (so-called "umbrella") coverage, so long as Tenant
maintains primary liability coverage of not less than $5,000,000, as the same
may be increased pursuant to Section 4.2.2.1(a) hereof.

            (c)   Landlord acknowledges that Tenant shall have the right, at its
sole election, and at Tenant's sole expense, independently to obtain all risk
fire and casualty and boiler/sprinkler damage insurance similar to that
described in Sections 4.2.2.2(b) and (c) below for the purpose of providing
Tenant with insurance proceeds to fund any Tenant's Restoration Fund described
in Section 6.1(h) below. However, Tenant shall not be obligated to file any
claim or use any proceeds from such insurance for the benefit of Landlord.

            4.2.2.2 INSURANCE TAKEN OUT BY LANDLORD.

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

            (a)   Comprehensive general liability insurance for the Building,
the Lot and the Complex of the same nature and type as described in Section
4.2.2.1(a) of this Lease, and with the same policy limits or such higher policy
limits as Landlord may reasonably determine; and

            (b)   All risk, fire and casualty insurance on a one hundred percent
(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 Complex Common Areas;
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 office 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, the Lot or
the Complex.

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

                                      -25-
<PAGE>

            4.2.2.3 TENANT REIMBURSEMENT OF INSURANCE TAKEN OUT BY LANDLORD.

            Tenant shall from time to time reimburse Landlord within thirty (30)
days of Landlord's invoice for Tenant's Proportionate Fraction for Building and
Tenant's Proportionate Fraction for Complex 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, the Building, the Lot, other
buildings in the Complex, and Complex Common Areas, 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. 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, the Lot or the Complex 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
Commonwealth of Massachusetts 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
thirty (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 (and such mortgagees of Landlord and members or shareholders of
Landlord if such mortgagees, members and/or shareholders may be named as
additional insureds without additional premium charges to Tenant or, if
additional premium charges are required, if Landlord reimburses Tenant for such
additional premium charges) as additional insureds and shall be noncancellable
with respect to the interest of Landlord, Landlord's Managing Agent and such
additional insureds without at least thirty (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, the Building, the Lot or the Complex or with respect 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

                                      -26-
<PAGE>

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 from such insurance. Tenant shall not acquire
as insured under any insurance carried by Landlord on the Premises, the
Building, the Lot or the Complex 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 FOR PREMISES.

      Landlord and Tenant agree that the Design Documents shall include separate
metering or submetering of all or certain Utility Services (as hereinafter
defined) for the Premises and the Expansion Spaces. Accordingly, to the extent
such Utility Services are separately metered or submetered, 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 (singularly, "Utility Service" and collectively, "Utility 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.
Landlord shall be under no obligation to furnish any utilities to the Premises
except as may be shown on the Final Design Documents and Landlord shall not be
liable for any interruption or failure in the supply of any such Utility
Services 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 Utility Services to the Premises
but otherwise shall have no liability to Tenant.

      To the extent permitted by law, Landlord shall have the right at any time
and from time to time during the Term to contract for or purchase one or more
Utility Services from any company or third party, including without limitation,
electricity, steam, chilled water and natural gas (collectively "Utilities")
providing Utilities (the "Utility Service Provider" or "Utility Service
Providers"), which contracts or purchases, in Landlord's reasonable opinion, are
likely to result in a reduction in costs for Utility Services to the occupants
of the Building or of the Complex taken as a whole. Tenant, at no cost to
Tenant, agrees to reasonably cooperate with Landlord and the Utility Service
Providers and at all times and, as reasonably necessary, and on reasonable
advance notice, shall allow Landlord and the Utility Service Providers
reasonable access to any utility lines, equipment, feeders, risers, fixtures,
wiring and any other such machinery or personal property within the Premises and
associated with the delivery of Utility Services. Tenant may, but shall not be
required to, purchase Utilities from respective Utility Service Providers.

                                      -27-
<PAGE>

      4.2.4 COMMON AREA MAINTENANCE AND EXPENSES.

      Landlord shall maintain the interior and exterior common areas and
facilities of the Building and the Lot (collectively, "Building Common Areas")
and interior and exterior common areas and facilities of the Complex
(collectively, "Complex Common Areas") in the same quality and condition as
other comparable first class office buildings in Cambridge, including without
limitation, keeping the Building Common Areas and the Complex Common Areas clean
and free of debris, keeping the sidewalks, driveways and parking areas
reasonably clear of snow and ice, maintaining the exterior landscaping,
lighting, parking areas and sidewalks of the Lot and Complex, maintaining
passenger elevator service, providing Utility Services to the Premises (if any
of such Utility Services are not separately metered or submetered), Building
Common Areas and Complex Common Areas, including hot water for lavatory purposes
and cold water (at temperatures supplied by the provider thereof) for drinking,
lavatory and toilet purposes, providing cleaning and janitorial services to the
Premises on Monday through Friday, excluding holidays, provided the same are
kept in order by Tenant in accordance with cleaning standards from time to time
agreed upon by Landlord and Tenant, and providing security services for the
Building, Lot and Complex to a standard comparable to security services provided
at other first class office buildings in Cambridge. Landlord's obligations with
respect to the foregoing shall be subject to Tenant's right to self-manage the
Building as set forth in Section 10.11(h) hereof. Tenant shall maintain the
interior of the Premises, including the mechanical, electrical and plumbing
systems of the Premises in good order, repair and condition (provided that if
Tenant shall fail to effect such repairs or maintenance or Tenant shall elect to
have Landlord perform such repairs or maintenance, Landlord may or shall, as
applicable, 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 Building Maintenance and Operation Charge", the "Annual Complex
      Maintenance and Operation Charge" and the "Nonstandard Charge" as
      hereinafter defined.

                  (a)   The "Annual Building Maintenance and Operation Charge"
                  shall be equal to Tenant's Proportionate Fraction for Building
                  on account of all costs incurred by Landlord during the then
                  current fiscal year in operating the Building and Lot and
                  providing maintenance, including without limitation
                  maintenance, operation and repair of the Lot and the Building
                  and all heating, plumbing, electrical, air conditioning and
                  mechanical fixtures and equipment serving Building Common
                  Areas, Utility Services for Building Common Areas, maintenance
                  of Lot and Building signage, elevators, landscaping, snow
                  removal, trash dumpster rental, trash removal, management fees
                  (which management fees shall have a

                                      -28-
<PAGE>

                  commercially reasonably relationship to the scope of services
                  to be provided by such manager and which may be based upon a
                  percentage of rent payable by tenants of the Building),
                  amortization of equipment to the extent used for Building or
                  Lot operation and maintenance, and all costs incurred by
                  Landlord in order for Landlord to comply with the Development
                  Approvals and Subsequent Approvals and which are recurring or
                  properly categorized as operating or maintenance costs
                  ("Impositions") and equitably attributable or allocated to the
                  Building or Lot.

                  (b)   The "Annual Complex Maintenance and Operation Charge"
                  shall be equal to Tenant's Proportionate Fraction for Complex
                  on account of all costs incurred by Landlord during the then
                  current fiscal year in operating the Complex Common Areas and
                  providing Complex Common Areas maintenance, including without
                  limitation, maintenance, operation and repair of all heating,
                  plumbing, electrical, air conditioning and mechanical fixtures
                  and equipment serving Complex Common Areas, Utility Services
                  for Complex Common Areas, maintenance of Complex Common Area
                  signage, elevators, landscaping, snow removal, trash dumpster
                  rental, trash removal, management fees (which management fee
                  shall have a commercially reasonable relationship to the scope
                  of services to be provided by such manager and which may be
                  based upon a percentage of rent payable by tenants of the
                  Complex), amortization of equipment to the extent used for
                  Complex Common Areas operation and maintenance and all
                  Impositions equitably attributable to or allocated to the
                  Complex.

                  (c)   Anything herein to the contrary notwithstanding, the
                  Annual Maintenance Charge shall not include (1) leasing and
                  sales commissions for the Building or any portion thereof or
                  any other building in the Complex, (2) fees paid in connection
                  with any tenant improvement costs for the Building or any
                  other building in the Complex, (3) such other fees and
                  commissions paid in connection with the leasing, re-leasing,
                  extension or renewal of leases for the Building or any other
                  building in the Complex, (4) costs incurred with respect to
                  the operation and maintenance of any other building in the
                  Complex which is leased or available for lease, any rentable
                  space therein or any common areas or facilities in such
                  buildings (except to the extent that Landlord maintains a
                  management office therein or in the Building, in which event
                  Tenant shall pay Tenant's Proportionate Fraction for Complex
                  of the fair market rental value thereof as equitably
                  determined by Landlord), (5) any cost or expense attributable
                  to the underground garage(s) within the Complex, (6) costs and
                  expenses which are properly attributable to a particular
                  building (other than the Building) or a particular tenant
                  thereof, (7) third party management fees for the Building and
                  Complex included in the Annual Maintenance Charge to the
                  extent such fees exceed market rate fees, (8) any management
                  or supervisory fee of Landlord if a third party is managing
                  the Building or Complex (but if Landlord is self-managing the
                  Building or Complex,

                                      -29-
<PAGE>

                  Landlord shall be entitled to reimbursement of its reasonable
                  costs for managing the Building and Complex provided that in
                  no event may Landlord's costs and fees for self-managing the
                  Building or Complex exceed the costs and fees that a
                  market-rate third party management company would charge for
                  providing comparable services), (9) in the event that any
                  capital repair, improvement or replacement to the Building
                  Common Areas or the Complex Common Areas 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 Building Maintenance and Operation
                  Charge or Annual Complex Maintenance and Operation Charge, as
                  applicable, provided that replacement of a capital item shall
                  be of substantially the same quality and/or usefulness as the
                  capital item being replaced or shall be expected to reduce the
                  Annual Maintenance Charge or otherwise provide some other
                  economic benefit to the operation of the Premises, Building,
                  Lot or Complex, as applicable, such as conserving energy or
                  environmental resources, or if such capital item is required
                  by any law enacted after the date hereof, (10) wages,
                  salaries, or other compensation or benefits paid to any
                  persons above the grade of Building manager and Complex
                  manager (or equivalent position), (11) debt service, (12)
                  capital expenditures (except to the extent expressly permitted
                  under this Section), (13) depreciation and amortization
                  (except to the extent expressly permitted under this Section),
                  (14) legal and accounting fees relating to (A) disputes with
                  occupants of the Building or Complex, or (B) disputes with
                  purchasers, prospective purchasers, mortgages or prospective
                  mortgagees, (15) any rent under any ground or underlying
                  lease, (16) any fines or penalties incurred due to violations
                  of law by Landlord or any tenant or other occupant of the
                  Building or Complex, (17) any amount incurred to any entity
                  affiliated with Landlord to the extent the same exceeds the
                  amount which would have been incurred on an arm's length basis
                  in the absence of such affiliation, (18) any interest, fines
                  or penalties incurred or resulting from late payment by
                  Landlord of any operating expense, (19) costs incurred in
                  connection with the Environmental Remediation, (20) any
                  amounts incurred for repairs or other work occasioned by fire,
                  windstorm or other casualty to the extent Landlord is
                  reimbursed by insurance or would have been reimbursed by
                  insurance had Landlord maintained the insurance it is required
                  to maintain under this Lease, (21) costs incurred in
                  connection with the permitting, financing or construction of
                  the Building and Complex, and (22) any so-called asset
                  management fees.

                  (d)   Notwithstanding anything herein to the contrary, if
                  Landlord furnishes or makes available any service, utility or
                  facility to less than all of the tenants of the Building or
                  Complex, as

                                      -30-
<PAGE>

      applicable, Landlord shall allocate all costs incurred by Landlord on
account of such services, utilities or facilities to the tenants of the Building
or Complex, as applicable, to whom or to which such services, utilities or
facilities are furnished or made available (all such costs being herein referred
to as the "Nonstandard Costs"). The "Nonstandard Charge" shall be equal to
Tenant's share of Nonstandard Costs equitably allocated to Tenant by Landlord.
By way of example, if Landlord furnishes trash services and/or cleaning services
to office tenants of the Building, but not to retail tenants, Landlord shall
allocate the cost thereof to office tenants and Tenant shall pay Tenant's share
thereof as equitably allocated by Landlord. In no event shall any cost incurred
by Landlord and payable by Tenant for insurance, Utility Services for the
Premises which are separately metered or submetered, and taxes and assessments
be included in the Annual Maintenance Charge payable by Tenant.

      Attached hereto as Exhibit B-2 is a list of the Annual Maintenance Charge
categories for the Building Common Areas and the Complex Common Areas,
insurance, Mitigation Expenses and taxes and assessments. Landlord shall not
make any change in the categories set forth thereon without obtaining Tenant's
prior consent thereto, which consent shall not be unreasonably withheld, delayed
or conditioned. Tenant, however, shall have no right to approve the amount of
any costs which may be incurred by Landlord with respect to such categories,
except to the extent set forth in Section 10.11(h) hereof. 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 five percent (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 in each
Lease Year (the "Additional Rent Adjustment Date"), and Tenant's payments shall
thereupon be adjusted accordingly. Not later than ninety (90) 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 Charge for the said fiscal year or fraction thereof, and thereupon
any balance owed by Tenant shall be paid to Landlord within twenty (20) days
after Tenant receives written notice thereof, and any excess paid by Tenant
under this Section shall be paid to Landlord, or credited to Tenant, on the next
rent payment date. Tenant shall have the right for a period of one (1) year
following its receipt of Landlord's Statement to examine Landlord's books and
records concerning the Annual Maintenance Charge. Such examination may be made
only by an independent certified public accounting firm approved by Landlord,
which approval shall not be unreasonably withheld, conditioned or delayed.
Landlord may withhold its approval of any examiner of Tenant who or which is
being paid by Tenant, in whole or in part, on a contingent fee basis. As a
condition to performing any such examination, Tenant and its examiner shall be
required to execute and deliver to Landlord an agreement, in form acceptable to
Landlord, agreeing to keep confidential any information which it discovers about
Landlord, the Building or the Complex in connection with such examination. If
the Annual Maintenance Charge due was less than the Annual Maintenance Charge
paid by Tenant, Landlord shall either promptly refund to Tenant the difference
or credit same against rent next

                                      -31-
<PAGE>

due was less than ninety-five percent (95%) of the Annual Maintenance Charge
paid by Tenant, Landlord shall reimburse Tenant for the reasonable third-party
costs of reviewing Landlord's books and records.

      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, provided
that any such change in the fiscal year shall not result in any inequitable
shifting of, or increase in, amounts payable to Landlord by Tenant. 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 twenty (20) days after Landlord
delivers such estimate to Tenant. 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.

      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.

                                      -32-
<PAGE>

      Not later than ninety (90) 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, within twenty (20) days thereafter. 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.

      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 (but not more frequently than once every three years), and upon any such
change all items referred to in this Section shall be appropriately apportioned,
provided that any such change in accounting periods shall not result in any
inequitable shifting of, or increase in, amounts payable to Landlord by Tenant.
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) twenty (20) 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 (or if
such due date is not a business day, on the first business day after such due
date), at Landlord's election it shall bear interest from the due date (or if
such due date is not a business day, from the first business day after such due
date) at the higher of (a)(i) the annual rate of interest payable by Landlord to
its mortgagee(s) or (ii) the prime commercial rate of Fleet National Bank or its
successor(s), as it may be adjusted from time to time, plus (b) four percent
(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.

                                      -33-
<PAGE>

                                    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.

            Except for (i) the period of time permitted by this Lease for Tenant
to perform Tenant's Work to prepare the Premises for Tenant's initial occupancy
as set forth in Section 3.2.1 and (ii) a period of six (6) months at the end of
the Term, and (iii) temporary vacancies of not more than forty percent (40%) of
the Premises at any one time, 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,
and to the extent set forth in Section 10.21 hereof, with Landlord's
cooperation. Without limitation, Tenant shall comply in all material respects
with all federal, state, and municipal laws, ordinances, and regulations
governing, and all Development Approvals, Subsequent Approvals and Title
Exceptions applicable to, Tenant's particular use or manner of use of the
Premises. 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 activities on the Premises; its, 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
thirty (30) 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 matters
described in this Section 5.1.2 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, the

                                      -34-
<PAGE>

Lot and the Complex 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, the Lot or the Complex, 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 of the 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, damages by fire or other insurable casualty or eminent
domain, and any damage directly caused by failure of Landlord to perform any of
its obligations 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. Unless otherwise agreed to by Landlord and Tenant, Tenant shall be
responsible for Utility Services 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, air
conditioning and other utility systems exclusively 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 the responsibility of Landlord pursuant to the terms of this Lease and
unless 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 Tenant's personal property of any kind
on or about the Premises; and to comply with the orders

                                      -35-
<PAGE>

and regulations of all governmental authorities with respect to zoning,
building, fire, health and other codes, regulations, ordinances or laws
applicable to the Premises. To the extent that applicable law requires Tenant to
make any capital repair, alteration, addition or replacement to the Premises,
Tenant may implement such capital repair, alteration, addition or replacement
over the longest period provided by applicable law.

      Except for typical office operating and cleaning supplies which are
stored, used and disposed of in compliance with all applicable laws, 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 facility for the storage or
disposal of such hazardous materials or (ii) Tenant has reason to believe that
such transportation may result in a public demonstration, protest or other
similar disturbance at the Building, the Lot or the Complex. If the use 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) injury, 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, injury, 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, injury, 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

                                      -36-
<PAGE>

            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 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, injury, 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 or on or under the Lot or Complex
prior to the date hereof, (B) migration of hazardous materials from any site
onto or under the Lot or Complex not caused by Tenant, (C) the use of any
hazardous materials at the Building, the Lot or Complex by Landlord, any other
tenant or occupant, or any so-called "midnight dumpers" or (D) the Use by any
party other than Tenant of hazardous materials at the Building, the Lot or
Complex 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.

            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 of hazardous materials by Tenant at the Premises.

      5.1.5 TENANT'S WORK.

      To procure at Tenant's sole expense all necessary permits and licenses
(but subject to Section 10.21 hereof) 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 new 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 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, in the Building, on the
Lot or in the Complex; 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 agreed to by
Landlord for an Approved Contractor 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.

      5.1.6 INDEMNITY.

      To defend, with counsel reasonably approved by Landlord, all actions
against Landlord, any officer, director, member, manager or stockholder of
Landlord, or a partner, trustee, stockholder, officer, director, employee, agent
or beneficiary of any member of Landlord, holders of mortgages secured by the
Premises, the Building or Lot and any other party having an interest in the
Premises ("Indemnified Parties") with respect to, and to pay, protect, indemnify

                                      -37-
<PAGE>

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, the Lot or Complex 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.

      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 twenty-four (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 be authorized or
required to perform pursuant to this Lease, and show the Premises to prospective
purchasers and lenders, and, during the last eighteen (18) 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,
and (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.

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<PAGE>

      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 3.2.1 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 at the time of Landlord's approval of such installation (but excluding
any Tenant's Work performed to prepare the Premises for Tenant's initial
occupancy) 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, reasonable wear
and tear, damage by fire or other insured casualty or eminent domain, and damage
directly caused by failure of Landlord to perform any of its obligations only
excepted. Subject to Section 3.2.1 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 fifteen (15) days' prior notice by Landlord, to
execute, acknowledge and deliver to Landlord, any prospective purchaser or
mortgagee of the Premises, Building, Lot or Complex 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, the Building, Lot or Complex. Any such
statement delivered pursuant to this Section 5.1.11 may be relied upon by any
such prospective purchaser or mortgagee of the Premises, or any prospective
assignee of any such mortgage.

                                      -39-
<PAGE>

      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, except for
consents or approvals pursuant to Section 5.2.1 hereof, 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, Lot and or the Complex, provided that such
amendments do not materially interfere with Tenant's right of use and enjoyment
of the Premises, the Lot or the Complex pursuant to this Lease. So long as
Tenant and Tenant's Affiliates are occupying at least sixty percent (60%) of the
r.s.f. in the Building or Tenant, Tenant's Affiliates or Tenant's subtenants are
occupying all of the r.s.f. in the Building, in each case excluding the Retail
Space, all such amendments shall be subject to Tenant's prior approval, which
shall not be unreasonably withheld, conditioned or delayed.

      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 to be set forth in the Design
Documents 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 Premises shall be
placed or maintained by Tenant in settings of cork, rubber, spring, or other
types of vibration or noise 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:

                                      -40-
<PAGE>

      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; no assignment or
subletting (including any assignment or sublease not requiring Landlord's
consent) shall affect the continuing primary liability of Tenant (which,
following assignment, shall be joint and several with the assignee); and 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 and Complex. Tenant shall not assign this Lease or
sublease any portion of the Premises to any other tenant in the Complex (unless
such assignee or subtenant is an Affiliate of Tenant) if Landlord has or expects
to have (within two years of the date of the proposed assignment or sublease)
vacant space in the Complex for the Permitted Uses or if the proposed assignee
or subtenant (other than an Affiliate of Tenant) has been shown space in the
Complex within six months prior to the date of the proposed assignment or
sublease. In addition, as to any assignee or subtenant of all or substantially
all of the Premises, Landlord's consent shall be required as to the
creditworthiness of the proposed assignee or subtenant in view of market
conditions then prevailing for leases having terms and conditions comparable to
this Lease and the obligations of such assignee or subtenant pursuant to this
Lease.

            If Tenant requests Landlord's consent to (i) assign this Lease or
sublet any portion of the Premises to any entity which is not an Affiliate of
Tenant (as herein defined) for the remainder of the Term or substantially all of
the remainder of the Term or (ii) sublease more than twenty-five percent (25%)
of the r.s.f. of the Premises for a sublease term of more than five years,
Landlord shall have the option, exercisable by written notice to Tenant given
within thirty (30) days after receipt of such request, to terminate this Lease
(in the event of a proposed assignment of the Lease ) or to terminate this Lease
only as to the portion of the Premises proposed to be subleased (in the event of
a sublease), in each case as of the date of commencement of the proposed
sublease or assignment. If Landlord does not exercise such right to terminate
this Lease as to, and recapture, the portion of the Premises affected by such
assignment or sublease, Tenant shall have the right to consummate the assignment
or sublease at such rent as Tenant may determine, provided however, that Tenant
shall pay to Landlord fifty percent (50%) of Sublease Profits (as herein
defined) on account thereof.

            Provided that Tenant is not then in default under this Lease beyond
any applicable notice, grace or cure period, Tenant shall have the right,
without Landlord's consent, to sublease up to twenty-five percent (25%) of the
r.s.f of the Premises, in the aggregate, to subtenants, provided that no such
sublease shall be for a term greater than the lesser of (x) five (5) years
(including any extensions provided for in such sublease) and (y) the remaining
term of this Lease (including any Extension Term previously exercised), and
provided further that in such case Tenant shall pay to Landlord fifty percent
(50%) of the Sublease Profits (as herein defined) with respect to all such
subleases. All other subleases by Tenant shall require Landlord's consent.

                                      -41-
<PAGE>

      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 and the then unamortized costs of Tenant's
Work incurred by Tenant to prepare the Premises for Tenant's initial occupancy
of the Premises. Sublease Costs shall be amortized on a straight line basis over
the term of the applicable sublease or assignment, except that the costs of
Tenant's Work incurred by Tenant to prepare the Premises for Tenant's initial
occupancy of the Premises shall be amortized on a straight line basis over the
initial Term of this Lease.

      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 and without payment of Sublease Profits to Landlord 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), 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) and the assignee or sublessee executes an
assignment and assumption agreement reasonably satisfactory to Landlord prior to
taking occupancy. From and after any sublease or assignment, Genzyme Corporation
shall continue to be primarily liable pursuant to this Lease. As used herein,
the term "Affiliate of Tenant" shall mean and refer to any entity controlled by,
controlling or under common control with Tenant.

      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; nor to serve or
allow the consumption of alcoholic beverages in the Premises unless Tenant
maintains Host Liquor liability insurance with an insurance company and with
limits of coverage satisfactory to Landlord.

      5.2.3 INSTALLATION, ALTERATIONS OR ADDITIONS.

      Except for Tenant's Work approved or deemed approved by Landlord pursuant
to this Lease or any Minor Alteration, not to make any installations,
alterations, or additions in, to or on the Premises nor to permit the making of
any openings in the walls, partitions, ceilings or floors of the Premises.

                                      -42-
<PAGE>

                                   ARTICLE VI

                               CASUALTY OR TAKING

6.1   DAMAGE BY FIRE.

      In the event of loss of, or damage to, the Premises or the Building or the
Garage by fire or other casualty, the rights and obligations of the parties
hereto shall be as follows:

            (a)    (i) If the Premises, or any part thereof, shall be damaged by
fire or other casualty, Tenant shall give prompt notice thereof to Landlord, and
Landlord, upon receiving such notice, shall proceed promptly and with due
diligence, subject to Force Majeure Events, to repair, or cause to be repaired,
such damage except as otherwise provided herein. With respect to portions of the
Building or Lot outside of the Premises that shall be damaged by fire or other
casualty, Landlord shall proceed promptly and with due diligence, subject to
Force Majeure Events, to repair, or to cause to be repaired, such damage after
such damage occurs except as otherwise provided herein; and (ii) if the Garage,
or any part thereof, shall be damaged by fire or other casualty and if the
Garage is then owned by Landlord or an Affiliate of Landlord (as defined in
Section 10.14 hereof) Landlord shall cause the Garage Owner to proceed promptly
and with due diligence, subject to Force Majeure Events, to repair or cause to
be repaired, such damage except as otherwise provided herein. If Landlord or an
Affiliate of Landlord is not the Garage Owner at the time of such damage by fire
or other casualty, Landlord shall use all reasonable efforts to cause the Garage
Owner to proceed promptly to repair such damage.

            (b)(i) If the Premises, or any part thereof, shall be rendered
untenantable by reason of such damage, whether to the Premises or to the
Building or if such damage materially interferes with Tenant's access to the
Premises, Annual Fixed Rent and Additional Rent shall proportionately (i.e.,
based on rentable square footage) abate for that portion of the Premises which
is untenantable for the period from the date of such damage or from the date
when material interference with Tenant's access due to such damage commences to
the date when such damage shall have been repaired or such access shall have
been restored, as applicable; and (ii) if the Garage or any part thereof shall
be rendered untenantable by reason of such damage or if such damage prevents
Tenant's access to the Garage then to the extent that Landlord does not provide
Tenant with substitute parking spaces in the Complex or within a reasonable
walking distance of the Premises ("Substitute Parking Spaces"), the Parking Fee
(as defined in Section 10.14) and/or the Valet Parking Fee (as defined in
Section 10.14), as applicable, shall proportionately abate (based upon the
number of Tenant's Parking Spaces or Valet Parking Spaces) for each of the
Garage Parking Spaces (as defined in Section 10.14) in excess of the Substitute
Parking Spaces which are unavailable to Tenant for the period from the date of
such damage or from the date when access to the Garage ceases due to such damage
to the date when such damage shall have been repaired or such access shall have
been restored, as applicable.

            (c)(i) If, as a result of fire or other casualty, the whole or a
substantial part of the Building is rendered untenantable, Landlord, within
ninety (90) days from the date of such fire or casualty, may terminate this
Lease by notice to Tenant, specifying a date not less than twenty (20) nor more
than forty (40) days after the giving of such notice on which the Term of this
Lease shall terminate. If Landlord does not so elect to terminate this Lease,
then Landlord shall

                                      -43-
<PAGE>

proceed with diligence to repair the damage to the Building and Premises and all
facilities serving the same, and the Annual Fixed Rent and Additional Rent shall
meanwhile proportionately abate, all as provided in Paragraph (b)(i) of this
Section 6.1. Landlord within one hundred twenty (120) days after the fire or
other casualty shall notify Tenant in writing whether or not, in its reasonable
judgment, the Building and the Premises can be restored to substantially their
condition prior to such damage and Utility Services restored within twelve (12)
months of the date of the casualty. If such notification shall state that such
restoration cannot be so accomplished, then Tenant may terminate this Lease
within thirty (30) days from Tenant's receipt of such notification. Furthermore,
if Tenant does not so terminate this Lease and if such damage is not repaired,
Utility Services are not restored and the Premises and the remainder of the
Building are not restored to substantially the same condition as they were prior
to such damage within twelve (12) months from the date of such damage, Tenant
within thirty (30) days from the expiration of such twelve (12) month period or
from the expiration of any extension thereof by reason of Force Majeure Events
(the "Casualty Restoration Completion Date"), may terminate this Lease by notice
to Landlord, specifying a date not more than forty-five (45) days after the
giving of such notice on which the term of this Lease shall terminate. The
period within which the required repairs may be accomplished shall be extended
by the number of days lost as a result of Force Majeure Events, provided however
that such period shall in no event be extended beyond six (6) months from the
Casualty Restoration Completion Date. Substantial part for purposes of this
Section 6.1(c)(i) and section 6.1(d) shall mean thirty-three and one-third
percent (33 1/3%) or more of the Building; and (ii) if as a result of fire or
other casualty, the whole or a substantial part of the Garage is rendered
untenantable, Landlord, within ninety (90) days from the date of such fire or
casualty, shall notify Tenant whether the Garage Owner will repair the damage to
the Garage. If the Garage Owner is unwilling to repair such damage and Landlord
does not provide substitute parking in the Complex or within reasonable walking
distance of the Premises, then Tenant may terminate this Lease by notice to
Landlord, specifying a date not less than twenty (20) nor more than forty (40)
days after the giving of such notice on which the Term of this Lease shall
terminate. "Substantial part" for this Section 6.1(c)(ii) shall mean
thirty-three and one-third percent (33 1/3%) or more of Tenant's Parking Spaces.

            If Tenant fails to terminate in accordance within the foregoing time
periods set forth in this Section 6.1, Tenant shall have waived its right to
terminate.

            (d)   If a substantial part of the Premises shall be rendered
untenantable by fire or other casualty during the last eighteen (18) months of
the then current Term of this Lease and Tenant has not exercised an Option to
Extend (as set forth in Section 10.12 hereof), Landlord may terminate this Lease
effective as of the date of such fire or other casualty upon notice to Tenant
given within sixty (60) days after such fire or other casualty.

            (e)   With respect to Sections 6.1 and 6.2 hereof, Landlord shall
not be required to repair or replace any of Tenant's trade fixtures, business
machinery, equipment, cabinet work, furniture, personal property or other
installations or improvements not originally installed by Landlord or otherwise
required to be insured by Tenant under the terms of this Lease, or any damage to
the Premises or Building caused by Tenant, or any damage to the Premises or
Building not covered by insurance proceeds or condemnation proceeds, or any
damage or destruction which Landlord is unable to restore due to Landlord's
inability, after exercising

                                      -44-
<PAGE>

reasonable and diligent efforts, to obtain final approval therefor from
applicable governmental authorities, and no damages, compensation or claim shall
be payable by Landlord for inconvenience, loss of business or annoyance arising
from any repair or restoration of any portion of the Premises or of the
Building.

            (f) The provisions of this Section 6.1 shall be considered an
express agreement governing any instance of damage or destruction of the
Building, the Premises or the Garage by fire or other casualty, and any law now
or hereafter in force providing for such contingency in the absence of express
agreement shall have no application.

            (g) In the event of any termination of this Lease pursuant to this
Section 6.1, the Term of this Lease shall expire as of the effective termination
date as fully and completely as if such date were the date herein originally
scheduled as the Term Expiration Date; provided that Landlord shall within
thirty (30) days thereafter refund any prepaid Rent.

            (h) If less than fifty percent (50%) of the Building shall be
rendered untenantable by fire or other casualty and Landlord reasonably
determines that the net proceeds of insurance recovered for such damage are not
adequate to restore the Building (or Premises or Utility Service) to
substantially their condition immediate prior to the damage or Landlord's
mortgagee refuses to release to Landlord sufficient insurance proceeds to
restore the Building (or Premises or Utility Service) to such condition,
Landlord shall so notify Tenant within ninety (90) days following such fire or
other casualty (the "Insurance Shortfall Notice"). Within thirty (30) days after
Tenant's receipt of an Insurance Shortfall Notice, Tenant may elect to (i)
terminate this Lease or (ii) (x) to require Landlord to restore the Building (or
Premises or Utility Service) to substantially their condition immediately prior
to the occurrence of such fire or other casualty and (y) to pay the difference
between the net proceeds of insurance recovered and the total cost of
restoration (the "Tenant Deficiency Election"). If Tenant does not give Landlord
notice of Tenant's Deficiency Election within such thirty (30) day period,
Landlord may terminate this Lease or Landlord may elect to restore the Building,
in each case pursuant to Section 6.1(c)(i) of this Lease. For purposes of this
Lease, the phrase "restored to substantially their condition immediately prior
to the occurrence of such fire or casualty" shall mean Landlord's Base Building
Improvements without the restoration of the improvements to the Premises that
constitute Tenant's Work, in respect to which Tenant acknowledges that Tenant,
and not Landlord, bears the risk of loss in respect to such fire or other
casualty.

            If Tenant makes Tenant's Deficiency Election, Tenant shall promptly
deposit with Landlord's mortgagee (or another "restoration trustee" mutually
acceptable to Landlord and Tenant) the amount reasonably estimated by Landlord's
architect to fund the estimated cost of the total restoration of the Building
(or Premises or Utility Service) to substantially their condition immediately
prior to the occurrence of the fire or other casualty after application of the
net proceeds of insurance recovered (the "Reconstruction Fund"); all amounts so
deposited by Tenant into the Reconstruction Fund shall be referred to herein as
"Tenant's Restoration Funds". Tenant shall have the right to approve the
construction cost for the restoration of the Building (or Premises or Utility
Service), which approval shall not be unreasonably withheld, conditioned or
delayed. Landlord's mortgagee (or another "restoration trustee" mutually
acceptable to Landlord and Tenant) shall advance proceeds from the
Reconstruction Fund, as needed, to prosecute the restoration as herein provided,
and any funds remaining in the Reconstruction Fund after final

                                      -45-
<PAGE>

completion and payment in full of all of the costs of restoration shall be
repaid to Tenant. If there is a shortfall in the funding of such restoration,
Tenant shall promptly deposit the required additional funds in the
Reconstruction Fund after Tenant's receipt of notice from Landlord or Landlord's
mortgagee (or another "restoration trustee" mutually acceptable to Landlord and
Tenant) stating the amount of the required additional funds and a description of
the restoration work related thereto. Landlord shall have no obligation to repay
to Tenant any portion of the Reconstruction Fund. During the period of such
restoration, Fixed Rent and Additional Rent shall proportionately abate as set
forth in Section 6.1(b)(i) hereof.

            (i)   Landlord's architect's certificate, given in good faith, shall
be deemed conclusive of the statements therein contained and binding upon Tenant
with respect to the extent of the damage to the Building and the performance and
completion of any repair or restoration work undertaken by Landlord pursuant to
Sections 6.1 or 6.2 hereof.

6.2   CONDEMNATION.

      In the event that the whole or any substantial part of the Building shall
be taken or appropriated by eminent domain or shall be condemned for any public
or quasi-public use, or (by virtue of any such taking, appropriation or
condemnation) shall suffer any damage (direct, indirect or consequential) for
which Landlord or Tenant shall be entitled to compensation, then (and in any
such event) this Lease and the Term hereof may be terminated at the election of
Landlord by a notice in writing of its election so to terminate which shall be
given by Landlord to Tenant within ninety (90) days following the date on which
Landlord shall have received notice of such taking, appropriation or
condemnation. In the event that a substantial part of the Premises or of the
means of access thereto or of the Garage Parking Spaces (as such term is defined
in Section 10.14 hereof) (unless replaced without undue delay by substitute
facilities within a reasonable walking distance from the Building) shall be so
taken, appropriated or condemned so as to substantially interfere with the
Permitted Uses of the Premises, then this Lease and the Term hereof may be
terminated at the election of Tenant by a notice in writing of its election so
to terminate which shall be given by Tenant to Landlord within sixty (60) days
following the date on which Tenant shall have received notice of such taking,
appropriation or condemnation. Substantial part for purposes of this Section 6.2
shall mean thirty-three and one-third percent (33 1/3%) or more of the Premises
or thirty-three and one-third percent (33 1/3%) or more of the Garage Parking
Spaces, as applicable.

      Upon giving of any such notice of termination (either by Landlord or
Tenant), this Lease and the Term hereof shall terminate as of the date on which
Tenant shall be required to vacate any part of the Premises or shall be deprived
of a substantial part of the means of access thereto. In the event of any such
termination, this Lease and the Term hereof shall expire as of the effective
termination date as fully and completely as if such date were the date herein
originally scheduled as the Term Expiration Date. If neither party elects to
terminate, Landlord will with reasonable diligence and at Landlord's expense,
restore the remainder of the Premises and Building, or the remainder of the
means of access and Garage Parking Spaces, to substantially the same condition
as practicable as existed prior to such taking, appropriation or condemnation in
which event a just proportion of the Annual Fixed Rent and Additional Rent,
according to the nature and extent of the taking, appropriation or condemnation
and the resultant injury sustained by the Premises and the means of access
thereto, shall be abated until what remains of the

                                      -46-
<PAGE>

Premises and the means of access thereto shall have been restored as fully as
may be for permanent use and occupancy by Tenant hereunder. In the event of any
taking of the Premises or any part thereof for temporary use, (i) this Lease
shall be and remain unaffected thereby, and (ii) Tenant shall be entitled to
receive for itself any award made for such use, provided, that if any taking is
for a period extending beyond the Term of this Lease, such award shall be
apportioned between Landlord and Tenant as of the Term Expiration Date.

6.3   AWARD.

      Irrespective of the form in which recovery may be had by law, all rights
to damages or compensation payable pursuant to Section 6.2 hereof shall belong
to Landlord in all cases except as set forth below in this Section 6.3 or in
Section 6.2 hereof. 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 Tenant's cost and expense, (ii) relocation
expenses or damages for loss of business (in excess of any such damages
attributable to the value of this lease) and (iii) the then unamortized cost of
Tenant's Work, in each case 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, in the case of the Fixed
Rent or regularly recurring items of Additional Rent, such as monthly payments
of the Annual Maintenance Charge, if such default shall continue for five (5)
days after written notice from Landlord to Tenant or, in the case of
nonrecurring items of Additional Rent, such as self-help costs incurred by
Landlord or amounts due as the result of the annual reconciliation of Annual
Maintenance Charge, if such default shall continue for twenty (20) days after
written notice from Landlord to Tenant (provided, however, that in all cases
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 written 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 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

                                      -47-
<PAGE>

thereafter, or (g) if Tenant fails to maintain the insurance required under
Section 4.2.2.1 hereof, (individually, an "Event of Default" and collectively,
"Events of Default") 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 prior to such time, if any, as Landlord has accepted a cure of such
Event of Default 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 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 commercially advisable or necessary to
relet the same, and no action of Landlord in accordance with the foregoing or
failure

                                      -48-
<PAGE>

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 for the
twelve (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 from Tenant 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 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
ten (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 within twenty (20) days after Tenant's receipt of a
written demand therefor, 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 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

                                      -49-
<PAGE>

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 or is otherwise deemed to be a mortgagee-in-possession under
applicable law, and such holder shall not in any event be liable to perform or
for failure to perform the obligations of Landlord under Article III. 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

                                      -50-
<PAGE>

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 a Subordination, Non-disturbance and Attornment
Agreement ("SNDA"), substantially in the form of Exhibit D hereto or any other
reasonable agreement which may be necessary to implement the provisions of this
Section 8.1 and Section 8.2 hereof.

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, the Building
or the Complex; provided, however, that Landlord shall have the option to
subordinate this Lease to any such mortgage of the Lot, the Building or the
Complex provided that Landlord obtains from the holder of record of any existing
or future mortgage an SNDA substantially in the form of Exhibit D hereto or
another form of agreement with Tenant and reasonably acceptable to 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 be 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 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

                                      -51-
<PAGE>

pursuant to Section 3.2 or Section 6.2 or (v) otherwise unreasonably limit,
impair or delay Tenant's rights hereunder. Tenant's failure or refusal to make
any such changes shall not constitute an Event of Default by Tenant pursuant to
this Lease.

                                   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 required to be furnished by
Landlord pursuant to this Lease (the cost of which is to be included in the
Annual Maintenance Charge).

      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
foundations, roof, exterior walls, exterior windows and waterproofing, floor
slabs, other structural columns, beams and other components, parking areas,
walks, landscaping, courtyard and any other Building Common Areas and Complex
Common Areas as may be necessary to keep them in the condition required by
Section 4.2.4 hereof. Landlord shall be responsible for the maintenance and
repair of the Utility Services 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, the Lot and the Complex (the appropriate 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 3.3 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 appropriate costs incurred
by Landlord in connection with the foregoing compliance obligations shall be
included in the Annual Maintenance Charge. All of the foregoing covenants

                                      -52-
<PAGE>

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 fifteen (15) days' prior notice from 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 SUBDIVISION.

            The Lot is currently a portion of the Complex and Landlord may
subdivide the Complex so as to, inter alia, establish the Lot as a separate
taxable parcel and establish easements which benefit and/or burden the Lot and
remaining portions of the Complex (collectively, the "Subdivision"). Upon or
after the Subdivision, Landlord may convey the Lot and assign this Lease to an
Affiliate of 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 so long as such Subdivision and Transfer do not
materially interfere with Tenant's use of the Premises or any of Tenant's rights
under this Lease to other portions of the Complex. After the Substantial
Completion Date, Landlord may convey the Lot and the Building and assign this
Lease to any person or entity.

                                      -53-
<PAGE>

                                    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,
with a copy to Genzyme Corporation, One Kendall Square, Building 1400,
Cambridge, Massachusetts 02139, Attention: General Counsel, and, if to Landlord,
at Landlord's Address or such other address as Landlord shall have last
designated by notice in writing to Tenant, with a copy to Patrick C. Toomey,
Esquire, Gadsby Hannah LLP, 225 Franklin Street, Boston, Massachusetts 02110.
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.

10.3  EASEMENTS; CHANGES TO LOT LINES.

      Landlord reserves the right, from time to time, to grant easements
affecting the Building, the Lot or the Complex and to change or alter existing
boundaries of the Lot or Complex for purpose of developing and using the Lot and
the Complex so long as such easements or such changes or alterations to existing
boundaries of the Lot or the Complex do not materially interfere with Tenant's
use of the Premises or any of Tenant's rights under this Lease to other portions
of the Complex.

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, 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 in the event that Tenant fails to do so within five
(5) days after Landlord's request therefor.

                                      -54-
<PAGE>

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, Building and Lot, but not upon
other assets of Landlord. Without limiting the generality of the foregoing, upon
any assignment of this Lease by Kendall Square, LLC to an Affiliate of Landlord,
Kendall Square, LLC shall have no further liability or obligation pursuant to
this Lease. No member, partner, trustee, stockholder, officer, director,
employee or beneficiary (or the members, 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 Building and Lot, including insurance proceeds and eminent
domain awards and the proceeds of any sale of the Building or Lot, and any rents
and profits from the Building in pursuit of its remedies upon an event of
default hereunder, and the general assets of the members, partners, trustees,
stockholders, officers, employees or beneficiaries (and the members, 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 party 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 (other
than the payment of money), 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 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 thirty (30) days following receipt
of written notice from Tenant or such additional time as is reasonably required
to correct any such default, or such other time as may be set forth in Section
3.2 hereof, after written 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 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.

                                      -55-
<PAGE>

      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 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 self-help should have been availed of by Tenant, or with respect to the
reasonableness of the expenses incurred by Tenant.

      Subject to the foregoing provisions of this Section 10.7, Landlord agrees
to pay on demand Tenant's expenses, including reasonable attorneys' fees,
incurred by Tenant in enforcing any obligation of Landlord under this Lease or
in curing any default by Landlord.

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
Trammel Crow Company and Insignia/ESG (the "Brokers") 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.
Landlord shall pay to the Brokers the real estate commission or fee due such
Brokers with respect to the transactions contemplated herein as and when due
pursuant to separate agreement(s) between Landlord and the Brokers.

10.9  APPLICABLE LAW AND CONSTRUCTION.

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

                                      -56-
<PAGE>

      There are no prior oral or written agreements between Landlord and Tenant
affecting this Lease. The Letter of Intent dated January 5, 2000 from Tenant to
Mr. David Clem of Lyme Properties, LLC shall be of no further force or effect.
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 in Article I above and their 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 EXPANSION OF PREMISES.

      (a) On the condition that this Lease is in full force and effect as of the
commencement of the fourth Lease Year, then, effective as of the commencement of
the fourth Lease Year, Landlord hereby leases to Tenant and Tenant hereby leases
from Landlord, subject to and with the benefit of the terms, covenants,
conditions and provisions of this Lease, approximately 25,000 r.s.f. in the
Building (the "First Expansion Space") in its then "AS IS" condition but subject
to Section 10.11(e) hereof, and on the condition that this Lease is in full
force and effect as of the commencement of the sixth Lease Year, then effective
as of the commencement of the sixth Lease Year, Landlord hereby leases to Tenant
and Tenant hereby leases from Landlord, subject to and with the benefit of the
terms, covenants, conditions and provisions of this Lease, approximately 25,000
r.s.f. in the Building (the "Second Expansion Space") in its then "AS IS"
condition but subject to Section 10.11(e) hereof. Landlord and Tenant agree that
the Premises initially demised to Tenant by Landlord pursuant to this Lease,
together with the First Expansion Space and the Second Expansion Space, will
constitute all of the Rentable Square Footage in the Building except for the
Retail Space.

      (b) On or prior to the date upon which Tenant is required to approve the
Schematic Design Documents, Landlord and Tenant shall mutually agree on the
location in the Building of the Premises initially demised to Tenant pursuant to
this Lease and the location in the Building of the First Expansion Space and the
Second Expansion Space (the "Expansion Spaces"). At such time as the parties
have agreed upon the location of the Expansion Spaces, the parties shall attach
hereto as Exhibit A-4 a plan showing such locations. The actual Rentable Square
Footage of the Expansion Spaces shall be determined pursuant to Section 2.3
hereof.

      (c) The Annual Fixed Rent Rate for each of the First Expansion Space and
the Second Expansion Space shall be calculated separately for each Expansion
Space and shall be (i) the Annual Fixed Rent Rate for the Premises initially
demised to Tenant plus (ii) the TI Factor

                                      -57-
<PAGE>

(as hereinafter defined) and shall be payable in each instance starting on the
later of the date on which possession of the First Expansion Space or the Second
Expansion Space, as applicable, is delivered to Tenant or the commencement of
the Term with respect thereto as set forth in Section 10.11(a) hereof. The "TI
Factor" shall mean the product of (1) the lesser of (A) all hard and soft costs,
determined on a Rentable Square Footage basis, incurred by Landlord for tenant
improvements to the First Expansion Space or the Second Expansion Space, as
applicable, or (B) $45.00 per r.s.f., multiplied by (2) twelve percent (12%).
For example, if Landlord incurs $45.00 per r.s.f. for tenant improvements to the
First Expansion Space, the TI Factor will be $5.40 per r.s.f. ($45.00 per r.s.f.
x .12 = $5.40 per r.s.f.). Except as set forth in Section 10.11(e) hereof,
Landlord shall have no obligation to make any tenant improvements to the
Expansion Spaces for Tenant. Notwithstanding any other provision of this Lease,
Tenant shall not be required to pay rent on either of the Expansion Spaces prior
to the respective dates determined above in this Section 10.11(c).

      (d) Commencing as of the tenth anniversary of the Commencement Date, the
Annual Fixed Rent (including the TI Factor) for the First Expansion Space and
the Second Expansion Space shall be increased by an amount determined by
multiplying such Annual Fixed Rent by twenty-one and nine tenths percent
(21.9%), as provided in Section 4.1(b) hereof.

      (e) Tenant improvements made to the Expansion Spaces by Landlord for the
initial tenants thereof shall be consistent with first class office space and
not overly partitioned with small or large offices and conference/training
rooms. Tenant may make alterations to the Expansion Spaces after the respective
commencement of the Term with respect thereto, but only in accordance with the
provisions of this Lease applicable to Tenant's Work, and Tenant shall occupy
the Expansion Spaces within six (6) months after the respective commencement of
the Term with respect thereto.

      (f) The term of this Lease with respect to the Expansion Spaces shall be
coterminous with the Term Expiration Date, such that the Lease of the Expansion
Spaces will terminate as of the Term Expiration Date.

      (g) Upon commencement of the Term with respect to the First Expansion
Space and the Second Expansion Space, as applicable, (i) the term "Premises" as
used herein shall mean the Premises initially demised to Tenant and the First
Expansion Space and Second Expansion Space, as applicable, and (ii) Tenant's
Proportionate Fraction for Building and Tenant's Proportionate Fraction for
Complex shall be recalculated as set forth in Section 2.3 hereof.

      (h) If upon commencement of the Term with respect to the Second Expansion
Space, and so long thereafter as Genzyme Corporation, Affiliate(s) of Tenant or
subtenants of Tenant occupy all of the r.s.f. of the Building, other than the
Retail Space, (i) Tenant shall have line item approval rights, not to be
unreasonably withheld, delayed or conditioned, with respect to major operating
expense categories for the Building set forth in Exhibit B-2 hereto or (ii)
Tenant may self-manage the Building, including the Retail Space, provided,
however, that during any period of self-management, Tenant shall be obligated to
manage, maintain and repair the Building in the same condition and to the same
extent required of Landlord pursuant to Section 4.2.4 and 9.1.2 hereof and
provided further, that Landlord shall have the right to approve any and all
contracts

                                      -58-
<PAGE>

and contractors to be retained by Tenant, such approval not to be unreasonably
withheld, delayed or conditioned.

      (i) Notwithstanding Section 10.11(a) hereof, Tenant shall have the option
to (i) lease the First Expansion Space as of the Commencement Date or (ii) both
Expansion Spaces as of the Commencement Date (as so determined, the "Early
Occupancy Space") by giving Landlord written notice thereof on or prior to that
date which is one (1) year prior to the Scheduled Substantial Completion Date.
In such event, (a) the Annual Fixed Rent for the Early Occupancy Space shall be
determined on the basis of the Annual Fixed Rent Rate set forth in Section 1.1
and not pursuant to Section 10.11(c) hereof and (b) Landlord shall have no
obligation to make any tenant improvements to the Early Occupancy Space.

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
(including the Expansion Spaces). If Tenant shall desire to exercise either
Option to Extend, it shall give Landlord a notice (the "Inquiry Notice") of such
desire not later than twenty-one (21) 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 (b) below) for
the applicable Extension Period shall be determined in accordance with
Subsection (e) below. After the applicable Fair Market Rent has been so
determined, Tenant may exercise each Option to Extend by giving Landlord written
notice (the "Exercise Notice") of its election to do so not later than (x) the
date by which Fair Market Rent has been determined pursuant to this Section
10.12 or (y) eighteen (18) months prior to the expiration of the Initial Term of
this Lease, or the preceding Extension Period, as the case may be, whichever is
earlier. If Tenant fails to timely give either the Inquiry Notice or the
Exercise Notice to Landlord with respect to any Option to Extend, at the sole
election of Landlord, Tenant shall be conclusively deemed to have waived such
Option to Extend hereunder.

      (b) For purposes of this Section 10.12, "Fair Market Rent" shall mean the
average of (1) ninety-five percent (95%) of the fair market rental value for
unfinished, shell office space in a comparable office building in the Kendall
Square, Cambridge, Massachusetts office market area (the "Relevant Market") and
(2) ninety-five percent (95%) of the fair market rental value for the Premises,
including the Expansion Spaces, and shall take into account all other relevant
factors in the Relevant Market, including the ten (10) year term of the
applicable Extension Period. In no event shall the Fair Market Rent for the
first Extension Period be less than the Annual Fixed Rent for the Premises,
including the Expansion Spaces, for the fifteenth Lease Year and in no event
shall the Fair Market Rent for the second Extension Period be less than the
Annual Fixed Rent for the Premises, including the Expansion Spaces, for the
twenty-fifth Lease Year.

      (c) 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 each
Extension Period (i) this Lease is in full force and effect, (ii) there is no
uncured Event of Default of Tenant under this Lease, (iii) Tenant has not
assigned this Lease

                                      -59-
<PAGE>

(or agreed to assign this Lease) or subleased (or agreed to sublease) more than
forty percent (40%) of the Rentable Floor Area of the Premises, in either case
to any person or entity other than an Affiliate of Tenant.

      (d) All of the terms, provisions, covenants, and conditions of this Lease
shall continue to apply during each Extension Period, except that (i) the Annual
Fixed Rent during each Extension Period (the "Extension Rent") shall be equal to
the Fair Market Rent for the Premises determined in accordance with Subsection
(b) above and the procedure set forth in Subsection (e) below and (ii) Tenant
shall not have any other Options to Extend.

      (e) 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 either Option to Extend, Landlord shall give Tenant notice of
Landlord's determination of the Fair Market Rent for the applicable Extension
Period. Such determination shall separately identify the two factors used to
calculate Fair Market Rent. 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. If Tenant shall notify
Landlord of Tenant's objection to Landlord's determination of Fair Market Rent,
the Fair Market Rent shall be determined by appraisal 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.

      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 appraisal 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
"Appraiser"). Within five (5) days thereafter, Landlord shall by notice to
Tenant appoint a second Appraiser. In determining the Fair Market Rent, each of
the Appraisers appointed by Landlord and Tenant shall separately identify the
two separate factors used to calculate Fair Market Rent. Each Appraiser shall
determine the Fair Market Rent for the applicable Extension Period within thirty
(30) days after Landlord's appointment of the second Appraiser. On or before the
expiration of such thirty (30) day period, the two Appraisers shall confer to
compare their respective determinations of the Fair Market Rent. If the
difference between the amounts so determined by the two Appraisers 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 Appraisers shall have ten (10) days thereafter to appoint a
third Appraiser (the "Third Appraiser"), 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 Appraisers. If the two Appraisers are unable to agree upon the Third
Appraiser within such ten (10) day period, such Third Appraiser shall be
appointed by the then President of the Greater Boston Real Estate Board upon
request of either Landlord or Tenant. Each party shall bear the cost of the
Appraiser selected by such party. The cost for the Third Appraiser, if any,
shall be shared equally by Landlord and Tenant. Each Appraiser and

                                      -60-
<PAGE>

Landlord shall be given reasonable access to the Premises for purposes of
determining Fair Market Rent.

10.13 ARBITRATION.

      All disputes arising from or related to the performance of Landlord's Work
or the interpretation of the Design Documents shall be submitted to and resolved
in arbitration under the Construction Industry Rules of the American Arbitration
Association, before the Arbitrator (as hereinafter defined), in Boston,
Massachusetts. Any award entered by the Arbitrator shall be final and binding
upon the parties thereto. Judgment upon any award rendered by the Arbitrator may
be entered in any court having competent jurisdiction.

      Notwithstanding the foregoing, disputes with respect to whether (i) the
Schematic Design Documents are in compliance with the Preliminary Design
Concept, (ii) the Design Development Documents are in compliance with the
Schematic Design Development Documents, or (iii) the Final Design Documents are
in compliance with the Design Development Documents, shall be summarily decided
by the Arbitrator and disputes concerning the achievement of the Substantial
Completion Date shall be determined by the Arbitrator, in each case without
resort to formal arbitration under the Construction Industry Rules of the
American Arbitration Association, and the Arbitrator's decision thereon shall be
binding on the parties. Otherwise, such disputes shall be resolved in
arbitration as set forth herein.

      Arbitration proceedings under this Section 10.13 shall, upon motion of any
party, be consolidated with arbitration proceedings pending between other
parties relating to the Landlord's Work, the same transaction, the same subject
matter, and/or involving related substantive rights. All parties hereby consent
to such consolidation. In addition to the foregoing, any party to any agreement,
whose rights or performance is involved in an arbitration proceeding hereunder,
may be joined as a party in any arbitration proceeding instituted hereunder.
Consolidation and joinder hereunder shall be by order of the Arbitrator. If the
Arbitrator shall fail, upon motion, to make such an order, any party may apply
to the Superior Court for Middlesex County, Massachusetts for such an order.
Neither party nor the Arbitrator may disclose the existence, content or results
of any arbitration hereunder without the prior written consent of both parties.
All administrative fees and expenses of the arbitration shall be borne equally
by the parties. Each party shall bear the expense of its own counsel, experts,
witnesses, and preparation and presentation of proofs.

      There shall be a single arbitrator, who shall be experienced as an
arbitrator and who shall be either a registered engineer, architect, or general
contractor engaged in business in Massachusetts, for at least ten (10) years,
and familiar with issues generally similar to those in dispute, and who shall
not have had any dealings with either party within the five-year period
immediately preceding such dispute (the "Arbitrator"). If Landlord and Tenant
are unable to agree upon the Arbitrator within ten (10) days after submission of
a dispute to arbitration, then either Landlord or Tenant may request that the
then President of the Boston Bar Association select the Arbitrator. Each party
shall bear half the cost of the Arbitrator. The right to arbitrate shall not be
deemed to be a limitation of the rights or remedies of either party in aid of
arbitration under any and all applicable laws, unless expressly waived, by such
party hereto. All arbitration proceedings shall be conducted in Boston,
Massachusetts.

                                      -61-
<PAGE>

10.14 PARKING.

      Tenant shall pay monthly, in advance, as Additional Rent the then fair
market value (the "Parking Fee") (adjusted no more frequently than annually) for
each of Tenant's Parking Spaces to be leased to Tenant. In no event shall the
Parking Fee exceed the lowest monthly rate being charged to tenants of the
Complex by Landlord or any Affiliate of Landlord for parking in the Garage.
After the Commencement Date, all of Tenant's Parking Spaces leased hereby may
only be utilized by Tenant's employees, visitors, sublessees of the Premises or
assignees of the Lease, visiting or working at the Premises. All of Tenant's
Parking Spaces and Valet Parking Spaces (as hereinafter defined) shall be
located in the underground parking structure to be constructed and/or owned by
Landlord or an Affiliate of Landlord (as hereinafter defined) south of Kendall
Street, as shown on Exhibit A hereto (the "Garage"). As used herein, the term
"Affiliate of Landlord" shall mean a person or entity controlled by, controlling
or under common control with Landlord. The owner of the Garage from time to time
is herein referred to as the "Garage Owner", and the Tenant's Parking Spaces and
the Valet Parking Spaces are herein referred to collectively as the "Garage
Parking Spaces." Landlord covenants and agrees that if the Garage is conveyed by
Landlord to any other person or entity, including an Affiliate of Landlord, such
conveyance shall be subject to a lease, permanent easement or similar instrument
by and between Landlord and the Garage Owner so that the Tenant shall have,
throughout the Term, the right to use the Garage Parking Spaces, subject to the
terms of this Lease.

      Landlord shall also lease to Tenant and Tenant shall lease from Landlord
one valet parking space per 1,000 r.s.f. of the Rentable Square Footage as
determined from time to time (the "Valet Parking Spaces") at a rate per space
equal to (i) the Parking Fee plus (ii) the incremental costs associated with
valet parking services for the Garage, which incremental costs shall be
pro-rated if valet parking spaces are provided to other tenants of the Complex
(as so determined, the "Valet Parking Fee"). Tenant shall pay monthly, in
advance, as Additional Rent the Valet Parking Fee for each Valet Parking Space.
Employees of Tenant visiting the Complex from other facilities may use the
Garage, subject to availability, at the daily rate charged by the Garage Owner,
provided, however, Landlord and Tenant agree to implement a parking ticket
validation system so that employees of Tenant visiting the Complex from other
facilities of Tenant may use any unused Valet Parking Spaces. The Garage Owner
shall have the right, from time to time but not more often than every six (6)
months, to relocate, on a temporary basis as may be necessary to effect repairs
and improvements to the Garage or for other business reasons, 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 further 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. Anything herein to the contrary
notwithstanding, Landlord shall have the right to relocate all or any number of
the Garage Parking Spaces to a garage which may be constructed by Landlord or an
Affiliate of Landlord in the northerly portion of the Complex provided that
access from the Building to such other garage is substantially as convenient as
access between the Building and the Garage, whereupon provisions of this Lease
applicable to the Garage, the Garage Owner and the Garage Parking Spaces shall
apply, mutatis mutandis, to such garage, garage owner and the Garage Parking
Spaces located therein.

                                      -62-
<PAGE>

      Neither Landlord nor the Garage Owner shall be responsible for money,
jewelry, automobiles or other personal property lost in or stolen from the
Garage, regardless of whether such loss or theft occurs when the Garage or other
areas therein are locked or otherwise secured against entry, or liable for any
loss, injury or damage to persons using the Garage or automobiles or other
property therein, it being agreed that the use of the Garage and the Garage
Parking Spaces shall be at the sole risk of Tenant and its employees, visitors
and guests. Landlord and the Garage Owner shall have the right from time to time
to promulgate reasonable rules and regulations regarding the Garage, the Garage
Parking Spaces and the use thereof, including, but not limited to, rules and
regulations controlling the flow of traffic to and from various parking areas,
the angle and direction of parking and the like, which rules and regulations and
any additions and amendments thereto Garage Owner shall use reasonable efforts
to consistently apply to all users of the Garage. Tenant shall comply with and
cause its employees, visitors and guests to comply with all such rules and
regulations as well as all reasonable additions and amendments thereto.

      Except for emergency repairs using authorized repair services, no person
using the Garage Parking Spaces shall perform any work on any automobiles while
located in the Garage. Except in connection with an approved assignment of the
Lease or an approved subletting of all or a portion of the Premises in
accordance with the terms hereof, Tenant shall not assign or sublease any of the
Garage Parking Spaces. Landlord shall have the right to terminate this Lease
with respect to any Garage Parking Spaces that Tenant sublets or assigns in
violation of the foregoing sentence. Landlord or the Garage Owner may elect to
provide parking cards or keys to control access to the Garage. In such event,
Landlord or the Garage Owner shall provide Tenant with one card or key for each
Garage Parking Space that Tenant is leasing hereunder, provided that Landlord or
the Garage Owner shall have the right to require Tenant or its employees to
place a reasonable deposit on such access cards or keys and to pay a reasonable
fee for any lost or damaged cards or keys. Tenant, at its sole cost and expense,
may obtain extra cards and keys from Landlord or the Garage Owner if any cards
are lost, stolen or destroyed.

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 Tenant, except
as may be required by applicable law.

10.16 SIGNAGE.

      Landlord shall provide a standard building directory in the office lobby
of the Building listing Tenant as a tenant of the Building. Tenant shall have
the exclusive right, at its sole cost and expense, to install and maintain signs
on the exterior of the Building 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 Ordinance, as

                                      -63-
<PAGE>

amended from time to time and the PUD Permit. 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. Notwithstanding the foregoing, Landlord shall have the right
to install, and to allow tenants of the Retail Space to install, signage on the
windows and doors of the Retail Space and additional signage on the exterior of
the Building at a height of up to fifteen (15) feet above ground level, subject
to limitations of applicable law, including, without limitation, the Cambridge
Zoning Ordinance and the Permits and Approvals.

10.17 BUILDING B LEASE.

      Simultaneously with the execution hereof, Landlord and Tenant shall enter
into an option to lease approximately 150,000 r.s.f in a building to be
constructed by Landlord now known as Building B in the area of the Complex
approximately shown on Exhibit A hereto (the "Option to Lease") in accordance
with and subject to the terms of the Option to Lease.

10.18 RIGHT OF FIRST REFUSAL.

      Provided that at the time Tenant exercises its rights under this Section
10.18: (i) this Lease remains in full force and effect, (ii) there is not then
outstanding an uncured Event of Default of Tenant under this Lease, and (iii)
Tenant and/or Tenant's Affiliates are occupying at least seventy-five percent
(75%) of the r.s.f of the Premises, then Tenant shall have a continuing right of
first refusal ("ROFR") to lease any office, research or laboratory space in the
Complex then owned by Landlord or Landlord's Affiliate (collectively, the "ROFR
Space"), (a) which may become available for lease after the initial term of any
lease(s) for the ROFR Space, as any of such lease(s) may be extended or renewed
pursuant to the terms of the initial lease(s) thereof or (b) which may become
available for lease upon the expiration or termination of lease(s), including
extensions and renewals thereof, entered into by Landlord after Tenant has
failed or declined to enter into a lease of such ROFR Space. Landlord shall
notify Tenant in writing (the "Availability Notice") at least ten (10) months in
advance of the expected date upon which the ROFR Space will become available,
provided however, that if ROFR Space becomes available due to a termination of a
lease for ROFR Space prior to the stated termination of a lease for ROFR Space,
Landlord's Availability Notice shall be sent to Tenant promptly after Landlord
becomes aware of the expected date of availability of such ROFR Space. Upon
receipt by Tenant of an Availability Notice, Tenant shall have ten (10) business
days within which to send to Landlord notice of the exercise of Tenant's ROFR
(the "ROFR Notice"), in which event Landlord and Tenant shall negotiate in good
faith for a period of fifteen (15) days (the "ROFR Negotiation Period") after
the date of such ROFR Notice with respect to the terms by which Tenant would
lease the ROFR Space from Landlord. If Tenant fails or refuses to send the ROFR
Notice or if Landlord and Tenant are unable to agree on the lease terms for the
ROFR Space within such fifteen (15) day period, thereupon Landlord may, at any
time during the one hundred eighty (180) day period following the expiration of
the ROFR Negotiation Period, lease all or any portion of the ROFR Space which
was the subject of the Availability Notice to any third party on terms and
conditions no more favorable to such third party than the final terms offered by
Landlord to Tenant during the ROFR Negotiation Period. If Landlord has not
leased the ROFR Space to a third party within such 180-day period, or if
Landlord proposes to offer the ROFR Space for lease to a third party on terms
and conditions more favorable to such third party

                                      -64-
<PAGE>

than the final terms offered to Tenant during the ROFR Negotiation Period,
Landlord shall first re-offer the ROFR Space to Tenant hereunder before Landlord
may offer the ROFR Space to any third party, and Tenant shall have five (5)
business days within which to accept such offer. Any person dealing with ROFR
Space may without further inquiry conclusively rely upon a representation in a
certificate of Landlord or Landlord's Affiliate as to whether the provisions of
this Section have been satisfied.

10.19 RETAIL TENANTS.

      Provided that Tenant or an Affiliate of Tenant is occupying at least sixty
percent (60%) of the Rentable Square Footage of the Premises, Tenant shall have
the right to approve tenant(s) of the Retail Space, which approval shall not be
unreasonably conditioned, withheld or delayed.

10.20 ACCESS

      Commencing on the Commencement Date, Tenant shall have access to the
Building twenty-four (24) hours per day, 365/366 days per year and during those
hours that the Building is not accessible to the general public, via a security
card system provided by Landlord. Tenant agrees to use and comply with any
security systems including the security card systems employed by Landlord from
time to time during the Term. Landlord may provide a manned security desk within
the lobby of the Building comparable to that of other buildings in the Complex,
it being understood by Tenant that the provision of a security desk and security
card systems are no warranty, representation or guaranty by Landlord as to the
safety or security of persons and property within the Building or the Premises.
Notwithstanding the foregoing sentence, Tenant shall be solely responsible for
the security of all persons and property within the Premises and for access to
the Premises including any security card system serving the Premises. Landlord
and Tenant agree to use good faith efforts to coordinate any security card
system from time to time implemented by Landlord for the Building and/or Complex
and any security card system implemented from time to time by Tenant for the
Premises and other premises of Tenant outside of the Complex.

10.21 COOPERATION.

      Landlord agrees that it shall, at Tenant's expense, cooperate with,
support, consult with, and provide information in its possession to Tenant in
seeking, applying for and obtaining any and all consents, permits, licenses,
certificates, waivers, special permits, approvals and the like required, or
deemed necessary or appropriate by Tenant, in connection with (a) Tenant's use
and occupancy of the Premises for the Permitted Use and/or (b) Tenant's exercise
of its rights and/or performance of its obligations under this Lease. Landlord
agrees that such cooperation shall include, without limitation, the co-signing
of applications, the providing of support and information that can reasonably be
made available by the record owner of the Premises but not by other parties;
providing letters of support or other supporting information or evidence for
submission to hearings or proceedings before any zoning, planning, land use, or
regulatory board or authority, or any license or permit-granting or permitting
office, board or authority. Notwithstanding the foregoing, Landlord shall not be
required to take any action (and Landlord may oppose any action proposed by
Tenant) if Landlord determines, in its sole discretion, that any action or
series of actions proposed by Tenant could have an adverse impact on the
Building,

                                      -65-
<PAGE>

Lot or Complex or any of the Development Approvals, Subsequent Approvals or any
rights or obligations of Landlord thereunder or under this Lease.

            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK]

                                      -66-
<PAGE>

WITNESS the execution hereof under seal as of the 20th day of August, 2000.

                         LANDLORD: KENDALL SQUARE, LLC

                                   By: Lyme Properties LLC, a New Hampshire
                                       limited liability company, its Manager

                                   By: /s/ David E. Clem
                                       ______________________________________
                                       David E. Clem, Member

                           TENANT: GENZYME CORPORATION

                                   By: /s/ Henri Termeer
                                       ______________________________________
                                       Henri Termeer, President and
                                       Chief Executive Officer

                                      -67-<PAGE>
                                                                   Exhibit 10.10

                            FIRST AMENDMENT TO LEASE

      This FIRST AMENDMENT TO LEASE (this "First Amendment") is executed by and
between KS Parcel D, LLC (the "Landlord"), a Delaware limited liability company,
and Genzyme Corporation (the "Tenant").

      Reference is hereby made to that certain lease dated August 28, 2000 (the
"Lease"), by and between Kendall Square, LLC ("Kendall Square"), as landlord,
and Tenant, as tenant, with respect to a portion of the Building on the Lot (as
such terms are defined in the Lease) located in Cambridge, Massachusetts.

      WHEREAS, as contemplated by Section 9.1.6 of the Lease, on or about July
19, 2002, Kendall Square subdivided the Complex so as to, inter alia, establish
the Lot as a separate taxable parcel, and in connection with such Subdivision
(as defined in the Lease), Kendall Square conveyed the Lot together with Lot A
in the Complex and assigned the landlord's interest under the Lease to KS Parcel
A/D, LLC, an Affiliate of Kendall Square; and on or about December 3, 2002, KS
Parcel A/D, LLC, further conveyed the Lot, and further assigned the Landlord's
interest under the Lease, to Landlord, also an Affiliate of Kendall Square (such
conveyances of the Lot and assignments of the landlord's interest under the
Lease are referred to herein collectively as the "Transfer").

      WHEREAS, as contemplated by Section 2.3 of the Lease, Landlord and Tenant
have reached a mutually acceptable agreement regarding the Rentable Square
Footage of the Premises and Building.

      WHEREAS, Landlord and Tenant now desire to amend the Lease to reflect the
Subdivision and the boundaries of the Lot and the Transfer, to reflect the
agreed upon Rentable Square Footage to modify certain provisions of the Lease
and otherwise to ratify and confirm the terms of the Lease, as more particularly
set forth below.

      NOW, THEREFORE, in consideration of the mutual promises herein contained,
and other good and valuable consideration, the receipt and legal sufficiency of
which is hereby acknowledged, Landlord and Tenant agree as follows:

      1. Each capitalized term which is used but not defined herein, or any term
which is defined in the Lease and used herein, shall have the meaning ascribed
thereto in the Lease.

      2. The legal description of the Lot to be attached to the Lease as Exhibit
A-1 is attached to this First Amendment as Exhibit A-1 and the plan of the Lot
to be attached to the Lease as Exhibit A-2 is attached to this First Amendment
as Exhibit A-2.

      3. Landlord and Tenant acknowledge that the actual process and procedures
followed in selecting Landlord's Design Team and Landlord's Contractor and in
designing and agreeing upon the Schematic Design Documents, the Design
Development Documents and the Final Design Documents differed, with the consent
and approval of both Landlord and Tenant, from the specific process and
procedures prescribed in Section 3.1.1 of the Lease. Landlord and

<PAGE>

Tenant hereby confirm that, prior to the date of this First Amendment, they have
agreed as follows:

      (i)   Landlord has selected and Tenant has approved Behnisch, Behnisch &
            Partners as Landlord's Architect for the Building.

      (ii)  Landlord has selected and Tenant has approved Turner Construction
            Company as the Construction Cost Estimator and as the Construction
            Manager/General Contractor for the Building.

      (iii) The Building Permit for the Building has been issued, the
            Substantial Completion Date occurred on April 30, 2003, and
            construction of the Tenant Work has commenced, and notwithstanding
            Section 1.1 or any other provision of the Lease to the contrary, the
            parties have agreed that the Term Commencement Date shall be deemed
            to have occurred on August 1, 2003.

      (iv)  The Environmental Remediation of the Lot commenced on June 26, 2000,
            and the Response Action Outcome Statement for the Lot required under
            the Massachusetts Contingency Plan was filed with the Massachusetts
            Department of Environmental Protection on April 19, 2003.

      (v)   The Certification of Landlord's LSP and Estoppel Certificates of
            COM/Energy (as required under Paragraphs 2(b) and 3 of the
            Environmental Agreement) were completed on December 12, 2002.

      (vi)  The Indemnity Expiration Date (as defined in the Release and
            Indemnity Agreement) with respect to the Lot occurred on or before
            December 12, 2002.

Landlord and Tenant hereby agree that as of the date of this First Amendment,
the only item remaining to be completed on the Progress Schedule is substantial
completion of those items of Base Building Improvements that constitute Post
Completion Items (as defined below in Paragraph 7 of this First Amendment).
Accordingly, Exhibit B to the Lease is hereby deleted in its entirety.

      4. Anything herein or in the measurement standard set forth in Section 2.3
of the Lease to the contrary notwithstanding, Landlord and Tenant have agreed
that for purposes of the Lease the Rentable Square Footage of the Building shall
be deemed to be 349,325 square feet, the Rentable Square Footage of the Premises
shall be deemed to be 343,000 square feet and the Rentable Square Footage of the
Retail Space shall be deemed to be 6,325 square feet, and the Rentable Square
Footage of the Building, Premises and Retail Space as agreed-upon above shall
not be subject to remeasurement by Landlord or Tenant. To reflect the foregoing
agreement regarding the Rentable Square Footage of the Building, Premises and
Retail Space and certain other agreements between the parties, Landlord and
Tenant hereby agree to amend the definitions set forth below as follows:

      (i)   The definition of "Initial Estimated Annual Additional Rent For
            Building" in Section 1.1 is amended to read $3,391,500.

                                     - 2 -
<PAGE>

      (ii)  The definition of "Initial Estimated Annual Additional Rent for
            Complex" in Section 1.1 is amended to read $484,500.

      (iii) The definition of "Tenant's Proportionate Fraction for Building" in
            Section 2.3 is amended to read 98.19%.

      (iv)  The definition of "Tenant's Proportionate Fraction for Complex" in
            Section 2.3 is amended to be the percentage equal to 94.17% of the
            ratio, expressed as a percentage, of the Rentable Square Footage of
            the Premises to the Rentable Square Footage of all buildings in the
            Complex (other than any garages in the Complex and any building or
            portion thereof which is not available for lease such as an
            information kiosk), including the Building, which Landlord is
            permitted to develop in the Complex pursuant to the PUD Approval. As
            of the date of this First Amendment, Tenant's Proportionate Fraction
            for Complex is 24.6%.

      (v)   The requirement in the definition of "Building" in Section 1.1 and
            in the second grammatical paragraph of Section 3.1.1 that the
            Building's ground floor retail space must contain at least 15,000
            r.s.f. is hereby waived.

      5. In consideration of the agreed-upon Rentable Square Footage of the
Building, Landlord and Tenant have agreed that, anything herein or in the
definitions of the components of Project Cost set forth in Section 3.1.1 of the
Lease to the contrary notwithstanding, the definitions of the components of
Project Cost set forth in Section 3.1.1 are amended to be as follows:

      (i)   Tenant's pro rata share of Mitigation Expenses (shown on the
            attached Project Budget Form as the "Impact Fees" line item) shall
            be equal to (a) the actual amount of Mitigation Expenses allocated
            solely to the Building and the Lot plus (b) (1) the actual amount of
            Mitigation Expenses allocable to the entire Complex multiplied by
            (2) Tenant's Proportionate Fraction for Complex.

      (ii)  Tenant's pro-rata share of infrastructure and sitework costs (shown
            on the attached Project Budget Form as the "Sitework" line item)
            shall be equal to the actual amount of infrastructure and sitework
            costs allocable to the entire Complex multiplied by Tenant's
            Proportionate Fraction For Complex.

      (iii) The broker's fee (shown on the attached Project Budget Form as the
            "Broker's Fee" line item) shall be the fixed amount of $2,137,500
            and shall be deemed to have been fully incurred as of the date(s) of
            payment thereof.

      (iv)  The Project Cost line item for Land and Environmental Remediation
            shall be subdivided into two components. The first component shall
            consist of the Land and Environmental Remediation charge (calculated
            at $60 per Rentable Square Foot) for the first 329,325 square feet
            of the Building's Rentable Square Footage; Interest on Landlord's
            Equity shall accrue on this first component of the Land and
            Remediation line item as of the execution date of the Lease. The
            second component shall consist of the Land and Environmental
            Remediation charge (calculated at $60 per Rentable Square Foot) for
            the remaining 20,000 square feet

                                     - 3 -
<PAGE>

            of the Building's Rentable Square Footage and no Interest on
            Landlord's Equity shall accrue on this second component.

      (v)   The Developer's Fee (shown on the attached Project Budget form as
            the "Development Fee/Supervision" line item) shall be the fixed
            amount of $4,365,440.

The Project Budget Form attached as Exhibit B-1 to the Lease is hereby deleted
and the Project Budget Form attached to this First Amendment as Exhibit B-1 is
hereby inserted in place thereof. The Project Budget Form attached to this First
Amendment shall be the Final Project Budget contemplated by Section 3.1.1 of the
Lease. Landlord and Tenant acknowledge that such Final Project Budget is merely
a budget, and that the actual amount of Annual Fixed Rent payable by Tenant
under the Lease shall be determined in accordance with the formula set forth in
Section 4.1 of the Lease based upon the amount of Project Cost actually incurred
as of the date on which Annual Fixed Rent is calculated or is subsequently
adjusted as provided in the Lease and in this First Amendment.

      6. Landlord and Tenant acknowledge that pursuant to certain Change Orders
requested by Landlord and approved by Tenant and described on the attached
Schedule CO, certain items of work were transferred from Tenant's Work to Base
Building Improvements to facilitate overall coordination of the construction of
the Building (the "TI Scope Transfer Work"). Landlord and Tenant further
acknowledge that the cost of the TI Scope Transfer Work has been billed directly
to and paid by Tenant prior to the date of this First Amendment. Accordingly,
the cost of the TI Scope Transfer Work is expressly excluded from Project Cost
but Landlord and Tenant specifically agree that Landlord is entitled to receive
a developer's fee of two and one-half percent (2 1/2%) rather than five percent
(5%) with respect to such TI Scope Transfer Work, and that such 2 1/2%
developer's fee has not been billed directly to Tenant but rather has been
included in the fixed Developer's Fee as set forth in Section 5(v) above.

      7. Pursuant to Section 10.11(i) of the Lease, Tenant hereby exercises its
Early Occupancy Space right and option to lease both Expansion Spaces as of the
Commencement Date. Landlord hereby acknowledges that Tenant has duly exercised
its option to lease both Expansion Spaces as Early Occupancy Space pursuant to
Section 10.11(i) of the Lease and expressly waives the requirement of Section
10.11(i) that Tenant was to have given notice of such exercise not less than one
year prior to the Scheduled Substantial Completion Date. Accordingly, commencing
on the Term Commencement Date (i) the Premises shall consist of all of the
Rentable Square Footage in the Building, other than the Rentable Square Footage
of the Retail Space and (ii) the Annual Fixed Rent for the Premises (including
both Expansion Spaces) shall be determined on the basis of the Annual Fixed Rent
Rate set forth in Section 1.1 of the Lease. In addition, as a result of Tenant's
exercise of its Early Occupancy Space right with respect to both Expansion
Spaces, Landlord and Tenant agree that a plan showing the locations of the
Expansion Spaces is no longer required to be attached to the Lease as Exhibit
A-4.

      8. In order to facilitate Tenant's Work to be performed in connection with
Tenant's initial occupancy of the Premises and to coordinate and integrate the
simultaneous performance of Landlord's Work and such Tenant's Work, Landlord and
Tenant hereby agree that, anything herein or in the Lease to the contrary
notwithstanding, (i) those items of Base Building

                                     - 4 -
<PAGE>

Improvements set forth on Schedule A attached to this First Amendment shall be
substantially completed by Landlord after the Substantial Completion Date (the
"Post Completion Items"), (ii) substantial completion of the Post Completion
Items shall not be a condition precedent to the occurrence of the Substantial
Completion Date, (iii) the Post Completion Items shall be substantially
completed with all due diligence by Landlord after the Substantial Completion
Date but during the performance of such Tenant's Work in accordance with the
Final Design Documents, as affected by Change Orders, without material deviation
therefrom, and (iv) all hard and soft costs incurred by Landlord in connection
with the construction of the Post Completion Items shall be included in the
Project Cost and (v) Landlord shall use reasonable and diligent efforts to
complete Punch List Items with respect to Landlord's Work and Post Completion
Items within thirty (30) days after completion of Tenant's Work. Promptly
following substantial completion of the Post Completion Items, Landlord shall
deliver to Tenant a certification by Landlord's Architect (confirmed by Tenant's
Architect) that the Post-Completion Items and all other Base Building
Improvements have been substantially completed in accordance with the Final
Design Documents, as affected by Change Orders, without material deviation
therefrom; the date by which the Post Completion Items have been substantially
completed is referred to herein as the "Final Completion Date".

      Since completion of the Post Completion Items will be deferred until after
the Substantial Completion Date as described above, Landlord and Tenant further
agree as follows:

      (i)   that clause (i) of the definition of Substantial Completion Date
            appearing in Section 3.2 of the Lease is hereby amended by deleting
            from such clause (i) the phrase "Base Building Improvements" and
            inserting in place thereof the phrase "Base Building Improvements
            exclusive of the Post Completion Items (which Post Completion Items
            shall be substantially completed after the Substantial Completion
            Date)"; and

      (ii)  Landlord shall be obligated to provide a full accounting of Project
            Cost within ninety (90) days after the Final Completion Date instead
            of within ninety (90) days after the Substantial Completion Date.
            Accordingly, the three grammatical sentences of Section 3.1.1 of the
            Lease appearing at the bottom of page 22 are revised to read as
            follows:

            "Within ninety (90) days after the Final Completion Date, Landlord
            shall deliver to Tenant a full accounting of the Project Cost
            incurred as of the Final Completion Date; Tenant, from the date
            hereof through the date which is twelve (12) months after receipt of
            such full accounting of Project Cost, may review all of Landlord's
            books and records relating to the incurrence and payment of the
            Project Cost in order to verify and confirm the accuracy thereof. If
            Landlord incurs any cost or expense properly includable in Project
            Cost after the Final Completion Date (including, without limitation,
            any Mitigation Expenses) which is not included in the full
            accounting, Landlord shall notify Tenant thereof and Landlord shall
            provide Tenant with such documentation with respect thereto as
            Tenant reasonably may request. Landlord and Tenant acknowledge and
            agree that the incurrence of additional costs and expenses by
            Landlord after the Final Completion Date which are properly
            includable in Project Cost will result in an

                                     - 5 -
<PAGE>

            increase in the Annual Fixed Rent Rate, which increase shall be
            effective as of the date of incurrence thereof."

      9. Landlord and Tenant agree the amount of Rent payable by Tenant as of
the Term Commencement Date shall be $15,151,962.00 per annum, payable in monthly
installments of $1,262,663.50 each. Such estimated amount of Annual Fixed Rent
is based upon the estimated amount of Project Cost incurred or to be incurred by
December 31, 2003, shown in the "Current Costs" column on the Final Project
Budget Form attached hereto as Exhibit B-1. Landlord and Tenant acknowledge and
agree that such estimated amount of Annual Fixed Rent shall be subject to
adjustment (i) after Tenant receives the full account of Project Cost required
by Section 3.1.1, (ii) upon the exercise by Tenant of its right under Section
3.1.1 to "review all of Landlord's books and records relating to the accuracy
and payment of the Project Cost in order to verify and confirm the accuracy
thereof" and (iii) upon the incurrence of additional costs and expenses by
Landlord after the Final Completion Date which are properly includable in
Project Cost. Upon any such adjustment any balance owed by Tenant or excess paid
by Tenant shall be paid to Landlord or credited to Tenant, as the case may be,
within twenty (20) days thereafter.

      As indicated on the Final Project Budget Form attached hereto as Exhibit
B-1, the full budgeted amount of several Project Cost line items, such as the
line items for Sitework and Impact Fees, as the case may be, are not expected to
be incurred until 2004 or later; the estimated amount of those costs is shown in
the "Deferred Costs" column on the Final Project Budget Form attached hereto as
Exhibit B-1. Upon the request of Tenant from time to time (but not more
frequently than semi-annually) Landlord shall provide Tenant with a status
report on the progress and estimated completion date of such items of deferred
work and an updated estimate of the projected or actual costs of completion of
such items of deferred work. Landlord and Tenant confirm and agree that, as
provided in Section 3.1.1 of the Lease, at such time after December 31, 2003, as
Landlord may incur any cost or expense (such as the budgeted items mentioned
above) properly includable in Project Cost, Landlord shall notify Tenant thereof
and Landlord shall provide Tenant with such documentation with respect thereto
as Tenant reasonably may request; Landlord and Tenant acknowledge and agree that
the incurrence of additional costs and expenses by Landlord after December 31,
2003 which are properly includable in Project Cost will result in an increase in
the Annual Fixed Rent Rate, which increase shall be effective as of the date of
incurrence of such additional costs and expenses.

      10. Effective as of November 1, 2003, Sections 1.1 and 10.1 of the Lease
shall be amended by deleting the phrase "One Kendall Square , Building 1400,
Cambridge, Massachusetts 02139" wherever it appears in said Sections and by
inserting in place thereof the phrase "500 Kendall Street, Cambridge,
Massachusetts 02142."

      11. The first sentence of Section 10.11(h) of the Lease, is hereby amended
by deleting the words "If upon commencement of the Term with respect to the
Second Expansion Space, and so long thereafter" and by inserting in place
thereof the words "As long".

      12. Section 1.1 of the Lease is hereby amended by deleting the definition
of Lease Year and inserting the following in place thereof:

                                     - 6 -
<PAGE>

      The first Lease Year shall commence on the Commencement Date and end on
      the last day of the month in which the first (1st) anniversary of the
      Commencement Date shall occur (unless the Commencement Date shall occur on
      the first day of a month, in which case the first Lease Year shall end on
      the day before the first (1st) anniversary of the Commencement Date).
      Subsequent Lease Years shall commence on the day after the last day of the
      first Lease Year or an anniversary thereof, and shall end on an
      anniversary of the last day of the first Lease Year.

      13. Section 4.1(a) of the Lease is hereby amended by deleting the words
"from time to time direct in writing" appearing in the third line and the words
"by notice to Tenant from time to time direct" appearing in the sixth and
seventh lines and by inserting in place of each such deletion the words "from
time to time direct upon not less than fifteen (15) business days' prior written
notice to Tenant..."

      14. Section 6.1 (c)(i) of the Lease is hereby amended by deleting the
third, fourth, fifth and sixth sentences thereof and inserting the following in
place thereof:

      Landlord, within one hundred twenty (120) days after the fire or other
      casualty, shall notify Tenant in writing of the period of time within
      which, in Landlord's reasonable judgment, the Building and the Premises
      can be restored to substantially their condition prior to such damage and
      Utility Services restored to the Utility Switching Points (the "Casualty
      Restoration Period"). If such notification shall state that such Casualty
      Restoration Period will extend more than twelve (12) months after the date
      of the casualty, then Tenant may terminate this Lease within thirty (30)
      days from Tenant's receipt of such notification. Furthermore, if Tenant
      does not so terminate this Lease and if such damage is not repaired,
      Utility Services are not restored and the Premises and the remainder of
      the Building are not restored to substantially the same condition as they
      were prior to such damage within the Casualty Restoration Period set forth
      in such notification, Tenant, within thirty (30) days after the expiration
      of the Casualty Restoration Period or from the expiration of any extension
      thereof by reason of Force Majeure Events as hereinafter set forth, may
      terminate this Lease by notice to Landlord, specifying a date not more
      than forty-five (45) days after the giving of such notice on which the
      term of this Lease shall terminate. The Casualty Restoration Period shall
      be extended by the number of days lost as a result of Force Majeure
      Events, provided however that the Casualty Restoration Period shall in no
      event be extended more than six (6) months due to Force Majeure Events (as
      so extended due to Force Majeure Events, the "Casualty Restoration
      Completion Date").

      15. The first sentence of Section 10.12(b) is hereby amended to read as
follows:

            For purposes of this Section 10.12, "Fair Market Rent" shall mean
            the product of (a) the Rentable Square Footage of the Premises
            multiplied by (b) the average calculated on a rentable square foot
            basis, of (1) eighty-nine and one-half percent (89.5%) of the fair
            market rental rate for unfinished, shell office space in a
            comparable office building in the Kendall Square, Cambridge,
            Massachusetts

                                     - 7 -
<PAGE>

            office market area (the "Relevant Market") and (2) eighty-nine and
            one-half percent (89.5%) of the fair market rental rate for the
            Premises, and shall take into account all other relevant factors in
            the Relevant Market, including the ten (10) year term of the
            applicable Extension Period.

      16. Anything in the Lease to the contrary notwithstanding, as used in the
Lease, as amended hereby, the term "Tenant's Parking Spaces" shall mean 700
nonreserved parking spaces and the foregoing number of Tenant's Parking Spaces
shall not be subject to redetermination pursuant to Sections 2.1, 10.14 or any
other provisions of the Lease; in addition, Landlord and Tenant hereby agree
that Tenant shall have no Valet Parking spaces, and all provisions of the Lease
with respect to Valet Parking Spaces are hereby deleted.

      17. Anything in the Lease to the contrary notwithstanding, Tenant shall
have no right to offer Tenant Financing.

      18. Landlord and Tenant hereby agree that the Option to Lease Building B
has expired without the exercise by Tenant of its rights with respect thereto
and the Option to Lease Building B shall be of no further force or effect. Upon
the request of Landlord, Tenant shall execute and deliver to Landlord a notice
of termination of the Option to Lease Building B in recordable form.

      19. The Lease, as amended hereby, is hereby ratified and confirmed in all
respects.

                                     - 8 -
<PAGE>

      Executed under seal effective as of the 1st day of August, 2003.

                                   LANDLORD:

                                   KS PARCEL D, LLC

                                   By: Kendall Square, LLC, its Manager

                                       By:  Lyme Properties, LLC, its Manager

                                       By: /s/ David E. Clem
                                          --------------------------------------
                                          David E. Clem, Member

                                   TENANT:

                                   GENZYME CORPORATION

                                   By: /s/ Evan M. Lebson
                                      ------------------------------------------
                                   Name: Evan M. Lebson
                                   Title Vice President and Treasurer

                                     - 9 -

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