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                                                                 EXHIBIT 10.25

                            FIFTH AMENDMENT TO LEASE


     FIFTH AMENDMENT TO LEASE (the "Fifth Amendment") dated as of the 9th day of
October, 1997 by and between Mortimer B. Zuckerman, Edward H. Linde and David
Barrett, Trustees of Fourteen Cambridge Center Trust under Declaration of Trust
dated February 4, 1982 and recorded with the Middlesex South District Registry
of Deeds in Book 14707, Page 96 and not individually (hereinafter called the
"Landlord") and Biogen, Inc. (successor to Biogen Research Corp., successor to
B. Leasing, Inc.). Biogen, Inc., is the Tenant under the Lease and is
(hereinafter called "Tenant").


                                 R E C I T A L S

     By lease dated October 4, 1982, as amended by First Amendment To Lease
dated January 19, 1989 (the "First Amendment"), by Second Amendment To Lease
dated March 8, 1990 (the "Second Amendment"), by Third Amendment To Lease dated
September 25, 1991 (the "Third Amendment") and by Fourth Amendment to Lease
dated October 6, 1993 (the "Fourth Amendment") (said Lease as so amended being
hereinafter called the "Lease"), Landlord did lease to Tenant and Tenant did
hire and lease from Landlord the "Site" and "Building" known as and numbered
Fourteen Cambridge Center, Cambridge, Massachusetts. The Site and the Building
are defined in Section 1.2 of the Lease and are collectively therein and herein
interchangeably called the "Demised Premises" or the "Premises".

     The Lease provided for an Original Lease Term which was scheduled to expire
on February 28, 1998. Pursuant to Section 1 of the Fourth Amendment, the Term
was extended for one (1) period of five (5) years (the "First Extended Term")
expiring on February 28, 2003.

     Landlord and Tenant have agreed to extend the Term of the Lease, to
restructure certain obligations of Landlord and Tenant under the Lease and to
make certain other modifications to the Lease and are entering into this
instrument to set forth the same.

     NOW, THEREFORE, in consideration of One Dollar ($1.00) and other good and
valuable consideration paid by each of the parties hereto to the other, the
receipt and sufficiency of which is hereby acknowledged, and in further
consideration of the provisions herein, Landlord and Tenant hereby agree as
follows:

     1.   The Lease Term (also in the Lease sometimes called the "Term"), which
          but for this Fifth Amendment is scheduled to expire on February 28,
          2003, is hereby presently extended for a period commencing on March 1,
          2003 and expiring on the date which is fifteen (15) years subsequent
          to the date (such date being hereinafter referred to as the "Fifth
          Amendment Effective Date") that this Fifth 

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          Amendment has been executed on behalf of both Landlord and Tenant
          (plus the partial month, if any, at the end of such fifteen (15) year
          period) (herein called the "Second Extended Term") unless extended or
          sooner terminated in accordance with the provisions of the Lease (as
          herein amended). The present extension of the Lease Term for the
          Second Extended Term shall be on all of the same terms and conditions
          set forth in the Lease except as otherwise provided in this Fifth
          Amendment. All references in the Lease (as herein amended) to the
          "Term" or "Lease Term" shall mean and be references to the Original
          Term as extended by the First Extended Term and as herein presently
          extended by the Second Extended Term.

     2.   Section 3.2 of the Lease is hereby deleted in its entirety and
          Landlord and Tenant acknowledge and agree that Tenant's only options
          to extend the Term beyond the expiration of the Second Extended Term
          shall be as provided in Section 3 of this Fifth Amendment.

     3.   Provided that at the time of the exercise there exists no "Event of
          Default" (defined in Section 15.1 of the Lease) and that the Lease is
          still in full force and effect, Tenant shall have the right to extend
          the Lease Term upon all of the same terms, conditions and provisions
          contained in the Lease (except for the Annual Fixed Rent which shall
          be adjusted during option periods as hereinafter set forth) for four
          (4) successive periods of five (5) years each as hereinafter set
          forth. Each of the four (4) option periods is sometimes herein
          referred to as an Extended Term.

          If Tenant desires to exercise any one of the aforementioned options to
          extend the then current term of the Lease, then Tenant shall give
          notice to Landlord, not earlier than eighteen (18) months nor later
          than fourteen (14) months prior to the expiration of the then current
          term of the Lease of Tenant's request for Landlord's quotation as of
          the commencement date of the applicable extension period of the annual
          fair market rental rate of the Demised Premises for the applicable
          Extended Term, such quotation to be based on the use of the Demised
          Premises as a first class office building of comparable age in the
          Cambridge Center Development Area (including such inflation indicators
          as may from time to time be in effect but determined without regard to
          any right to extend but taking into account Tenant's payments of
          Impositions and Tenant's obligations to maintain and pay for the
          maintenance of the Demised Premises as set forth in the Lease as
          herein amended). Within one hundred five (105) days after Landlord's
          receipt of Tenant's notice requesting such a quotation, Landlord shall
          notify Tenant of Landlord's quotation. In order to exercise its rights
          hereunder, Tenant shall, within fifteen (15) days after receipt of
          Landlord's quotation, by written notice to Landlord either (i) accept
          such quotation and give notice that it exercises its option to extend
          the Lease Term (as it may previously have been extended), in which
          case the Lease Term (as it may previously have been extended) shall be

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          extended for the applicable Extended Term or (ii) make a request to
          Landlord for a broker determination of the Annual Fixed Rent for the
          applicable Extended Term (based on the criteria set forth in this
          Section 3), which determination shall be made as follows:

               Tenant's notice requesting a broker determination of the Annual
               Fixed Rent shall include the name of a commercial real estate
               brokerage firm selected by Tenant with at least ten (10) years
               experience dealing in properties of a nature and type generally
               similar to the Demised Premises located in the Downtown
               Boston-Cambridge area ("Tenant's Extended Term Broker"). Within
               ten (10) days after Landlord's receipt of Tenant's notice
               requesting a broker determination and stating the name of such a
               commercial real estate brokerage firm, Landlord shall give
               written notice to Tenant of Landlord's selection of a commercial
               real estate brokerage firm having at least the experience
               referred to above ("Landlord's Extended Term Broker"). Within ten
               (10) days thereafter the two (2) firms so selected shall select a
               third such commercial real estate brokerage firm also having at
               least the experience referred to above (the "Third Extended Term
               Broker"). Within ten (10) days after the selection of the third
               commercial real estate brokerage firm, the three (3) firms so
               selected, by majority opinion, shall attempt to determine the
               Annual Fixed Rent on the basis of the criteria set forth above in
               this Section 3. If the brokers are able to agree at least by
               majority on a determination of such Annual Fixed Rent the brokers
               shall send a notice to Landlord and Tenant by the end of such ten
               (10) day period of such determination which shall be hereinafter
               called the "Broker Determination." If the brokers are unable to
               agree at least by majority on a determination of such Annual
               Fixed Rent, the brokers shall send a notice to Landlord and
               Tenant by the end of such ten (10) day period of such inability
               and within fourteen (14) days after the selection of the Third
               Extended Term Broker, both Tenant's Extended Term Broker and
               Landlord's Extended Term Broker shall make separate
               determinations of the Annual Fixed Rent on the basis of the
               criteria set forth above in this Section 3 ("Tenant's Broker's
               Rent Determination" and "Landlord's Broker's Rent Determination"
               respectively). Within fourteen (14) days after both Tenant's
               Broker's Rent Determination and Landlord's Broker's Rent
               Determination have been made, the Third Extended Term Broker
               shall select either Landlord's Broker's Rent Determination or
               Tenant's Broker's Rent Determination (the "Third Broker's Rental
               Selection") and the Third Broker's Rental Selection shall, in its
               entirety and without regard to the other Rent Determinations,
               constitute the "Broker Determination." In making the selection,
               in no event shall the Third Extended Term Broker have any power
               to amend, modify, compromise, average or blend either Landlord's
               Broker's Rent Determination or Tenant's Broker's Determination,
               it being the intent of Landlord and Tenant that the Third

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               Extended Term Broker shall simply select one of either Landlord's
               Broker's Rent Determination or Tenant's Broker's Rent
               Determination which, in the Third Extended Term Broker's
               judgement, most accurately and fairly defines the Annual Fixed
               Rent for the Premises for the Extended Term based on the
               requirements of this Section 3. The Third Extended Term Broker
               shall advise Landlord and Tenant in writing by the expiration of
               said fourteen (14) day period of the Third Broker's Rental
               Selection as so determined. In no event shall the Annual Fixed
               Rent as determined in any manner pursuant to this Section be less
               than the Annual Fixed Rent for the Lease Year immediately
               preceding the commencement of the applicable Extended Term.

     Tenant shall have the right to extend the Lease Term for the applicable
     Extended Term by written notice to Landlord given within fifteen (15) days
     after Tenant's receipt of the Broker Determination.

     Upon the giving of notice by Tenant to Landlord either (i) within the
     fifteen (15) day period hereinbefore referred to accepting Landlord's
     quotation of the Annual Fixed Rent for the then applicable Extended Term
     and exercising Tenant's option to extend or (ii) within the fifteen (15)
     day period also hereinbefore referred to accepting the Broker Determination
     and exercising Tenant's option to extend, then this Lease and the Lease
     Term hereof shall be extended, for an additional term of five (5) years,
     without the necessity for the execution of any additional documents (except
     that Landlord and Tenant agree to enter into an instrument in writing
     setting forth the Annual Fixed Rent as determined in the relevant manner
     set forth above); and in such event all references herein to the Lease Term
     or the term of the Lease shall be construed as referring to the Lease Term,
     as so extended, unless the context clearly otherwise requires, and except
     that in no event shall the Lease Term be extended for more than twenty (20)
     years after the expiration of the Second Extended Term. In no event shall
     Tenant have the right to exercise its second five (5) year extension option
     unless it has timely exercised its first five (5) year extension option,
     nor shall Tenant have the right to exercise its third five (5) year
     extension option unless it has timely exercised its first and second five
     (5) year extension options, nor shall Tenant have the right to exercise its
     fourth five (5) year extension option unless its has timely exercised its
     first, second and third extension options, nor shall Tenant have the right
     to exercise more than one (1) option at a time.

4.   (A) For the period prior to the Fifth Amendment Effective Date, Tenant
     shall continue to pay Annual Fixed Rent as provided in the Lease as
     previously amended.

     (B) During the period commencing on the Fifth Amendment Effective Date and
     ending on the last day of the sixtieth (60th) month following the Fifth


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     Amendment Effective Date, Annual Fixed Rent shall be payable at the annual
     rate equal to the product of (i) $18.95 and (ii) the 67,362 square feet of
     Gross Building Area of the Building, being a total of $1,276,509.90 per
     year.

     (C) During the period commencing on the first day of the sixty-first (61st)
     month following the Fifth Amendment Effective Date and ending on the last
     day of the one hundred and twentieth (120th) month following the Fifth
     Amendment Effective Date, Annual Fixed Rent shall be payable at the annual
     rate equal to the product of (i) $21.25 and (ii) the 67,362 square feet of
     Gross Building Area of the Building, being a total of $1,431,442.50 per
     year.

     (D) During the period commencing on the first day of the one hundred and
     twenty-first (121st) month following the Fifth Amendment Effective Date and
     ending on the last day of the one hundred eightieth (180th) month following
     the Fifth Amendment Effective Date, being the expiration of the Second
     Extended Term, Annual Fixed Rent shall be payable at the annual rate equal
     to the product of (i) $23.50 and (ii) the 67,362 square feet of Gross
     Building Area of the Building, being a total of $1,583,007.00 per year.

     (E) During the extension option periods (if exercised), Annual Fixed Rent
     shall be payable by Tenant as provided in Section 3 hereinabove.

5.   For the portion of the Lease Term prior to the Fifth Amendment Effective
     Date, Section 3 of the Fourth Amendment shall be unchanged. For the portion
     of the Lease Term on and after the Fifth Amendment Effective Date, Sections
     3(A) and 3(D) of the Fourth Amendment are hereby deleted in their entirety
     (except for the first paragraph of said Section 3(A) which shall remain in
     full force and effect) and Section 3(b) of the Second Amendment (as amended
     by Section 3(A) of the Fourth Amendment) is hereby amended so that the
     monthly rates per vehicle for Tenant's parking charges shall be as follows:

     PERIOD                               MONTHLY RATE PER CAR


     From the Fifth Amendment Effective   The lesser of (i) $105.00, subject to 
     Date through December 31, 1999       escalation as provided in Section 3(B)
                                          and 3(C) of the Fourth Amendment or   
                                          (ii) the monthly amount (hereinafter  
                                          the "Specified Monthly Garage Rate")  
                                          charged per space by the operator of  
                                          the North Garage (whether or not such 
                                          operator is an affiliate of Landlord) 
                                          to any single tenant leasing parking  
                                          rights                                
                                          
                                          
                                          
                                          
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                                          for fifty (50) or more vehicles in the
                                          North Garage.

     From January 1, 2000 through         The lesser of (i) $140.00, subject to 
     December 31, 2004                    escalation as provided in Section 3(B)
                                          and 3(C) of the Fourth Amendment or   
                                          (ii) the Specified Monthly Garage     
                                          Rate.                                 
                                                        
     From January 1, 2005 through         The lesser of (i) $180.00, subject to
     December 31, 2009                    to escalation as provided in Section  
                                          3(B) and 3(C) of the Fourth Amendment
                                          or (ii) the Specified Monthly Garage 
                                          Rate.                                
                                          

     From January 1, 2010 through the     The lesser of (i) $215.00, subject to
     expiration of the Second Extended    escalation as provided in Section
     Term                                 3(B) and 3(C) of the Fourth Amendment 
                                          or (ii) the Specified Monthly Garage  
                                          Rate.                                 
                                          

     During the extension option periods, if exercised, the parking charges
     shall be the prevailing monthly rates per space charged by the operator of
     the North Garage (whether or not such operator is an affiliate of
     Landlord).

6.   Sections (A) (B), (C) and (D) of Section 16.31 of the Lease added by
     Section 4 of the Fourth Amendment are hereby deleted in their entirety and
     replaced with the correspondingly lettered Sections as follows:

          (A) Upon and subject to the terms and conditions contained in this
          Section and provided that (i) the Lease (as herein amended) shall be
          in full force and effect, (ii) there shall be no "Event of Default"
          (defined in Section 15.1 of the Lease) either at the time of the
          giving of the "Tenant's Option Exercise Notice" (defined below) or on
          the "Closing Date" (as hereinafter defined and as it may be extended
          hereunder) and (iii) Tenant has neither assigned the Lease nor sublet
          the Demised Premises (except only as provided in Subsection (L) below)
          Landlord hereby grants to Tenant the right and option to purchase the
          Demised Premises. Landlord and Tenant hereby agree that, subject to
          compliance with the terms and conditions contained in this Section
          (including, but not limited to Items (i), (ii) and (iii) set forth
          immediately above), the within granted option to purchase the Demised
          Premises shall remain superior to the rights of any other person to
          purchase or otherwise acquire the Demised Premises through the
          expiration of the Second Extended Term whether or not 

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          Tenant's option has been exercised as of such purchase or acquisition,
          it being covenanted and agreed (a) that the within granted option to
          purchase the Demised Premises shall not prevent any sale, conveyance
          or other transfer of the Demised Premises or any interest therein but
          any such sale, conveyance or other transfer shall be subject to the
          within granted option to purchase the Demised Premises upon and
          subject to the terms and conditions hereof and (b) that the within
          granted option to purchase the Demised Premises shall not prevent any
          foreclosure, deed in lieu of foreclosure or the exercise of any other
          rights under any mortgage now or hereafter encumbering the Demised
          Premises but that any person acquiring title to the Demised Premises
          as a result of foreclosure, deed in lieu of foreclosure or by the
          exercise of any such other rights shall be subject to the within
          granted option to purchase the Demised Premises upon and subject to
          the terms and conditions hereof.

          (B) (i) In order to exercise the within granted option to purchase the
          Demised Premises, Tenant shall give written notice to Landlord
          ("Tenant's Option Exercise Notice") at any time on or before the date
          which is eighteen (18) months prior to the expiration of the Second
          Extended Term (time being of the essence) of Tenant's exercise of its
          purchase option hereunder and its request for Landlord's quotation of
          the annual fair market purchase price of the Demised Premises as of
          the Closing Date, such quotation to be based on the use of the Demised
          Premises as a first class office building of comparable age in the
          Cambridge Center Development Area. Within forty-five (45) days after
          Landlord's receipt of Tenant's notice requesting such a quotation,
          Landlord shall notify Tenant of Landlord's quotation. If Tenant
          desires to exercise its option to purchase the Demised Premises,
          Tenant shall, within fifteen (15) days after receipt of Landlord's
          quotation, by written notice to Landlord ("Tenant's Purchase Price
          Notice") either (x) accept such quotation, in which case the "Purchase
          Price" for the Demised Premises shall be the price as quoted by
          Landlord or (y) make a request to Landlord for a broker determination
          of the purchase price for the Demised Premises, which determination
          shall be made as follows:

               Tenant's notice requesting a broker determination of the purchase
               price shall include the name of a commercial real estate
               brokerage firm selected by Tenant with at least ten (10) years
               experience dealing in properties of a nature and type generally
               similar to the Demised Premises located in the Downtown
               Boston-Cambridge area ("Tenant's Sale Broker"). Within ten (10)
               days after Landlord's receipt of Tenant's notice requesting a
               broker determination and stating the name of such a commercial
               real estate brokerage firm, Landlord shall give written notice to
               Tenant

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               of Landlord's selection of a commercial real estate brokerage
               firm having at least the experience referred to above
               ("Landlord's Sale Broker"). Within ten (10) days thereafter the
               two (2) firms so selected shall select a third such commercial
               real estate brokerage firm also having at least the experience
               referred to above (the "Third Sale Broker"). Within ten (10) days
               after the selection of the third commercial real estate brokerage
               firm, the three (3) firms so selected, by majority opinion, shall
               attempt to determine the purchase price for the Demised Premises
               on the basis of the criteria set forth hereinabove. If the
               brokers are able to agree at least by majority on a determination
               of such purchase price the brokers shall send a notice to
               Landlord and Tenant by the end of such ten (10) day period of
               such determination which shall constitute the "Purchase Price"
               hereunder. If the brokers are unable to agree at least by
               majority on a determination of such purchase price, the brokers
               shall send a notice to Landlord and Tenant by the end of such ten
               (10) day period of such inability and within fourteen (14) days
               after the selection of the Third Sale Broker, both Tenant's Sale
               Broker and Landlord's Sale Broker shall make separate
               determinations of the purchase price for the Demised Premises on
               the basis of the criteria set forth hereinabove ("Tenant's
               Broker's Sale Determination" and "Landlord's Broker's Sale
               Determination" respectively). Within fourteen (14) days after
               both Tenant's Broker's Sale Determination and Landlord's Broker's
               Sale Determination have been made, the Third Sale Broker shall
               select either Landlord's Broker's Sale Determination or Tenant's
               Broker's Sale Determination (the "Third Broker's Sale Selection")
               and the Third Broker's Sale Selection shall, in its entirety and
               without regard to the other Sale Determinations, constitute the
               "Purchase Price" hereunder. In making the selection, in no event
               shall the Third Sale Broker have any power to amend, modify,
               compromise, average or blend either Landlord's Broker's Sale
               Determination or Tenant's Broker's Sale Determination, it being
               the intent of Landlord and Tenant that the Third Sale Broker
               shall simply select one of either Landlord's Broker's Sale
               Determination or Tenant's Broker's Sale Determination which, in
               the Third Sale Broker's judgement, most accurately and fairly
               defines the purchase price for the Demand Premises based on the
               criteria set forth hereinabove. The Third Sale Broker shall
               advise Landlord and Tenant in writing by the expiration of said
               fourteen (14) day period of the Third Broker's Sale Selection as
               so determined.

               Within five (5) days after the date either (i) Tenant notifies
               Landlord that it accepts Landlord's quotation of the Purchase
               Price pursuant to (x) hereinabove or (ii) the date of Tenant's
               receipt of notification from the 


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<PAGE>   9

               brokers of the Purchase Price as provided in (y) hereinabove,
               Tenant shall pay to Landlord a deposit in the amount equal to ten
               percent (10%) of the Purchase Price in good funds payable to
               Landlord (the "Deposit"); provided, however, that the Deposit
               shall be promptly endorsed or otherwise paid over to the "Escrow
               Agent" (defined in subsection (B)(ii) hereof) and shall be held
               and applied by the Escrow Agent in accordance with the provisions
               of said subsection (B)(ii) hereof. It is hereby covenanted and
               agreed that if Tenant shall not give to Landlord Tenant's Option
               Exercise Notice or Tenant's Purchase Price Notice or pay the
               Deposit to Landlord within the time periods referenced above
               (time being of the essence), the within granted option to
               purchase the Demised Premises shall automatically cease, expire
               and be null and void without any action of the parties and
               without any liability or obligation to or against any of the
               parties, provided, however, that the Lease shall remain in full
               force and effect in accordance with its terms for the remainder
               of the Term. If Tenant shall timely give to Landlord Tenant's
               Option Exercise Notice and Tenant's Purchase Price Notice and pay
               the Deposit to Landlord, the Closing Date shall be as set forth
               in subsection (D) hereof.

               (B) (iii) The "Escrow Agent" shall be the General Counsel of
               Boston Properties, Inc., or such law firm, title insurance
               company or other institutional escrow agent as Landlord shall
               select. Landlord shall promptly pay over the Deposit to the
               Escrow Agent so selected and shall cause the Escrow Agent holding
               the Deposit to acknowledge to Tenant receipt of the Deposit
               within a reasonable period of time after the Escrow Agent
               receives the Deposit. The Deposit shall be held in such interest
               bearing account in such banking institution in the City of Boston
               and upon such terms and conditions relating to the deposit of
               funds and maintenance of accounts as the Escrow Agent shall
               determine. The type of account, the rate of interest, the terms
               and conditions relating to the deposit of funds and maintenance
               of accounts and the banking institution shall be as solely
               selected by the Escrow Agent and the Escrow Agent shall have no
               liability to Landlord or Tenant respecting the selection of the
               type of account, the rate of interest, the terms and conditions
               relating to the deposit of funds and maintenance of accounts
               and/or the banking institution. Further, the Deposit shall be
               held by the Escrow Agent subject to the terms of this Section
               16.31 and shall be duly accounted for on the "Closing Date"
               (hereinafter defined) as it may be extended pursuant to this
               Section 16.31 or on the earlier termination of this Section
               16.31. All interest earned on the Deposit shall be paid to
               Landlord with no credit against the purchase price for such
               interest being given to Tenant; provided, however, that if
               pursuant to the terms of this Section 16.31 the Deposit shall be
               returned to 


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<PAGE>   10

               Tenant, the interest earned on the Deposit at the time of such
               return of the Deposit shall be paid over to Tenant.

                    If for any reason the closing does not occur and either
               party makes a written demand upon the Escrow Agent for delivery
               of the Deposit and the interest earned thereon, the Escrow Agent
               shall give written notice to the other party of such demand. If
               the Escrow Agent does not receive a written objection from the
               other party to the proposed payment within ten (10) days after
               the giving of such notice, the Escrow Agent is hereby authorized
               to make such delivery or payment. If the Escrow Agent does
               receive such written objection within such ten (10) business day
               period or if for any other reason the Escrow Agent in good faith
               shall elect not to make such payment, the Escrow Agent shall
               continue to hold the Deposit until otherwise directed by written
               instructions from both Landlord and Tenant or a final judgment of
               a court. However, the Escrow Agent shall have the right at any
               time to deposit the Deposit with the clerk of such court of
               competent jurisdiction in the Commonwealth of Massachusetts that
               the Escrow Agent shall select. The Escrow Agent shall give
               written notice of such deposit to Landlord and Tenant. Upon such
               deposit the Escrow Agent shall be relieved and discharged of all
               further obligations and responsibilities hereunder.

                    The Landlord and Tenant acknowledge that the Escrow Agent
               shall act solely as a stakeholder at Landlord's and Tenant's
               request and for their convenience, that the Escrow Agent shall
               not be deemed to be the agent of either of the parties, and the
               Escrow Agent shall not be liable to either of the parties for any
               act or omission on its part unless taken or suffered in bad
               faith, in willful disregard of this Section 16.31 or involving
               gross negligence. Landlord and Tenant shall jointly and severally
               indemnify and hold the Escrow Agent harmless from and against all
               costs, claims and expenses, including reasonable attorneys' fees,
               incurred in connection with the performance of the Escrow Agent's
               duties hereunder, except with respect to actions or omissions
               taken or suffered by the Escrow Agent in bad faith, in willful
               disregard of this Section 16.31 or involving gross negligence on
               the part of the Escrow Agent. The parties agree that
               notwithstanding the obligations of the Escrow Agent under this
               Agreement, the Escrow Agent (if an attorney or law firm) shall be
               permitted to represent Landlord or Tenant (as the case may be) in
               connection with the transaction evidenced by this Section 16.31
               or in connection with any matters arising from or related to this
               Section 16.31, the consummation of this Section 16.31 or any
               claimed breach of this Section 16.31 by any party.

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<PAGE>   11


               (C) (i) The Purchase Price payable for the Demised Premises shall
               be payable on the closing by Federal Funds immediately available
               to the Landlord at BankBoston, Boston, Massachusetts or such
               other bank as may be stipulated by Landlord by written notice to
               Tenant. Upon closing, credit shall be given by Landlord to Tenant
               for and in the amount of the Deposit (but not for the interest
               earned thereon). The parties acknowledge and agree that
               "Impositions" (as defined in Article VI of the Lease) and the
               cost of the maintenance, repair and replacement of the Demised
               Premises pursuant to Sections 8.1 and 10.1 of the Lease, as such
               Sections are amended by this Fifth Amendment, are and shall
               continue to be paid entirely by Tenant. Accordingly, no credit or
               adjustment shall be given to Tenant at closing (or otherwise)
               respecting Impositions and such maintenance, repair and
               replacement obligations.

               (C) (ii) Fixed Rent and all Additional Rent (except respecting
               Impositions and certain maintenance, repair and replacement
               obligations, provision for which is made in Section 16.31 (C)(i)
               above) shall be paid through the "Closing Date" (referred to in
               Section 16.31 (D)) as it may be extended pursuant to the
               provisions of this Section 16.31 and in the event that Tenant has
               prepaid amounts of the foregoing (excluding Impositions) for
               periods extending beyond said Closing Date (as it may be so
               extended), appropriate credit shall be given to Tenant for
               amounts thereof (excluding Impositions and such maintenance,
               repair and replacement obligations) prepaid for periods extending
               beyond said Closing Date (as it may be so extended).

               (D) The closing (the "Closing") shall be held at the Middlesex
               South District Registry of Deeds, 208 Cambridge Street,
               Cambridge, Massachusetts 02141 (or at such other location in
               which said Registry of Deeds may be located), or at such other
               place in the City of Boston as the parties may agree, at 10:30
               A.M. on that date (the "Closing Date") which is the expiration
               date of the Second Extended Term; provided, however, if said day
               shall be a Saturday, Sunday or Legal Holiday, the Closing Date
               shall be the next following business day on which the Middlesex
               South District Registry of Deeds shall be open for the
               transaction of business. In addition to the provisions of Section
               16.31 (A) above, Landlord covenants that, so long as this Section
               16.31 shall be in full force and effect and Tenant has not
               assigned this Lease nor sublet the Premises (except only as
               provided in Subsection (L) below) and Tenant shall not have
               wrongfully failed to close on its purchase of the Demised
               Premises and there shall be no "Event of Default" (defined in
               Section 15.1 (a) of the Lease (as herein amended), Landlord shall
               not encumber the Demised Premises from time to time with any
               mortgages (and other financing documents) in the aggregate
               principal amount greater than the purchase price. It is agreed
   
                                      -11-

<PAGE>   12

               that time is of the essence with respect to the provisions and
               agreements in this Section 16.31.

6.1  Section (E) of Section 16.31 of the Lease added by Section 4 of the Fourth
     Amendment is hereby amended as follows:

               (A)  the words "and with the benefit of" are inserted after the
                    words "subject to" in the second line of the introductory
                    paragraph;

               (B)  the following words are added at the end of subsection
                    (viii):", provided, that Landlord, in Landlord's sole
                    discretion, may elect at any time to terminate the "East
                    Garage Sublease;"

6.2  Section (L) of Section 16.31 of the Lease added by Section 4 of the Fourth
     Amendment is hereby amended by adding the following at the end thereof:

               Notwithstanding the foregoing, if Tenant shall have timely given
               Tenant's Option Exercise Notice and Tenant's Purchase Price
               Notice as provided in Subsection (B) of this Section 16.31, as
               amended by Section 6 of the Fifth Amendment, Tenant may designate
               a nominee to take title to the Demised Premises by notice given
               to Landlord at least seven (7) days prior to the Closing Date,
               provided that all terms and conditions of this Section 16.31, as
               amended, and the Lease applicable to such sale of the Demised
               Premises shall apply to such nominee in addition to Tenant.

7.   Exhibit I attached to the Fourth Amendment to Lease is hereby deleted in
     its entirety and replaced with Exhibit I attached hereto.

8.   The time period of "March 1, 1998 through February 28, 2002" referenced in
     the third (3rd) and fourth (4th) lines of Section (A)(1) of Section 16.32
     of the Lease added by Exhibit II of the Fourth Amendment is hereby deleted
     and replaced with the "Fifth Amendment Effective Date through the date
     which is twelve (12) months prior to the expiration of the Second Extended
     Term."

9.   (A) It is the purpose and intent of Landlord and Tenant that the Lease as
     hereby amended shall constitute, and be construed as, an absolutely net
     lease, whereby under all circumstances and conditions (whether or not now
     or hereafter existing or within the contemplation of the parties) the
     Annual Fixed Rent shall be a completely net return to Landlord throughout
     the Term of the Lease and Tenant shall be responsible for all costs of
     keeping and maintaining the Demised Premises, the Building and the Site in
     first class condition and in compliance with all existing and future laws,
     including, without limitation, all costs of capital and operating repairs
     and replacements, including the roof, structural, mechanical and other
     systems of the Building; and unless and to the extent specifically
     otherwise 

                                      -12-

<PAGE>   13

     provided in the Lease as herein amended Tenant shall indemnify and hold
     harmless Landlord from and against any and all expenses, costs,
     liabilities, obligations and charges whatsoever, which shall arise or be
     incurred or become due, during the Term of the Lease, with respect to or in
     connection with, the Demised Premises, the Building and the Site, and the
     operation, management, maintenance and repair and replacement thereof.

     (B) Unless expressly otherwise provided in the Lease, the Lease shall not
     terminate, nor shall Tenant have any right to terminate this Lease, nor
     shall Tenant be entitled to any abatement or reduction of Annual Fixed
     Rent, Additional Rent or any other sums payable by Tenant under the Lease,
     nor shall the obligations of Tenant hereunder be affected by reason of (i)
     any damage to or destruction of all or any part of the Demised Premises,
     the Building or the Site, from whatever cause (except only and to the
     extent specifically otherwise provided in Article XIV of the Lease), (ii)
     the taking of the Demised Premises, the Building or the Site, or any
     portion thereof, by eminent domain or otherwise for any reason (except only
     and to the extent specifically otherwise provided in Article XIV of the
     Lease), (iii) the prohibition, limitation or restriction of Tenant's use of
     all or any part of the Demised Premises, the Building or the Site, from
     whatever cause, or any interference with such use, or (iv) any other cause
     whether similar or dissimilar to the foregoing, any present or future law
     to the contrary notwithstanding. It is the intention of the parties that
     Landlord shall have no obligation or covenant under the Lease with respect
     to or in connection with the operation, management, maintenance, repair and
     replacement of the Demised Premises, the Building and the Site except for
     (i) the obligation to provide the services set forth in Section 7.1 of the
     Lease, as amended by Section 2 of the Third Amendment and Section 10 of
     this Fifth Amendment, subject to reimbursement by Tenant as provided in
     Section 7.1.1 of the Lease, as amended by Section 3 of the Third Amendment
     and Section 10 of this Fifth Amendment, (ii) the obligation to provide
     Tenant the right to use, in common with others, the common internal
     roadways, sidewalks and pedestrian walks in the Parcel 2 Development Area
     as provided in Section 7.2 of the Lease, subject to reimbursement by Tenant
     as provided in Section 7.1.1 of the Lease, as amended by Section 3 of the
     Third Amendment and Section 10 of this Fifth Amendment, (iii) the
     obligation to maintain insurance as provided in Section 13.2 of the Lease,
     subject to reimbursement from Tenant as provided in Section 7.1.1 of the
     Lease, as amended by Section 3 of the Third Amendment and Section 10 of the
     Fifth Amendment and (iv) the obligation to provide Tenant's parking rights
     in the North Garage as provided in Section 16.5 of the Lease, as amended by
     Section 3 of the Second Amendment, Section 3 of the Fourth Amendment and
     Section 5 of the Fifth Amendment and (v) Landlord's obligations set forth
     in Section 13.1 (B) of this Fifth Amendment respecting certain "Hazardous
     Materials" (as defined in said Section 13.1) at the Site. Subsections (i),
     (ii) and (v) above are herein collectively referred to as "Landlord's
     Repair Obligations". In addition, it is the 

                                      -13-

<PAGE>   14


     intention of the parties hereto that the obligations of Tenant under the
     Lease shall be separate and independent covenants and agreements, that
     Annual Fixed Rent, Additional Rent and all other sums payable by Tenant
     under the Lease shall continue to be payable in all events and that the
     obligations of Tenant under the Lease shall continue unaffected, unless the
     requirement to pay or to perform the same shall have been excused pursuant
     to an express provision of the Lease.

     (C) In order to effectuate the intention of Landlord and Tenant as set
     forth in this Section 9, the Lease shall be amended as provided in Sections
     10, 11, 12, 13 and 13.1 below.

10.  (A) For purposes of calculating Tenant's payments for operating costs
     pursuant to Section 7.1.1 of the Lease, as amended by Section 3 of the
     Third Amendment, for that portion of the Lease Term prior to the Fifth
     Amendment Effective Date, the terms of said Section 7.1.1, as amended by
     Section 3 of the Third Amendment, shall be unchanged.

     (B) For the purposes of calculating Tenant's payments for the operating
     costs for that portion of the Lease Term on and after the Fifth Amendment
     Effective Date, said Section 7.1.1, as amended by Section 3 of the Third
     Amendment, shall be amended by deleting the definition of "Operating
     Expenses" in its entirety and substituting the following therefor:

          "Operating Expenses" means the cost of operation of the Demised
          Premises reasonably and necessarily incurred by Landlord in performing
          Landlord's obligations under the Lease, as amended; including without
          limitation, all costs of performing Landlord's obligations under
          Section 7.1 of the Lease, as amended by Section 2 of the Third
          Amendment; premiums for insurance carried with respect to the Demised
          Premises as set forth in Section 13.2 of the Lease; compensation and
          all fringe benefits, workmen's compensation insurance premiums and
          payroll taxes paid to, for or with respect to all persons engaged in
          the operating, maintaining or cleaning of the exterior of the Building
          (exclusive of cleaning the windows of the Building) or the Site; the
          cost of any letters of credit or deposits necessary to secure any
          utility services if required and to the extent not otherwise paid for
          by Tenant; cost of building and cleaning supplies and equipment; cost
          of maintenance, repairs and replacements (other than reimbursement
          from contractors under guarantees); cost of snow removal and care of
          landscaping, cost of carrying insurance with respect to and of
          providing streetlights and electricity therefor, repairing, and
          otherwise maintaining and replacing roadways, sidewalks and pedestrian
          ways within or abutting the Site or the Parcel 2 Development Area;
          payments under service contracts with independent contractors; and all
          other reasonable and necessary amounts paid in connection with
          performing 

                                      -14-

<PAGE>   15

          Landlord's obligations under the Lease, as amended, respecting the
          operation, maintenance and cleaning of the exterior of the Building,
          the Site and areas outside of the Site but within the Parcel 2
          Development Area , including, without limitation, the cost of all
          capital expenditures; provided, however, with respect to the insuring,
          repairing, removing snow from, landscaping and otherwise maintaining
          and replacing roadways, sidewalks and pedestrian walks or ways outside
          of the Site but within the Parcel 2 Development Area there only shall
          be included within Operating Expenses "Tenant's Proportionate Share"
          (hereinafter defined) thereof. "Tenant's Proportionate Share" shall be
          a fraction, the numerator of which shall be the Gross Building Area of
          the Building and the denominator of which shall be the sum of (i) the
          Gross Building Area of the Building plus (ii) the gross building area
          of other buildings under construction or completed from time to time
          in the Parcel 2 Development Area. Notwithstanding the foregoing, there
          shall be excluded from "Operating Expenses" the cost of performing
          Landlord's obligations under Section 13.1 (B) of the Fifth Amendment
          respecting certain Hazardous Materials at the Site.

11.  (A) For the portion of the Lease Term prior to the Fifth Amendment
     Effective Date, Tenant shall continue to perform the repairs and
     maintenance set forth in Section 8.1 of the Lease as amended by Section 4
     of the Third Amendment.

     (B) For the portion of the Lease Term on and after the Fifth Amendment
     Effective Date, said Section 8.1, as amended by Section 4 of the Third
     Amendment, shall be amended and restated as follows:

          Tenant shall, throughout the Lease Term, at Tenant's sole cost and
          expense, keep and maintain the Demised Premises and every part thereof
          including, without limitation, all mechanical, utility and other
          systems and appurtenances of the Building and the structural and
          nonstructural elements of the roof, interior and exterior walls,
          foundation, floor slabs and all other elements of the Building and all
          above ground and below ground storage tanks in, on or under the
          Building, the Site or the Demised Premises in first class condition
          and repair and in compliance with all applicable existing and future
          laws, excepting only (i) normal and reasonable wear and use, (ii)
          damage caused by fire or other casualty or as a consequence of the
          exercise of the power of eminent domain to the extent Tenant is
          relieved of its obligations pursuant to Article XIV of the Lease, and
          (iii) those repairs (or services) to be made by Landlord as
          "Landlord's Repair Obligations" (as defined in Section 9 of the Fifth
          Amendment). In addition to the foregoing, Tenant shall, throughout the
          Lease Term, at Tenant's sole cost and expense (x) provide to the
          Demised Premise steam, water, sewer, electricity, gas, oil, elevator
          service, rubbish 

                                      -15-


<PAGE>   16

          removal, security systems and service, telephone service and pest
          control services, (y) provide to the interior of the Building cleaning
          service and (z) clean the windows of the Building. Landlord shall have
          no responsibility for providing any of (x), (y) and (z) above. Tenant
          shall not commit or suffer to be committed any waste upon or about the
          Demised Premises, and except for Landlord's Repair Obligations, shall
          promptly at its cost and expense make all necessary replacements,
          restorations, renewals and repairs to the Demised Premises and
          appurtenances thereto, whether interior or exterior, ordinary or
          extraordinary, foreseen as well as unforeseen, necessary to keep the
          Demised Premises in good and lawful order and condition, and such
          repairs, replacements, restorations and renewals shall, to the maximum
          extent possible, be at least equivalent in quality to the quality of
          the original work or the property replaced, as the case my be. Tenant
          shall undertake preventive maintenance as well, and a reasonable
          policy of inspection to ascertain the need for repairs and
          replacements shall be adhered to by Tenant throughout the entire Lease
          Term. Tenant specifically covenants and agrees to replace all glass
          damaged with glass of the same kind and quality. Except for Landlord's
          Repair Obligations, and except to the extent Tenant is relieved of its
          obligations for damage caused by fire or other casualty or by eminent
          domain pursuant to Article XIV of the Lease, Tenant's obligations to
          maintain the Demised Premises is absolute and without limitation.
          Except for Landlord's Repair Obligations, Landlord shall have
          absolutely no liability or obligation whatsoever to make any repairs,
          replacements, alterations, restorations or renewals of any nature or
          description to the Demised Premises (including the improvements
          thereon). Landlord, at Landlord's expense, shall cooperate with Tenant
          in Tenant's performance of its obligations under this Section 8.1 to
          the extent necessary because of Landlord's ownership of the fee
          interest in the Demised Premises. Landlord and Tenant represent to the
          other that as of the date of this Fifth Amendment that they have not
          received written notice from any governmental authority or any insurer
          of the Demised Premises that the Demised Premises is currently in
          violation of any applicable law or insurance underwriting standard.
          Landlord and Tenant shall promptly forward to the other party any
          written notice they receive after the date of this Fifth Amendment
          from any governmental authority or any insurer of the Demised Premises
          that the Demised Premises is in violation of any applicable law. Prior
          to Tenant making any submission or filing or sending any notice to any
          governmental authority in connection with the performance of its
          obligations under this Section 8.1, Tenant shall first submit such
          submission, filing or notice to Landlord for Landlord's approval,
          which approval shall not be unreasonably withheld or delayed.

                                      -16-

<PAGE>   17

12.  (A) For the portion of the Lease Term prior to the Fifth Amendment
     Effective Date, Landlord shall continue to perform its obligations set
     forth in Section 8.2 of the Lease.

     (B) For the portion of the Lease Term on and after the said Fifth Amendment
     Effective Date, Section 8.2 of the Lease shall be deleted in its entirety
     and Landlord shall have no obligation to perform any of its obligations
     formerly set forth therein. At the request of Tenant (i) Landlord shall
     make available to Tenant the plans of the Demised Premises in Landlord's
     possession which are useful or necessary for Tenant to perform its
     maintenance, repair and replacement obligations under the Lease and (ii)
     Landlord shall make available to and assign to Tenant, to the extent
     assignable, Landlord's interest in any warranties covering portions of the
     Demised Premises which Tenant is required to maintain, repair or replace
     under the Lease, provided that Landlord shall also retain the benefit of
     such warranties.

13.  (A) For the portion of the Lease Term prior to the Fifth Amendment
     Effective Date, Section 10.1 of the Lease shall be unchanged.

     (B) For the portion of the Lease Term on and after the Fifth Amendment
     Effective Date, Section 10.1 of the Lease shall be amended and restated as
     follows:

          Tenant shall, at its sole cost and expense, all during the Lease Term,
          promptly comply with all present and future laws, ordinances, rules
          and regulations of any duly constituted governmental authority
          ("Governmental Authority") relating to the use or occupancy of the
          Demised Premises. Tenant shall promptly pay all fines, penalties and
          damages that may arise out of or be imposed because of its failure to
          comply with the provisions of this Section 10.1.

13.1 (A) Tenant covenants during the Lease Term and for such further time as
     Tenant occupies any part of the Demised Premises, (i) Tenant shall not, nor
     shall Tenant permit its employees, invitees, agents, independent
     contractors, contractors, assignees or subtenants to, keep, maintain, store
     or dispose of (into the sewage or waste disposal system or otherwise) or
     engage in any activity which might produce or generate any substance which
     is or may hereafter be classified as a hazardous material, waste or
     substance (collectively "Hazardous Materials"), under federal, state or
     local laws, rules and regulations, including, without limitation, 42 U.S.C.
     Section 6901 et seq., 42 U.S.C. Section 9601 et seq., 42 U.S.C. Section
     2601 et seq., 49 U.S.C. Section 1802 et seq. and Massachusetts General
     Laws, Chapter 21E and the rules and regulations promulgated under any of
     the foregoing, as such laws, rules and regulations may be amended from time
     to time (collectively "Hazardous Materials Laws") in violation of Hazardous

                                      -17-

<PAGE>   18

     Materials Laws, (ii) Tenant shall immediately notify Landlord of any
     incident in, on or about the Premises, the Building or the Site that would
     require the filing of a notice under any Hazardous Materials Laws, (iii)
     Tenant shall comply and shall cause its employees, invitees, agents,
     independent contractors, contractors, assignees and subtenants to comply
     with each of the foregoing and (iv) Landlord shall have the right to make
     such inspections (including testing) as Landlord shall elect from time to
     time to determine that Tenant is complying with the foregoing.

     (B) Landlord covenants during the Lease Term, at Landlord's cost and
     expense, to comply with the requirements of Hazardous Materials Laws
     arising because of Hazardous Materials (a) which are in, on, under or about
     the Site as of the Commencement Date, (b) which migrate to the Site after
     the Commencement Date from any adjoining property or (c) which are caused
     or created by Landlord, its agents, contractor or employees.
     Notwithstanding the first sentence of this Subsection 13.1(B), (i) if
     Tenant exercises Tenant's option to purchase the Demised Premises pursuant
     to Section 16.31 of the Lease, as added to the Lease by Section 4 of the
     Fourth Amendment and amended by Section 6 of this Fifth Amendment, as
     provided in said Section 16.31 the Demised Premises shall be conveyed in
     "as is" condition and Landlord and Tenant's rights and obligations with
     respect to the presence of Hazardous Materials in, on, under or about the
     Demised Premises in connection with such purchase and after such purchase
     shall be as set forth in said Section 16.31, (ii) Tenant, at Tenant's cost
     and expense shall be responsible for compliance with Hazardous Materials
     Laws required because of Hazardous Materials which migrate to the Demised
     Premises from another site where (x) Tenant or an affiliate of Tenant or
     their agents, contractors or employees caused or created the contamination
     on such other site or (y) Tenant or an affiliate of Tenant own or
     previously owned such other site and (iii) Landlord and Tenant do not
     intend for the first sentence of this Subsection 13.1(B) to amend or alter
     any of the rights or obligations of Landlord or any affiliate of Landlord
     or Tenant or any affiliate of Tenant under the "Option Documents" (as
     hereinafter defined), and in the event of any conflict between the first
     sentence of this Subsection 13.1(B) and the Option Documents, the terms of
     the Option Documents shall control. The "Option Documents" shall be deemed
     to be the Acquisition, Option and Cooperation Agreement dated as of October
     6, 1993 between North Parcel Limited Partnership (an affiliate of Landlord
     and Tenant) and Tenant (the "Option Agreement") and the "Transaction
     Documents" (as defined in the Option Agreement) defined therein, including,
     without limitation, the Indemnity Agreement Regarding Hazardous Materials
     dated October 6, 1993 from Biogen, Inc. and Biogen Realty Limited
     Partnership (an affiliate of Tenant), the ground Lease dated October 6,
     1993 between Biogen Realty Limited Partnership and North Parcel Limited
     Partnership respecting Tract V of Parcel 2 of the Kendall Square Urban
     Renewal Area, the Exclusive Easement and Option Agreement dated October 6,
     1993 between the Cambridge Redevelopment Authority and North Parcel Limited
     Partnership respecting Tract 


                                      -18-

<PAGE>   19

     VI of Parcel 2 of the Kendall Square Urban Renewal Area and the
     Unconditional Guaranty and Indemnity of Biogen, Inc. dated October 6, 1993.

14.  Tenant acknowledges that the beneficial interest in Landlord has been
     transferred to Boston Properties Limited Partnership, a Delaware limited
     partnership, the sole general partner of which is Boston Properties, Inc.,
     a Delaware corporation which is a publicly traded real estate investment
     trust and such transfer is a "REIT Transaction" (as defined in Section
     16.32 (A) (2) of the Lease added to the Lease pursuant to Section 5 of the
     Fourth Amendment) and is not subject to Tenant's rights under said Section
     16.32. To reflect such transfer the following amendments are made to the
     Lease:

     (A)  The words "(including, without limitation, Boston Properties Limited
          Partnership, Boston Properties, Inc. and their officers, directors and
          employees)" are substituted in the place of the following:

               (i)  the parenthetical in the ninth (9th) and tenth (10th) lines
                    of Section 16.31 (H) of the Lease added to the Lease
                    pursuant to Section 4 of the Fourth Amendment, and

               (ii) the parenthetical in the tenth (10th) and eleventh (11th)
                    lines from the end of Section 9 of the Fourth Amendment.

     (B)  The words "(including, but not limited to, Boston Properties Limited
          Partnership, Boston Properties, Inc., Mortimer B. Zuckerman, Edward H.
          Linde and/or affiliates of any of the foregoing)" are substituted in
          place of the following:

          (i)  the parenthetical in the ninth (9th) through twelfth (12th) lines
               of Section 16.31 (Q) of the Lease;

          (ii) the parenthetical in third (3rd) through sixth (6th) lines of
               Section 16.32 (A) (2) of the Lease, and

          (iii) the parenthetical in the fifth (5th) through the eighth (8th)
               lines from the end of Section 16.32 (A) (3) of the Lease.

     (C)  The words "(or any affiliates of Landlord, including, but not limited
          to, Boston Properties Limited Partnership, Boston Properties, Inc.,
          Mortimer B. Zuckerman, Edward H. Linde and other affiliates of any of
          the foregoing)" are substituted in place of the following:

          (i)  the parenthetical in the seventh (7th) through the eleventh
               (11th) lines of Section 16.32 (D) (1) of the Lease;


                                      -19-

<PAGE>   20

          (ii)   the parenthetical on the sixth (6th) line through the tenth
                 (10th) lines from the end of Section 16.32 (D) (3) (a) of the
                 Lease; and

          (iii)  the parenthetical on the fifth (5th) through the ninth (9th)
                 lines from the end of Section 16.32 (D) (3) (b) of the Lease.

     (D)  Subsection (iii) of both Sections 16.32 (D) (4) (a) and 16.32 (D) (4)
          (b) of the Lease are deleted in their entirety and replaced with the
          following:

          "(iii) any offer to sell, sale or transfer of the Demised Premises or
                 any interest in the Demised Premises to any firm, entity, or
                 business organization in which at the time of the sale or
                 transfer Boston Properties, Inc. and/or Boston Properties 
                 Limited Partnership directly or indirectly own(s) at least 
                 fifty percent(50%) of or otherwise controls.

15.  Landlord and Tenant each represents and warrants to the other that it has
     not dealt with any real estate brokers or other persons or entities which
     have been, are or will be entitled to any broker's or finder's fee or any
     similar commission or fee in connection with this Fifth Amendment
     (including, without limitation, the transactions contemplated by Sections
     16.31 and 16.32 of the Lease and added to the Lease pursuant to the Fourth
     Amendment and as herein amended) except Fallon, Hines & O'Connor (the
     "Recognized Broker"). Landlord and Tenant each agree to indemnify, hold
     harmless, protect and defend the other from and against any and all loss,
     damage, liability and expense, including costs and reasonable attorneys'
     fees which such other party incurs or sustains by reason of the breach by
     the indemnifying party of its foregoing warranties and representations.
     Landlord covenants and agrees that it shall be solely responsible for and
     shall pay to the Recognized Brokerage a commission in the amount of One
     Hundred Thirty Five Thousand Dollars ($135,000.00) in connection with this
     Fifth Amendment which shall be the sole fee or commission due the
     Recognized Broker for the transaction evidenced by this Fifth Amendment.
     The provisions hereof shall survive the expiration or any termination of
     this Lease (as herein amended), the expiration or any termination of
     Section 16.31 and/or Section 16.32 of the Lease and/or the delivery of any
     deed of the Demised Premises to Tenant pursuant to Section 16.31 of the
     Lease, Section 16.32 of the Lease or otherwise.

16.  Concurrently with the execution of this Fifth Amendment to Lease, Landlord
     shall deliver to Tenant a Trustees' Certificate respecting the authority to
     enter into this Fifth Amendment To Lease and Tenant shall deliver to
     Landlord a corporate vote evidencing the authority of Tenant to enter into
     this Fifth Amendment to Lease.


                                      -20-

<PAGE>   21


17.  All capitalized terms and words used in this Amendment shall have the same
     meaning as set forth in the Lease unless a contrary meaning is expressly
     set forth herein.

18.  Except as expressly amended hereby, the Lease and its terms and provisions
     shall remain unchanged and in full force and effect.

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

WITNESS:                                       LANDLORD:

--------------------                           ---------------------------
                                               EDWARD H. LINDE, TRUSTEE OF
                                               FOURTEEN CAMBRIDGE CENTER 
                                               TRUST AND NOT INDIVIDUALLY


                                               TENANT:

                                               BIOGEN, INC.

                                               By: _______________________
                                               Name: DAVID C. DLESK
                                               Title: (VICE PRESIDENT)
                                                     ---------------------

ATTEST:

By:______________________________              By:________________________

Name:____________________________              Name:_________________________
Title:    CLERK (ASSISTANT CLERK)              Title: TREASURER (OR ASSISTANT
      ---------------------------                    ------------------------  
                                                      TREASURER)
                                               ------------------------------ 



                                      -21-