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                                                                 Exhibit 10.27

                                                                        11.10.00

                   PURCHASE AND SALE AND ASSIGNMENT AGREEMENT

         THIS PURCHASE AND SALE AND ASSIGNMENT AGREEMENT ("Agreement") is
made as of the 28th day of November, 2000, by and among Serono, Inc., a
Delaware corporation ("Seller"), Transkaryotic Therapies, Inc., a Delaware
corporation ("Buyer"), and joined in for the limited purposes set forth
herein by Stewart Title Insurance Company, as escrow agent ("Escrow Agent").

                                   BACKGROUND

         A. Seller is the owner of the following real and personal property
(collectively, the "Property"):

         (1) The land with the improvements thereon ("Improvements") on a
certain parcel of land located at 78-80 Pacella Park Drive, Randolph, Norfolk
County, Massachusetts, as more particularly described in EXHIBIT A hereto,
together with all easements, rights and privileges appurtenant thereto (the
"Real Property");

         (2) All personal property, fixtures and equipment now located in or
attached to the Real Property, including without limitation the personal
property, fixtures and equipment listed in EXHIBIT A-1 hereto; and

         (3) All existing intangible property used by Seller in connection with
the operation of the foregoing, including, without limitation, all contract
rights, guarantees, licenses, permits and warranties.

         B. Seller is the tenant under a lease of certain premises located at 76
Pacella Park Drive, Randolph, Norfolk County, Massachusetts ("Leased Premises")
dated October 16, 1984, as amended by a First Amendment dated as of August 1,
1987, an undated Second Amendment, and a Third Amendment dated as of January 27,
1993 (as so amended, the "Lease").

         C. Seller is prepared to sell, transfer and convey the Property to
Buyer, to assign its interest under the Lease to Buyer, and to provide a
guaranty of certain obligations of Buyer under the Amended Lease (as defined in
Section 4.1.9 below), and, subject to the terms of Section 4.1.9 below, Buyer is
prepared to purchase and accept the same from Seller and to assume Seller's
obligations under the Lease, all for the purchase price and on the other terms
and conditions hereinafter set forth.

                              TERMS AND CONDITIONS

         In consideration of the mutual covenants and agreements herein
contained, and intending to be legally bound hereby, the parties hereto agree:

         1. SALE AND PURCHASE; ASSIGNMENT AND ASSUMPTION. Seller hereby agrees
to sell, transfer and convey the Property to Buyer, and to assign its interest
under the Lease to Buyer, and Buyer hereby agrees to purchase and accept the
Property from Seller, and to assume the

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obligations of Seller under the Lease, in each case for the purchase price and
on and subject to the other terms and conditions set forth in this Agreement.

         2. PURCHASE PRICE. The purchase price for the Property (the "Purchase
Price") shall be Nine Million and 00/100 Dollars ($9,000,000.00), and the
consideration for the assignment of the Lease shall be the assumption thereof by
Buyer, which, subject to the terms and conditions hereinafter set forth, shall
be paid to Seller by Buyer as follows:

            2.1. DEPOSIT. By December 1, 2000, Buyer shall deliver to Escrow
Agent, in immediately available funds, to be held in escrow and delivered in
accordance with this Agreement, a cash deposit in the amount of Two Hundred
Fifty Thousand and 00/100 Dollars ($250,000.00) (such amount, with all interest
earned thereon, shall hereinafter be referred to as the "Deposit"). The Deposit
shall be held in an insured, interest bearing account by Escrow Agent, with
interest credited to Buyer, and shall be applied to the Purchase Price
hereunder. The Deposit shall be non-refundable (except as provided in Sections
4.1.4.2, 4.1.5, 4.1.6, 5, 9 and 10.2 hereunder), but shall be applied to the
Purchase Price at the Closing.

            2.2. PAYMENT AT CLOSING. At the consummation of the transaction
contemplated hereby (the "Closing"), Buyer shall deliver to Escrow Agent cash in
an amount equal to the Purchase Price less the Deposit. The Purchase Price,
subject to adjustments and apportionments as set forth herein, shall be paid at
Closing by wire transfer of immediately available federal funds, transferred to
the order or account of Seller or such other person as Seller may designate in
writing.

         3. REPRESENTATIONS AND WARRANTIES OF SELLER. Seller represents and
warrants to Buyer as follows:

            3.1. AUTHORITY. Seller is a corporation duly formed and validly
existing under the laws of the Commonwealth of Massachusetts, and has all
requisite power and authority to enter into this Agreement and perform its
obligations hereunder. The execution and delivery of this Agreement have been
duly authorized.

            3.2. NO CONFLICT. The execution and delivery of this Agreement and
the consummation of the transactions contemplated hereunder on the part of
Seller do not and will not conflict with or result in the breach of any material
terms or provisions of, or constitute a default under, or result in the creation
or imposition of any lien, charge, or encumbrance upon any of the Property or
the Lease, or assets of the Seller by reason of the terms of any contract,
mortgage, lien, lease, agreement, indenture, instrument or judgment to which
Seller is a party or which is or purports to be binding upon Seller or which
otherwise affects Seller, which will not be discharged, assumed or released at
Closing, except for the agreements to be obtained from Trinet Property Partners
Limited Partnership ("Landlord") with respect to the Lease and the Leased
Premises under the provisions of Section 4.1.9 below. No action by any federal,
state or municipal or other governmental department, commission, board, bureau
or instrumentality, and no consent of any third party (other than Landlord) is
necessary to make this Agreement a valid instrument binding upon Seller in
accordance with its terms.

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            3.3. LEASES; SUBLEASES. There are no leases or occupancy agreements
currently in effect that affect the Property. The only lease which affects the
Leased Premises is the Lease, a true, correct, and complete copy of which is
attached hereto as SCHEDULE 3.3. There are no other agreements, written or oral,
with respect to the Lease, other than as set forth in SCHEDULE 3.3. There are no
subleases or occupancy agreements (other than the Lease) currently in effect
that affect the Leased Premises.

            3.4. NO CONDEMNATION. There is no pending or, to the best of
Seller's actual knowledge, contemplated condemnation, eminent domain or similar
proceeding with respect to all or any portion of the Property or the Leased
Premises.

            3.5. NO RIGHTS IN OTHERS. No person or other entity has any right or
option to acquire, lease, sublease, or occupy all or any portion of the Property
granted or arising by, through or under Seller. No person or other entity has
any right or option to acquire, lease, sublease, or occupy all or any portion of
the Leased Premises granted or arising by, through or under Seller, except for
Seller's option to purchase the Leased Premises set forth in the Lease.

            3.6. CONTRACTS. There are no construction, management, leasing,
service, equipment, supply, maintenance or concession agreements in effect with
respect to the Property or the Leased Premises, except for cleaning, landscaping
and security arrangements made by Seller which shall be terminated by Seller by
the Closing.

            3.7. COMPLIANCE. Seller has received no notice of, and to the best
of Seller's actual knowledge, there are no existing violations of any federal,
state, or municipal laws, ordinances, orders, codes, regulations or requirements
affecting the Property or the Leased Premises.

            3.8. LITIGATION. There is no action, suit or proceeding pending or,
to the best of Seller's actual knowledge, threatened against or affecting the
Property or the Leased Premises, or arising out of the ownership, management or
operation of the Property, the Leased Premises, this Agreement or the
transactions contemplated hereby.

            3.9. ENVIRONMENTAL MATTERS. To the best of Seller's actual
knowledge, Seller has not generated, stored, or disposed other than in
compliance with law, nor has Seller released or discharged, any hazardous
substances or wastes from or on the Property or the Leased Premises, nor, except
as disclosed in Schedule 3.9, has there been any generation, storage, release,
discharge or disposal of such hazardous substances or wastes from or on the
Property or the Leased Premises by any other party, other than the generation,
storage and disposal of Hazardous Substances used by Seller in its business
operations in accordance with applicable law. As used in this Agreement, the
terms "Hazardous Substances" and "Hazardous Wastes" shall have the meanings set
forth in the Comprehensive Environmental Response, Compensation and Liability
Act, as amended, and the regulations thereunder, the Resource Conservation and
Recovery Act, as amended, and the regulations thereunder, and the Federal Clean
Water Act, as amended, and the regulations thereunder, and such terms shall also
include asbestos, petroleum products, radioactive materials and any regulated
substances under any Federal, State or local environmental law, regulation or
ordinance.

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            3.10. FIRPTA. Seller is not a "foreign person" as defined in Section
1445(f)(3) of the Internal Revenue Code.

            3.11. CONTINUING OBLIGATIONS. There are no contractual or
governmental obligations in connection with the Property or the Leased Premises
which will be binding upon Buyer after Closing, except the Amended Lease (as
defined in Section 4.1.9 below), which are not recorded with the Norfolk
Registry of Deeds or filed in the Norfolk Registry District of the Land Court.

            3.12. BANKRUPTCY. Seller, as debtor, has not filed or been the
subject of any filing of a petition under the Federal Bankruptcy Law or any
insolvency laws, or any laws for composition of indebtedness or for the
reorganization of debtors.

         For the purpose of this Article 3, the term "to the best of Seller's
actual knowledge" shall mean the actual knowledge of Seller, after reasonably
diligent inquiry.

         4. CONDITIONS PRECEDENT. The respective obligations of Buyer and Seller
are expressly conditioned on the satisfaction of certain conditions as more
specifically set forth in this Section 4.

            4.1. CONDITIONS PRECEDENT TO BUYER'S OBLIGATIONS. All of Buyer's
obligations hereunder are expressly conditioned on the satisfaction at or before
the time of Closing hereunder, or at or before such earlier time as may be
expressly stated below, of each of the following conditions (any one or more of
which may be waived in writing in whole or in part by Buyer, at Buyer's option):

                 4.1.1 ACCURACY OF REPRESENTATIONS. All of the representations
and warranties of Seller contained in this Agreement shall have been true and
correct in all material respects when made, and shall be true and correct in all
material respects on the date of Closing with the same effect as if made on and
as of such date.

                 4.1.2 PERFORMANCE BY SELLER. Seller shall have performed,
observed and complied with all material covenants, agreements and conditions
required by this Agreement to be performed, observed and complied with on its
part prior to or as of Closing hereunder.

                 4.1.3 DOCUMENTS AND DELIVERIES. All instruments and documents
required on Seller's part to effectuate this Agreement and the transactions
contemplated hereby shall be delivered to Buyer and shall be in form and
substance consistent with the requirements herein.

                 4.1.4 ACCESS; CONDITIONS TO CLOSING.

                       4.1.4.1. During the term of this Agreement, Buyer, its
agents and representatives, after having provided Seller with reasonable prior
notice, shall be entitled to enter upon the Property and the Leased Premises to
perform inspections and tests of the Property and the Leased Premises, including
without limitation, surveys, environmental studies, engineering studies,
structural inspections, and similar examinations and tests, review of lease
documentation, title and survey matters, review of permits, approvals, and
governmental

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records, review of facility and safety records, and inventory of
furniture, fixtures and equipment, and facility related documentation, including
records of facility validation, facility maintenance, and facility and equipment
decommissioning. Buyer shall comply with all applicable provisions of the Lease
in making any such entry upon the Leased Premises. Buyer shall repair any damage
to the Property or the Leased Premises caused by any entry, tests or
investigations conducted by Buyer, and indemnify Seller from any and all
personal injury or property damage resulting therefrom. The foregoing
indemnification shall survive Closing or the termination of this Agreement.

                       4.1.4.2. Buyer may terminate this Agreement in its
sole discretion in the event of Buyer's dissatisfaction with the
environmental condition of the Property or the Leased Premises by giving
written notice of such dissatisfaction and election to Seller on any day
prior to and including December 1, 2000 (the period from the date hereof to
December 1, 2000, the "Environmental Evaluation Period"), in which event the
Deposit shall be returned forthwith to Buyer and, except as expressly set
forth herein, neither party shall have any further liability or obligation to
the other hereunder. Such notice shall state the basis for Buyer's
dissatisfaction, provided that in no event shall Seller have any right to
challenge Buyer's dissatisfaction, it being expressly understood that Buyer
may terminate this Agreement under the provisions of this Section 4.1.4.2 in
its sole discretion. Buyer shall deliver with such notice copies of any
reports, studies, evaluations, surveys or the like made or received by Buyer
with respect thereto, provided that Buyer shall have no duty to perform or
complete any such reports or other materials, nor shall Buyer be obligated to
cause the preparer thereof to permit Seller or any other party to rely
thereon. In the absence of such written notice by said time limit, the
contingency provided for in this Section 4.1.4.2 no longer shall be
applicable, and this Agreement shall continue in full force and effect, with
disposition of the Deposit becoming limited to the provisions of Sections
2.1, 5, 9, 10.1 and 10.2, as applicable.

                 4.1.5. SURVEY. Buyer shall approve or disapprove the Survey, as
hereinafter defined, in the following manner:

                       (a) NOTICE. Buyer shall be responsible for obtaining an
update of the survey of the Property delivered by Seller to Buyer, and shall be
responsible for obtaining a survey of the Leased Premises (collectively, the
"Survey"). On or before December 1, 2000, Buyer shall give notice to Seller of
Buyer's approval of the Survey or disapproval of any matters thereon ("Survey
Exceptions"). Buyer's failure to give any notice within the time limit shall be
deemed approval of the Survey, and this contingency shall be deemed satisfied.

                       (b) SURVEY CURE PERIOD. Seller shall have the right, but
not the obligation, to remove any Survey Exceptions within thirty (30) days
after Buyer's notice of disapproval ("Survey Cure Period"), either by obtaining
the surveyor's written agreement to make appropriate changes to the Survey or by
taking other steps reasonably satisfactory to Buyer to permit such changes to
the Survey.

                       (c) WAIVER OF UNCURED MATTERS. If Seller does not cure
any Survey Exceptions within the Survey Cure Period, Buyer shall have three (3)
business days to give Seller written notice that Buyer waives its objections to
the Survey Exceptions. If Buyer does not give such notice by said time limit,
this Agreement shall terminate whereupon the

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Deposit shall be returned forthwith to Buyer and, except as expressly set forth
herein, neither party shall have any further liability or obligation to the
other hereunder. If this Agreement is so terminated, Buyer shall provide to
Seller copies of the written documents obtained in the course of its
inspections.

                       (d) APPROVED SURVEY. The Survey, as approved by Buyer
pursuant to clause (a) above and with any waivers by Buyer pursuant to clause
(c) above, is referred to herein as the "Approved Survey".

                 4.1.6. TITLE. Buyer shall review and approve or disapprove the
condition of title to the Property and the Leased Premises in the following
manner:

                       (a) REPORT AND NOTICE. Seller has delivered to Buyer a
copy of Seller's policy of title insurance for the Property. Buyer has obtained
an update thereof, and has obtained a title examination of the Leased Premises
(collectively, the "Title Report"), a copy of which is attached hereto as
SCHEDULE 4.1.6. Buyer has given notice to Seller of Buyer's disapproval of
certain exceptions to title through the date of the Title Report (the title
exceptions listed as disapproved in such notice, the "Disapproved Exceptions"),
a copy of which notice is also included in Schedule 4.1.6.

                       (b) TITLE CURE PERIOD. Seller shall have the right, but
not the obligation, by December 1, 2000 (the "Title Cure Period"), to (i) remove
any Disapproved Exceptions or (ii) agree to use reasonable and diligent efforts
to remove any Disapproved Exceptions on or before the Closing. If Seller gives
notice, within the Title Cure Period, that Seller will make reasonable and
diligent efforts to remove any such Disapproved Exception on or before the
Closing, then Seller shall proceed with such efforts, but shall not be required
to spend or incur costs of more than $100,000 in doing so. If Seller should be
unable to accomplish such removal on or before the Closing, the title conditions
to Closing under this Section 4.1.6 shall be deemed unsatisfied, and Buyer shall
have the option in such regard set forth in Section 5.1. With respect to any
Exception consisting of a financial encumbrance in a liquidated, fixed amount,
such as a mortgage, deed of trust, or other debt security, or any delinquent
real estate taxes or mechanic's liens outstanding against the Property, such
matter shall automatically be deemed a Disapproved Exception; Seller hereby
covenants to remove any such Exception on or before the Closing, provided that
Seller may use the proceeds from the Purchase Price to do so as set forth in
Section 14.8.

                       (c) WAIVER OF UNCURED EXCEPTIONS. If Seller does not
remove or agree to remove any Disapproved Exception within the Title Cure
Period, Buyer shall have until December 10, 2000 to give Seller notice that
Buyer waives its objections to such Exception. If Buyer does not give such
notice by such time limit, this Agreement shall terminate whereupon the Deposit
shall be returned forthwith to Buyer and, except as expressly set forth herein,
neither party shall have any further liability or obligation to the other
hereunder.

                       (d) APPROVED TITLE. The condition of title, as approved
by Buyer pursuant to clause (a) above and with any waivers by Buyer pursuant to
clause (c) above, is referred to herein as the "Approved Title."

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                       (e) LATER CHANGES TO CONDITION OF TITLE. Buyer shall have
the right to approve or disapprove any exceptions to title that are revealed by
the Survey or become of record after there is an Approved Title.

                 4.1.7. MATERIAL ADVERSE CHANGE. Between the expiration of the
Inspection Period and the date of Closing, there shall have been no material
adverse change in the financial or physical condition of the Property or the
Leased Premises, and no change to title or survey matters from the Approved
Title and the Approved Survey shall have arisen, which change would be
unacceptable to Buyer's mortgagee or to a reasonably prudent buyer.

                 4.1.8. POSSESSION. The Property and the Leased Premises shall
be delivered to the Buyer at the Closing free of any tenants or other occupants.

                 4.1.9. AMENDED LEASE, GUARANTY, AND NON-DISTURBANCE AGREEMENT.
Prior to December 1, 2000, (a) Buyer and Landlord shall have reached agreement
as to the form of lease and notice thereof which will amend, restate, and
supercede the Lease at the time of Closing ("Amended Lease"), and Buyer, Seller
and Landlord shall have reached agreement as to the form of guaranty to be
provided by Seller to Landlord with respect to the Amended Lease ("Guaranty"),
on terms and conditions satisfactory to Buyer in its sole discretion, and (b)
all mortgagees of the Leased Premises and any other party having approval rights
with respect thereto shall have approved the Amended Lease (including, without
limitation, the option to purchase set forth therein) and the transaction
contemplated by this Agreement, and shall have reached agreement with Buyer as
to a commercially reasonable Subordination, Nondisturbance and Attornment
Agreement (collectively, the "Non-Disturbance Agreement") with respect to the
Amended Lease to be provided by such mortgagees at the time of Closing. Seller
retains the right to act in its sole discretion in agreeing or not agreeing to
(a) the particular terms of the Guaranty which Seller is to provide, and (b) if
Seller is required to execute the Non-Disturbance Agreement, any terms of such
agreement which affect Seller.

                 Seller and Buyer shall share equally the cost of Landlord's
legal expenses in connection with the Amended Lease, the Guaranty, and the
Non-Disturbance Agreement, up to a maximum payment by each party of no more than
Ten Thousand ($10,000.00) Dollars.

                 4.1.10 PERMITTING CONTINGENCY. Buyer shall have obtained from
the Town of Randolph Zoning Board of Appeals, the Randolph Board of Health, and
any other governmental body having jurisdiction amendments of the existing
special permits or new special permits for each of the Property and the Leased
Premises, permits for the use of recombinant DNA, and such other permits as are
required by the Town of Randolph to permit the use of the Property and the
Leased Premises for Buyer's intended biotechnology research, manufacturing,
office and storage activities, on terms and conditions satisfactory to Buyer in
its sole discretion, and all appeal periods therefrom shall have expired without
appeal having been taken ("Permitting Contingency"). Buyer agrees to use
diligent efforts to pursue such relief, including the timely filing of
applications, and attendance at all hearings related thereto. Seller hereby
agrees to cooperate in such proceedings, and to attend such hearings in support
of Buyer's requested relief. Buyer shall furnish Seller with copies of its
applications and other filings, and shall keep Seller advised as to the progress
of such proceedings.

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            4.2 CONDITIONS PRECEDENT TO SELLER'S OBLIGATIONS. All of Seller's
obligations hereunder are expressly conditioned on the satisfaction at or before
the time of Closing hereunder, or at or before such earlier time as may be
expressly stated below, of each of the following conditions (any one or more of
which may be waived in writing in whole or in part by Seller, at Seller's
option):

                 4.2.1. ACCURACY OF REPRESENTATIONS. All of the representations
and warranties of Buyer contained in this Agreement shall have been true and
correct in all material respects when made, and shall be true and correct in all
material respects on the date of Closing with the same effect as if made on and
as of such date.

                 4.2.2. AMENDED LEASE AND NON-DISTURBANCE AGREEMENT.
Simultaneous with the Closing, Buyer and Landlord shall have entered into the
Amended Lease and Non-Disturbance Agreement (and the mortgagees shall have
executed same), and Seller and Landlord shall have entered into the Guaranty, in
forms to be mutually agreed in accordance with Section 4.1.9.

                 4.2.3. GUARANTY REIMBURSEMENT AGREEMENT. Prior to December 1,
2000, and in conjunction with Buyer, Seller and Landlord agreeing as to the form
of Amended Lease and Guaranty pursuant to Section 4.1.9 above, Buyer and Seller
shall have reached agreement as to a form of contract whereby Buyer agrees to
reimburse Seller for Seller's expenditures, if any, under the Guaranty (the
"Guaranty Reimbursement Agreement"). Also, at the Closing, Buyer and Seller
shall mutually execute and deliver such Guaranty Reimbursement Agreement.

                 4.2.4. PERFORMANCE BY BUYER. Buyer shall have performed,
observed and complied with all material covenants, agreements and conditions
required by this Agreement to be performed, observed and complied with on its
part prior to or as of Closing hereunder.

                 4.2.5. DOCUMENTS AND DELIVERIES. All instruments and documents
required on Buyer's part to effectuate this Agreement and the transactions
contemplated hereby shall be delivered to Seller and shall be in form and
substance consistent with the requirements herein.

         5. FAILURE OF CONDITIONS.

            5.1. CONDITIONS TO BUYER'S PERFORMANCE. (a) Except as otherwise
provided in Section 5.1(b) below or Section 10.2 hereof, in the event Seller
shall not be able to convey title to the Property and leasehold title to the
Leased Premises on the date of Closing in accordance with the provisions of this
Agreement, or if the conditions to Buyer's performance set forth in Article 4
have not been satisfied, then Buyer shall have the option, exercisable by
written notice to Seller at or prior to Closing, of (i) accepting at Closing
such title as Seller is able to convey and/or waiving any unsatisfied condition
precedent, with no deduction from or adjustment of the Purchase Price and
proceeding to Closing, or (ii) declining to proceed to Closing. In the latter
event, except as expressly set forth herein, all obligations, liabilities and
rights of the parties under this Agreement shall terminate, and the Deposit
shall be returned to Buyer. Such election to terminate this Agreement and have
the Deposit returned shall be Buyer's sole remedy for Seller's inability to
perform its obligations hereunder or the failure of any conditions to Closing

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to be satisfied. Seller shall not have any liability to Buyer for any such
inability to perform or failure of conditions, nor any obligation to cure any
defects or other matters which may cause such inability or failure or which may
otherwise exist or apply with respect to the Property or the Leased Premises,
except as expressly set forth in this Agreement to the contrary.

            (b) PERMITTING CONTINGENCY. In the event that the Permitting
Contingency has not been satisfied by March 1, 2001, then Buyer shall have the
option, exercisable by written notice to Seller at or prior to Closing, of (i)
accepting at Closing such title as Seller is able to convey and/or waiving any
unsatisfied condition precedent, including the Permitting Contingency, with no
deduction from or adjustment of the Purchase Price and proceeding to Closing, or
(ii) declining to proceed to Closing. In the latter event, except as expressly
set forth herein, all obligations, liabilities and rights of the parties under
this Agreement shall terminate, and the Deposit shall be returned to Buyer. Such
election to terminate this Agreement and have the Deposit returned shall be
Buyer's sole remedy for the failure of the Permitting Contingency to be
satisfied. Seller shall not have any liability to Buyer for any such failure of
the Permitting Contingency to be satisfied, provided that Seller has performed
its obligations under Section 4.2.5 above.

            5.2. CONDITIONS TO SELLER'S PERFORMANCE. In the event that the
conditions to Seller's performance set forth in Article 4 have not been
satisfied by the Closing, then Seller shall have the option, exercisable by
written notice to Buyer at or prior to Closing, to decline to proceed to
Closing. In such event, except as expressly set forth herein, all obligations,
liabilities and rights of the parties under this Agreement shall terminate, and
the Deposit shall be returned to Buyer.

         6. REPRESENTATIONS OF BUYER. Buyer represents and warrants that:

            6.1. AUTHORITY. Buyer is a Delaware corporation, duly organized and
validly existing under the laws of the Commonwealth of Massachusetts and has all
requisite power and authority to enter into this Agreement and to perform its
obligations hereunder. The execution and delivery of this Agreement by Buyer has
been duly authorized.

            6.2. NO CONFLICT. The execution and delivery of this Agreement and
the consummation of the transactions contemplated hereunder on the part of Buyer
do not and will not violate any applicable law, ordinance, statute, rule,
regulation, order, decree or judgment, conflict with or result in the breach of
any material terms or provisions of, or constitute a default under, or result in
the creation or imposition of any lien, charge, or encumbrance upon any of the
property or assets of the Buyer by reason of the terms of any contract,
mortgage, lien, lease, agreement, indenture, instrument or judgment to which
Buyer is a party or which is or purports to be binding upon Buyer or which
otherwise affects Buyer, which will not be discharged, assumed or released at
Closing. No action by any federal, state or municipal or other governmental
department, commission, board, bureau or instrumentality is necessary to make
this Agreement a valid instrument binding upon Buyer in accordance with its
terms.

         7. CLOSING; DELIVERIES.

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            7.1. TIME OF CLOSING. The Closing shall take place on a date
specified by written notice from Buyer to Seller but no later than the earlier
to occur of (a) fifteen (15) days after satisfaction of the Permitting
Contingency, or (b) March 31, 2001, at the offices of Hale and Dorr LLP, 60
State Street, Boston, Massachusetts.

            7.2. SELLER DELIVERIES. At Closing, Seller shall deliver to Buyer
the following, and it shall be a condition to Buyer's obligation to close that
Seller shall have delivered the same to Buyer:

                 7.2.1. A Massachusetts Quitclaim Deed to the Real Property from
Seller to Buyer, duly executed and acknowledged by Seller, together with
authority of the signatory thereto, in form satisfactory to Buyer and its title
insurer in their reasonable discretion, and subject only to such title matters
as are approved by Buyer pursuant to Section 4.1.6.

                 7.2.2. A Bill of Sale to the Property (other than the Real
Property) from Seller to Buyer, duly executed and acknowledged by Seller, in
form reasonably satisfactory to Buyer, conveying the property subject thereto
without encumbrance.

                 7.2.3 An Assignment and Assumption of Lease with respect to the
Lease in the form attached hereto as SCHEDULE 7.2.3, duly executed by Seller.

                 7.2.4. Copies of all architectural and engineering drawings and
specifications and utilities layout plans for the construction of the
Improvements and the Leased Premises which are in the possession of Seller.

                 7.2.5. Such affidavits or letters of indemnity as the title
insurer shall require, consistent with such documentation as is customarily
required by title insurance companies in connection with commercial real estate
closings in the Commonwealth of Massachusetts, in order to issue, without extra
charge, an owner's policy of title insurance for the Property and a leasehold
policy of title insurance for the Leased Premises, free of any exceptions for
unfiled mechanics' or materialmen's liens for work performed by Seller prior to
Closing and free of any exception for parties in possession.

                 7.2.6. A Non-Foreign Affidavit as required by the Foreign
Investors in Property Tax Act ("FIRPTA"), as amended, in the form of EXHIBIT B,
duly executed by Seller.

                 7.2.7. A certification by Seller that all representations and
warranties made by Seller in this Agreement are true and correct on the date of
Closing.

                 7.2.8 The Amended Lease and the Notice of Lease with respect
thereto, duly executed by Landlord, and the Guaranty, duly executed by Seller,
together with evidence of authority of the signatories thereto.

                 7.2.9 The Non-Disturbance Agreement, duly executed by Seller
(if required) and all mortgagees of the Leased Premises.

                 7.2.10 All other instruments and documents reasonably required
to effectuate this Agreement and the transactions contemplated thereby.

                                      -10-
<PAGE>

            7.3. BUYER DELIVERIES. At Closing, Buyer shall deliver to Seller the
following, and it shall be a condition to Seller's obligation to close that
Buyer shall have delivered the same to Seller:

                 7.3.1. In accordance with Seller's instructions, a wire
transfer in the amount required under Article 2 hereof (subject to the
adjustments provided for in this Agreement), transferred in immediately
available funds to the order or account of Seller or to such other person or
persons as Seller shall designate in writing.

                 7.3.2 An Assignment and Assumption of Lease with respect to the
Lease in the form attached hereto as SCHEDULE 7.2.3, duly executed by Buyer.

                 7.3.3. A certification by Buyer that all representations and
warranties made by Buyer in this Agreement are true and correct on the date of
Closing.

                 7.3.4. The Amended Lease and the Notice of Lease with respect
thereto, duly executed by Buyer, together with evidence of authority of the
signatory thereto.

                 7.3.5. The Non-Disturbance Agreement, duly executed by Buyer.

                 7.2.6 The Guaranty Reimbursement Agreement, duly executed by
Buyer.

                 7.3.7. All other instruments and documents reasonably required
to effectuate this Agreement and the transactions contemplated thereby.

         8. TAXES; UTILITIES CHARGES; EXPENSES.

            8.1. TAXES, UTILITIES CHARGES AND LEASE ADJUSTMENTS. All estate
taxes, charges and assessments affecting the Property ("Taxes"), all charges for
water, electricity, sewer rental, gas, telephone and all other utilities
("Utilities Charges"), and all rents and other amounts due under the Lease
("Lease Adjustments") shall be prorated on a per diem basis as of the date of
Closing. If any Taxes have not been finally assessed as of the date of Closing
for the current fiscal year of the taxing authority, then the same shall be
adjusted at Closing based upon the most recently issued bills therefor, and
shall be re-adjusted when and if final bills are issued. If any Utilities
Charges or Lease Adjustments cannot conclusively be determined as of the date of
Closing, then the same shall be adjusted at Closing based upon the most recently
issued bills thus far and shall be re-adjusted within 180 days after the end of
the calendar year in which the Closing occurs after final Utilities Charges and
Lease Adjustments are determined.

            8.2. EXPENSES. Each party will pay all its own expenses incurred in
connection with this Agreement and the transactions contemplated hereby,
including, without limitation, (1) all costs and expenses stated herein to be
borne by a party, and (2) all of their respective accounting, legal and
appraisal fees. Buyer, in addition to its other expenses, shall pay at Closing
all recording charges incident to the recording of the deed for the Property,
and the Notice of Lease and Non-Disturbance Agreement with respect to the
Amended Lease, and premiums for Buyer's title insurance policy and survey.
Seller, in addition to its other expenses, shall pay at Closing all documentary
stamps, deed stamps and realty transfer taxes.

                                      -11-
<PAGE>

         9. DAMAGE OR DESTRUCTION; CONDEMNATION; INSURANCE.

            9.1 If at any time prior to the Closing, a fire or other casualty
results in damage to the Property or to the Leased Premises having a restoration
cost in excess of $50,000, and cannot be restored to its original condition
prior to Closing, or if all or any portion of the Property or the Leased
Premises is condemned or taken by eminent domain, then, at Buyer's option, this
Agreement shall terminate, and the Deposit shall be returned to Buyer, and,
except as expressly set forth herein, neither party shall have any further
obligation or liability to the other hereunder.

            9.2 If there is any damage or destruction or condemnation or taking,
as above set forth, and if Buyer elects not to terminate this Agreement as
therein provided, or if such casualty results in a restoration cost less than
$50,000, then (1) in the case of a taking, all condemnation proceeds paid or
payable to Seller shall belong to Buyer and shall be paid over and assigned to
Buyer at Closing, and (2) in the case of a casualty, Seller shall assign to
Buyer at the Closing all rights to any insurance proceeds paid or payable under
the applicable insurance policies, and shall pay over to Buyer or deduct from
the Purchase Price any deductible thereunder, in either event without adjustment
to the Purchase Price (except for the deductible as aforesaid). Seller shall
maintain its existing insurance in effect during the term of this Agreement, a
copy of which Seller shall furnish to Buyer.

         10. REMEDIES.

             10.1. BUYER DEFAULT. In the event Buyer breaches its obligation to
complete the purchase of the Property and assignment of the Lease, or to perform
any of its other obligations under this Agreement, then Seller shall, as its
sole remedy therefor, be entitled to receive the Deposit, plus all interest
earned and accrued thereon, as liquidated damages (and not as a penalty) in lieu
of, and as full compensation for, all other rights or claims of Seller against
Buyer by reason of such default. Thereupon this Agreement shall terminate and
the parties shall be relieved of all further obligations and liabilities
hereunder, except as expressly set forth herein. Buyer and Seller acknowledge
that the damages to Seller resulting from Buyer's breach would be difficult, if
not impossible, to ascertain with any accuracy, and that the liquidated damage
amount set forth in this Section represents both parties' best efforts to
approximate such potential damages.

            10.2. SELLER DEFAULT. In the event Seller breaches its obligation to
complete the sale of the Property and the assignment of the Lease, or to perform
any of its other obligations under this Agreement, Buyer may, as its sole remedy
therefor, either (i) enforce specific performance of this Agreement against
Seller, or (ii) terminate this Agreement, receive a return of the Deposit, and
recover from Seller all out-of-pocket expenditures reasonably and actually
incurred by Buyer in connection with this Agreement, up to a maximum of
Seventy-Five Thousand ($75,000.00) Dollars.

         11. NOTICES. All notices and other communications provided for herein
shall be in writing and shall be sent to the address set forth below (or such
other address as a party may hereafter designate for itself by notice to the
other parties as required hereby) of the party for whom such notice or
communication is intended:

                                      -12-
<PAGE>

            11.1. If to Seller:

                  Serono Laboratories, Inc.
                  100 Longwater Circle
                  Norwell, MA 02061
                  Fax No.  -
                  Attention: A. Peter Frank, Esq., General Counsel

                  With a copy to:

                  William A. Kuncik, P.C., Esq.
                  Nixon Peabody LLP
                  101 Federal Street
                  Boston, MA 02110

            11.2. If to Buyer:

                  Transkaryotic Therapies, Inc.
                  195 Albany Street
                  Cambridge, MA 02139
                  Fax No.: 617-491-7903
                  Attention: Mr. Daniel Geffken, Vice President and
                             Chief Financial Officer

                  With a copy to:

                  Hale and Dorr LLP
                  60 State Street
                  Boston, Massachusetts 02109
                  Fax No.: 617-526-5000
                  Attention: Katharine E. Bachman, Esq.

            11.3 If to the Escrow Agent to:

                 Stewart Title Insurance Company
                 99 Summer Street
                 Boston, MA 02110
                 Fax No.: 617-737-8370
                 Attention: Marie L. Franco, Esq.

Any such notice or communication shall be deemed given when deposited with the
U.S. Mail, registered or certified mail, return receipt requested, postage
prepaid, or when deposited with a recognized national overnight courier service.

         12. BROKERS. Buyer and Seller each represent to the other that it has
not dealt with any broker or agent in connection with this transaction. Each
party hereby indemnifies and holds harmless the other party from all loss, cost
and expense (including reasonable attorneys' fees) arising out of a breach of
its representation or undertaking set forth in this Section 12. The provisions
of this Section 12 shall survive Closing or the termination of this Agreement.

                                      -13-
<PAGE>

         13. ESCROW AGENT. Escrow Agent shall hold the Deposit in accordance
with the terms and provisions of this Agreement, subject to the following:

             13.1. OBLIGATIONS. Escrow Agent undertakes to perform only such
duties as are expressly set forth in this Agreement or any subsequent written
instructions duly executed by Seller and Buyer, and no implied duties or
obligations shall be read into this Agreement against Escrow Agent.

             13.2. RELIANCE. Escrow Agent may act in reliance upon any writing
or instrument or signature which it, in good faith, believes is genuine, valid
or authentic, and any statement or assertion contained in such writing or
instrument, and may assume that any person purporting to give any writing,
notice, advice or instrument in connection with the provisions of this Agreement
has been duly authorized to do so. Escrow Agent shall not be liable in any
manner for the sufficiency or correctness as to form, manner and execution, or
validity of any instrument deposited in escrow, nor as to the identity,
authority, or right of any person executing the same, and Escrow Agent's duties
under this Agreement shall be limited to those provided in this Agreement.

             13.3. INDEMNIFICATION. Unless Escrow Agent discharges any of its
duties under this Agreement in a negligent manner or is guilty of willful
misconduct with regard to its duties under this Agreement, Seller and Buyer
shall indemnify Escrow Agent and hold it harmless from any and all claims,
liabilities, losses, actions, suits or proceedings at law or in equity, or other
expenses, fees, or charges of any character or nature, which it may incur or
with which it may be threatened by reason of its acting as Escrow Agent under
this Agreement; and in such connection Seller and Buyer shall indemnify Escrow
Agent against any and all expenses including reasonable attorneys' fees and the
cost of defending any action, suit or proceeding or resisting any claim in such
capacity. The respective obligations of Seller and Buyer under this Section 13.3
shall be apportioned equally between Seller and Buyer, i.e., fifty percent (50%)
for each of Seller and Buyer.

             13.4. DISPUTES. If the parties (including Escrow Agent) shall be in
disagreement about the interpretation of this Agreement, or about their
respective rights and obligations, or the propriety of any action contemplated
by Escrow Agent, or the application of the Deposit, Escrow Agent shall hold the
Deposit until the receipt of written instructions from and signed by both Buyer
or Seller, or a final order of a court of competent jurisdiction. In addition,
in any such event, Escrow Agent may, but shall not be required to, file an
action in interpleader to resolve the disagreement. Escrow Agent shall be
indemnified for all costs and reasonable attorneys' fees in its capacity as
Escrow Agent in connection with any such interpleader action and shall be fully
protected in suspending all or part of its activities under this Agreement until
a final judgment in the interpleader action or a settlement agreement duly
executed by all necessary parties is received.

             13.5. COUNSEL. Escrow Agent may consult with counsel of its own
choice and have full and complete authorization and protection in accordance
with the opinion of such counsel. Escrow Agent shall otherwise not be liable for
any mistakes of fact or errors of judgment, or for any acts or omissions of any
kind, unless caused by its negligence or willful misconduct.

                                      -14-
<PAGE>

         14. MISCELLANEOUS.

             14.1. ASSIGNABILITY. Buyer may not assign or transfer all or any
portion of its rights or obligations under this Agreement to any other
individual, entity or other person without the prior written consent thereto by
Seller, which consent shall be in Seller's sole discretion, provided that Buyer
may assign or transfer such rights and obligations to any entity in which Buyer
or an affiliate thereof has a controlling interest without the consent of
Seller, provided that any such assignee or transferee of Buyer's obligations
duly executes an assumption agreement reasonably satisfactory to Seller.

             14.2. GOVERNING LAW; BIND AND INURE. This Agreement shall be
governed by the law of the Commonwealth of Massachusetts and shall be binding
upon and inure to the benefit of the parties hereto and their respective heirs,
executors, administrators, successors, assigns and personal representatives.

             14.3. TIME OF THE ESSENCE. Time is of the essence of this
Agreement.

             14.4. HEADINGS. The headings preceding the text of the paragraphs
and subparagraphs hereof are inserted solely for convenience of reference and
shall not constitute a part of this Agreement, nor shall they affect its
meaning, construction or effect.

             14.5. COUNTERPARTS. This Agreement may be executed simultaneously
in counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

             14.6. EXHIBITS. All Exhibits which are referred to herein and which
are attached hereto or bound separately and initialed by the parties are
expressly made and constitute a part of this Agreement.

             14.7. SURVIVAL; LIMITATION OF RECOVERY. Each of the warranties and
representations of Seller set forth in Article 3 and Buyer set forth in Article
6 shall survive the Closing and delivery of the deed and other closing documents
by Seller to Buyer, and shall not be deemed to have merged therewith; provided,
however, that (a) any suit or action for breach of any of the representations or
warranties set forth herein must be commenced within one (1) year after the
Closing or any claim based thereon shall be deemed irrevocably waived, and (b)
the maximum amount which either party may recover hereunder shall be Five
Hundred Thousand ($500,000.00) Dollars. The parties' indemnities under Sections
4.4.4.1 and 12 shall also survive the Closing. The delivery and acceptance of
the deed and other closing documents hereunder shall be deemed a full
performance and discharge of all of the parties' other agreements, obligations
and covenants hereunder.

             14.8. USE OF PROCEEDS TO CLEAR TITLE. To enable Seller to make
conveyance as herein provided, Seller may, at the time of Closing, use the
Purchase Price or any portion thereof to clear the title of any or all
encumbrances or interests, provided that provision reasonably satisfactory to
Buyer's attorney is made for prompt recording of all instruments so procured in
accordance with conveyancing practice in the Commonwealth of Massachusetts.

                                      -15-
<PAGE>

             14.9. SUBMISSION NOT AN OFFER OR OPTION. The submission of this
Agreement or a summary of some or all of its provisions for examination or
negotiation by Buyer or Seller does not constitute an offer by Seller or Buyer
to enter into an agreement to sell or purchase the Property, and neither party
shall be bound to the other with respect to any such purchase and sale until a
definitive agreement satisfactory to the Buyer and Seller in their sole
discretion is executed and delivered by both Seller and Buyer.

             14.10. ENTIRE AGREEMENT; AMENDMENTS. This Agreement and the
Exhibits hereto set forth all of the promises, covenants, agreements, conditions
and undertakings between the parties hereto with respect to the subject matter
hereof, and supersede all prior and contemporaneous agreements and
understandings, inducements or conditions, express or implied, oral or written,
except as contained herein. This Agreement may not be changed orally but only by
an agreement in writing, duly executed by or on behalf of the party or parties
against whom enforcement of any waiver, change, modification, consent or
discharge is sought.

             14.11 STANDSTILL AGREEMENT. Seller hereby covenants and agrees that
during the time that this Agreement is in effect, Seller will not: (i) market,
advertise or otherwise offer, solicit offers, or accept offers to sell the
Property or assign, sublet or otherwise transfer its interest under the Lease;
(ii) sell the Property or any portion thereof, or assign, sublet or otherwise
transfer its interest under the Lease, to any third party; or (iii) voluntarily
encumber the Property or its interest under the Lease in any way. Each party
hereby agrees not to issue any press releases or other statements to the public,
other than as necessary to satisfy the Permitting Contingency, with respect to
the transaction which is the subject of this Agreement until the Closing has
been consummated.

                            (signatures on next page)

                                      -16-
<PAGE>

         IN WITNESS WHEREOF, the parties have executed and delivered this
Agreement as of the date first above written.

SELLER:                             SERONO, INC., a
                                    Delaware corporation

                                    By: /s/ Peter Frank
                                        ---------------------------------------
                                        Name:  Peter Frank
                                        Title: Vice-President

BUYER:                             TRANSKARYOTIC THERAPIES, INC. a
                                   Delaware corporation

                                   By: /s/ Daniel E. Geffken
                                       ----------------------------------------
                                       Name:  Daniel E. Geffken
                                       Title: Vice President Finance and
                                              Chief Financial Officer

ESCROW AGENT:                      STEWART TITLE INSURANCE COMPANY

                                   By: /s/ Marie L. Franco
                                       ----------------------------------------
                                       Name:  Marie L. Franco
                                       Title: National Underwriting Counsel

                                      -17-
<PAGE>

                         LIST OF EXHIBITS AND SCHEDULES

Exhibit A                           Description of Land

Exhibit A-1                         Inventory of Personal Property, Fixtures,
                                    and Equipment

Exhibit B                           FIRPTA Affidavit

Schedule 3.3                        Lease

Schedule 3.9                        Environmental Information

Schedule 4.1.6                      Title Report and Exceptions

Schedule 7.2.3                      Form of Assignment and Assumption of Lease

                                      -18-
<PAGE>

                                    EXHIBIT A

                             DESCRIPTION OF THE LAND

                               (Follows this Page)

                                      -19-
<PAGE>

                                   EXHIBIT A-1

             INVENTORY OF PERSONAL PROPERTY, FIXTURES, AND EQUIPMENT
                                    107250-145

                                      -20-
<PAGE>

                                    EXHIBIT B

                                FIRPTA AFFIDAVIT

                               (Follows this Page)

                                      -21-
<PAGE>

                              NON-FOREIGN AFFIDAVIT

Section 1445 of the Internal Revenue Code provides that a transferee of a U.S.
real property interest must withhold tax if the transferor is a foreign person.
To inform the transferee that withholding of tax is not required upon the
disposition of a U.S. real property interest by _______________________________
("Seller"), the undersigned hereby certifies the following:

         1.       Seller is not a foreign person, foreign corporation, foreign
                  partnership, foreign trust, or foreign estate (as those terms
                  are defined in the Internal Revenue Code and Income Tax
                  Regulations);

         2.       Seller's U.S. taxpayer identification number is [___________];
                  and

         3.       Seller's address is ________________________________.

The undersigned understands that this certification may be disclosed to the
Internal Revenue Service by the transferee and that any false statement
contained herein could be punished by fine, imprisonment, or both. Under
penalties of perjury, the undersigned declares that it has examined this
certification and to the best of its knowledge and belief it is true, correct,
and complete, and further declares that it has authority to sign this document.

Date:  As of _____________, ____

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

                                      -22-
<PAGE>

                                  SCHEDULE 3.3

                                      Lease

                                  See attached

                                      -23-
<PAGE>

                                  SCHEDULE 3.9

                            Environmental Information

1. Environmental Evaluation dated September, 1992, prepared by Zecco, Inc. for
Seller

2. On or about October 23, 2000, Buyer was informed by its environmental
consultant that analytical results obtained from a surficial soil sample
collected beneath a thirty-gallon drum located in the wooded area in the rear of
the Premises indicated a concentration of Extractable Petroleum Hydrocarbons
("EPH") of 240 mg/kg. A copy of the communication to Buyer is attached hereto.
The Massachusetts Contingency Plan, 30 CMR 40.000 et seq., requires reporting to
the Massachusetts Department of Environmental Protection of EPH concentrations
in S1 soils which exceed 200 mg/kg unless such concentrations are timely
addressed by a Limited Removal Action, as described at 310 CMR 40.0318.

         In November, 2000 Seller had a Limited Removal Action performed in
accordance with 310 CMR 40.0318, since less than 100 cubic yards of the
above-described soil were required to be removed. This soil was removed,
transported and disposed of in accordance with applicable law. Seller hereby
agrees to provide Buyer with such documentation of the foregoing Limited Removal
Action as Buyer may reasonably request.

                                      -24-
<PAGE>

                                 SCHEDULE 4.1.6

                           Title Report and Exceptions

                                  See attached

                                      -25-
<PAGE>

                                 SCHEDULE 7.2.3

                   Form of Assignment and Assumption of Lease

                                  See attached

                                      -26-
<PAGE>

                       ASSIGNMENT AND ASSUMPTION OF LEASE

         THIS ASSIGNMENT AND ASSUMPTION OF LEASE (this "Assignment") is entered
into effective as of the [_______] day of [_______________] [_______], between
Serono, Inc., a Delaware corporation ("Assignor"), whose address is 100
Longwater Circle, Norwell, Massachusetts 02061, and Transkaryotic Therapies,
Inc., a Delaware corporation ("Assignee"), whose address is 195 Albany Street,
Cambridge, Massachusetts 02139.

         1.       REFERENCE TO PURCHASE AND SALE AND ASSIGNMENT AGREEMENT.
                  Reference is made to a Purchase and Sale and Assignment
                  Agreement dated November ___, 2000 (the "Purchase Agreement")
                  between Assignor, as seller, and Assignee, as buyer, pursuant
                  to which, inter alia, Assignor has agreed to assign to
                  Assignee, and Assignee has agreed to assume from Assignor,
                  Assignor's interest in the lease described in Exhibit A
                  attached hereto (the "Lease"). Capitalized terms used herein
                  and not otherwise defined herein shall have the meanings set
                  forth in the Purchase Agreement.

         2.       ASSIGNMENT. For good and valuable consideration received by
                  Assignor, the receipt and sufficiency of which are hereby
                  acknowledged, Assignor hereby grants, transfers and assigns to
                  Assignee all right, title and interest of Assignor as tenant
                  in, to and under the Lease, together with all security
                  deposits thereunder (the "Security Deposit").

         3.       ASSUMPTION. Assignee hereby assumes, and agrees to be bound
                  by, all of the covenants, agreements and obligations of
                  Assignor as tenant under the Lease, that shall arise or be
                  incurred, or that are required to be performed, on and after
                  the date of this Assignment.

         4.       INDEMNITY. Assignee shall indemnify, defend and hold harmless
                  Assignor from any loss, cost, claim, liability, expense
                  (including reasonable attorney's fees) or demand of whatever
                  nature under the Lease arising or accruing on or after the
                  date hereof. Assignor shall indemnify, defend and hold
                  harmless Assignee from any loss, cost, claim, liability,
                  expense (including reasonable attorney's fees) or demand of
                  whatever nature under the Lease arising or accruing before the
                  date hereof.

         5.       BINDING EFFECT. This Assignment shall be binding upon and
                  inure to the benefit of Assignor and Assignee and their
                  respective heirs, executors, administrators, successors and
                  assigns.

                                      -27-
<PAGE>

         IN WITNESS WHEREOF, Assignor and Assignee have executed this Assignment
as of the day and year first above written.

                                         SELLER:

                                         SERONO, INC.

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

                                         BUYER:

                                         TRANSKARYOTIC THERAPIES, INC.

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

                                      -28-
<PAGE>

                                    EXHIBIT A

                                      -29-<PAGE>

                                                                 Exhibit 10.28

          FIRST AMENDMENT TO PURCHASE AND SALE AND ASSIGNMENT AGREEMENT

         This FIRST AMENDMENT TO PURCHASE AND SALE AND ASSIGNMENT AGREEMENT
(this "Amendment") is dated as of February 8, 2001 by and between SERONO, INC.,
a Delaware corporation, as seller (the "Seller"), and Transkaryotic Therapies,
Inc., a Delaware corporation, as buyer (the "Buyer").

                                    RECITALS

         WHEREAS, Buyer and Seller are parties to that certain Purchase and Sale
and Assignment Agreement dated as of November 28, 2000, as modified by letter
agreements dated November 30, 2000, January 12, 2001, and January 31, 2001 (as
so amended, the "Agreement") for the lease of 76 Pacella Park Drive and the
conveyance of 78-80 Pacella Park Drive, Randolph, Massachusetts; and

         WHEREAS, Buyer and Seller desire to amend the Agreement as set forth
herein.

                                    AGREEMENT

         NOW, THEREFORE, in consideration of Ten Dollars ($10.00), and for other
good and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged and agreed, Buyer and Seller hereby agree as follows:

         1. CAPITALIZED TERMS. Capitalized terms used herein and not otherwise
defined shall have the meaning set forth in the Agreement.

         2. PURCHASE PRICE. Section 2.0 of the Agreement is hereby amended by
reducing the Purchase Price from Nine Million and 00/100 ($9,000,000.00) Dollars
to Eight Million Eight Hundred Thousand and 00/100 (8,800,000.00) Dollars.

         3. CLOSING. The parties hereby agree that the Closing under Section 7.1
of the Agreement shall occur at the offices of Hale and Dorr LLP, 60 State
Street, Boston, Massachusetts 02109 at 10:30 a.m. on February 8, 2001, and that
the recording and funding for the Closing shall be implemented on February 9,
2001 through an escrow with Stewart Title Guaranty Company pursuant to the
escrow provisions of the Agreement and an Escrow Agreement of even date
herewith.

         4. PERSONAL PROPERTY, FIXTURES AND EQUIPMENT. Seller hereby covenants
and agrees with Buyer that Seller shall deliver to Buyer at the Closing (or, to
the extent not feasible to do so at the Closing within thirty (30) days
thereafter) any documentation in Seller's possession for the major items of
personal property, fixtures, and equipment to be conveyed by Seller to Buyer
under the Agreement, including without limitation, CAD drawings, validation
documentation, manuals, warranties, and related materials, if and to the extent
that Seller has any of the foregoing. Seller further covenants and agrees to
make its personnel reasonably available to Buyer at Seller's expense for a
reasonable period following the Closing to facilitate the transfer

<PAGE>

to Buyer of regulatory and operating responsibility for such personal property,
fixtures and equipment.

         5. PH NEUTRALIZATION AND STEAM EQUIPMENT. Buyer shall hereafter be
solely responsible for the so-called "ph neutralization and steam equipment" in
the 76 Pacella property which connects into the 78-80 Pacella property,
including removal of such equipment at the expiration or earlier termination of
Buyer's new lease of the 76 Pacella property from Trinet Property Partners, L.P.
and any necessary repair, replacement and restoration work in the 76 Pacella
property in connection with such removal. Buyer's assumption of the foregoing
responsibility is the consideration for the reduction in the Purchase Price set
forth in Section 2 of this Amendment.

         6. MISCELLANEOUS. Except as amended hereby, the Agreement shall
continue in full force and effect. This Amendment may be executed in counterpart
facsimiles which, taken together, shall constitute a single effective
instrument.

                                     SELLER:

                                     SERONO, INC.

                                     By: /s/ A. Peter Frank
                                         --------------------------------------
                                         A. Peter Frank, its Vice President and
                                         General Counsel

                                     By:/s/ Monica Elliott
                                         --------------------------------------
                                         Monica Elliot, its Treasurer

                                     BUYER:

                                     TRANSKARYOTIC THERAPIES, INC.

                                     By: /s/ Daniel E. Geffken
                                         --------------------------------------
                                         Daniel E. Geffken, its Vice President,
                                         Finance and Chief Financial Officer

                                      -2-

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