Document:

Director Grantor Trust Agrmt Amendment 2002-1

                                          DIRECTOR GRANTOR TRUST AGREEMENT

                                                  AMENDMENT 2002-1
                                               Effective May 14, 2002

The Edison International Director Grantor Trust agreement is amended as follows:

To delete the following grantors: Edison EV, Edison Source, Edison Enterprises, Mission Land Company, The Mission
Group, and Edison Technology Solutions.

To add Section 3.02-5 to read as follows:
3.02-5  Prior to a Special  Circumstance,  the Trust  Committee  may direct the  Trustee in writing to  reimburse  a
Grantor for any benefit  payments  made directly by the Grantor to  Participants  in  accordance  with 3.02-1.  Such
reimbursement  directions will include a schedule showing the names of the Participants  paid, the amounts paid, and
the dates of payment and must be received by the Trustee within one year of payment.

Edison International                                    U.S. Trust of California N.A.

By:         [Beverly P. Ryder]                          By:      [Terry J. Colberg]
    ---------------------------------                       -----------------------
Title: Vice President and Secretary                     Title: [Vice President]

Southern California Edison Company                      Edison EV

By:         [Beverly P. Ryder]                          By:      [Kenneth S. Stewart]
    ---------------------------------                       -------------------------
Title: Secretary                                        Title: President

Edison Technology Solutions                             Edison Capital
                                                        Edison Enterprises
                                                        Edison Source
By:      [Kenneth S. Stewart]                           Edison Mission Energy
    ------------------------------
Title: President                                        Mission Land Company
                                                        The Mission Group

                                                        By:      [Kenneth S. Stewart]
                                                            -------------------------
                                                        Title: Attorney-in-Fact06302002 Form 10-Q Exhbit 10.21

                                            Exhibit 10.21

NOTE:  Information in this document marked with an "[*]" has been omitted and filed separately
with the Commission.  Confidential treatment has been requested with respect to the omitted
portions.

LICENSE AGREEMENT

between

TRANSKARYOTIC THERAPIES, INC.

and

CELL GENESYS, INC.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

LICENSE AGREEMENT

This License Agreement (the
"Agreement"), effective as of June 7, 2002 (the "Effective
Date"), is between Transkaryotic Therapies, Inc., a corporation
organized and existing under the laws of Delaware and having its principal place
of business at 195 Albany Street, Cambridge, MA 02139 ("TKT"),
and Cell Genesys, Inc., a corporation organized and existing under the
laws of Delaware and having its principal place of business at 342 Lakeside
Drive, Foster City, CA 94404 ("Cell Genesys").  TKT and CELL
GENESYS are sometimes hereinafter referred to each as a "Party" and
collectively as the "Parties."

WITNESSETH:

Whereas, CELL
GENESYS has certain Patent Rights  (as
hereinafter defined);

Whereas,
TKT desires to obtain a license under the
CELL GENESYS Patent Rights upon the terms and conditions set forth herein, and
CELL GENESYS desires to grant such a license;

Now,
Therefore, in consideration of the foregoing premises and the mutual
covenants herein contained, and intending to be legally bound, the Parties
hereby agree as follows:

ARTICLE I

DEFINITIONS

Unless specifically set forth to the contrary herein, the
following terms, whether used in the singular or plural, shall have the
respective meanings set forth below:

1.1 "Affiliate" shall mean, with respect to
a particular Party, a person, corporation, partnership, or other entity that
controls, is controlled by or is under common control with such Party.  For the
purposes of the definition in this Section 1.1, the word
"control" (including, with correlative meaning, the terms
"controlled by" or "under the common control with") means
the actual power, either directly or indirectly through one or more
intermediaries, to direct or cause the direction of the management and policies
of such entity, whether by the ownership of at least fifty percent (50%) of the
voting stock of such entity, or by contract or otherwise. 

1.2 "Designated Protein" shall mean one of
the [*], as described in or determined by the provisions set forth in the letter
dated June 7, 2002 from Robert H. Tidwell of CELL GENESYS to Michael J. Astrue
of TKT (the "Side Letter"), and subject to Sections 2.2 and 2.3
hereof.

1.3  [*]  

                          2.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

1.4 "Licensed Product(s)" shall mean [*]
Licensed Products and [*] Licensed Products.  "[*] Licensed Product"
shall mean [*] of a Designated Protein, the making, using, selling, offering for
sale or import of which is covered by a claim in the Patent Rights.  "[*]
Licensed Product" shall mean [*], the making, using, selling, offering for
sale or import of which is covered by a claim in the Patent Rights.  For
clarity, the term "Licensed Product(s)" does not include a composition
that includes a protein that is covered by, or that was made by a process
covered by, a claim in the Patent Rights, if that protein is not a Designated
Protein. 

1.5 "Patent Rights" shall mean: (a) all
patents and patent applications (including provisional and utility applications)
owned by CELL GENESYS, or licensed by CELL GENESYS with the right to sublicense,
as of the Effective Date that claim [*], including, but not limited to: [*]; and
also any new patent or patent application (i.e., not a continuing
application) filed after the Effective Date that is owned by CELL GENESYS and
[*] (b) divisions, continuations, continuations-in-part (but solely as to claims
that encompass subject matter disclosed in the parent application), extensions,
supplementary protection certificates, utility models, reexaminations, reissues
and renewals of the patents and patent applications in clause (a) above,; and
(c) foreign equivalents of the patents and applications in clauses (a) and (b)
above.  

1.6 "Third Party" shall mean a party other
than TKT, CELL GENESYS, or their respective Affiliates.

ARTICLE II

LICENSE GRANT

2.1 License Grant. Subject to the terms and conditions of
this Agreement, CELL GENESYS grants to TKT (i) an [*] under the Patent Rights to
[*], and (ii) a [*] under the Patent Rights to [*] solely for use to [*].

2.2 Identification of Designated Proteins. TKT shall
designate in writing each additional protein that TKT desires to designate as a
Designated Protein pursuant to the Side Letter (up to the maximum number and
within the timeframes set forth therein), but subject to the following.  Until
such time as TKT's rights to designate additional Designated Proteins as
provided in this Agreement have terminated, CELL GENESYS shall [*] of any
protein.

2.3 Substitution of Designated Proteins.
Notwithstanding Section 1.2, Section 2.2 and the Side Letter, if, on the
[*] of the Effective Date, CELL GENESYS has [*] then TKT shall have the right to
designate an [*]: (i) a method or composition for, or including, the activation
of a gene that is [*], (ii) a composition or a gene activation method that does
[*] and (iii) a method or composition for or including gene activation which
does [*].  Such additional [*] shall be identified on or before [*].

ARTICLE III

CONFIDENTIALITY/PRESS RELEASES 

                          3.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

3.1 Nondisclosure Obligation. All proprietary or
confidential information disclosed by or on behalf of one Party (the
"Disclosing Party") to the other Party (the "Receiving
Party") hereunder in writing and marked "Confidential" or the
equivalent (the "Proprietary Information") shall be maintained in
confidence by the Receiving Party and shall not be disclosed to a Third Party or
used for any purpose whatsoever except as set forth herein without the prior
written consent of the Disclosing Party.  All information disclosed by CELL
GENESYS or TKT pursuant to the Binding Letter of Intent dated April 21, 2002
(the "LOI") shall be deemed Proprietary Information of CELL GENESYS or
TKT, as the case may be, and shall be subject to the terms of this Article 3.
For purposes of clarity, it is the intent of the Parties that general financial
terms may be disclosed but that the number of Licensed Products and the nature
and category of proteins which may be designated under this Agreement , as well
as the nature of the milestones herein shall be considered Proprietary
Information of both Parties.  The identity of the Licensed Products and
Designated Proteins shall be TKT Proprietary Information.  However, the
foregoing obligations shall not apply to particular Proprietary Information
solely to the extent that the receiving Party can demonstrate with written
evidence that such information:  

	 is known by the Receiving Party at the time of its
receipt, and not through a prior confidential disclosure by or on behalf of the
Disclosing Party;

	 is properly in the public domain through no fault of the
Receiving Party;

	 is subsequently disclosed to the receiving Party by a
Third Party who may lawfully do so and is not directly or indirectly under an
obligation of confidentiality to the Disclosing Party; or

	 is developed by the Receiving Party independently of, and
without reference to or use of, Proprietary Information received from the
Disclosing Party.

3.2 Authorized Disclosures. A Party may disclose the
Proprietary Information of the other Party to the extent such disclosure:

	 is required to be disclosed by any governmental or other
regulatory agency in order to obtain patents, to obtain approval to conduct
clinical trials or to market a Licensed Product; provided, however, that
(i) prior written notice is promptly delivered to the disclosing Party in order
to provide an opportunity to challenge or limit the disclosure obligations, (ii)
such disclosure may be only to the extent necessary to obtain such patents or
approval, or to comply with regulations as appropriate and (iii) confidential
treatment shall be sought to the extent reasonably practicable; 

	 is necessary to be disclosed to employees, agents,
consultants and/or other Third Parties for purposes related to the development
and commercialization of Licensed Products (or for such Third Parties to
determine their interest in performing such activities) in accordance with this
Agreement on the condition that such Third Parties agree to be bound by
confidentiality obligations at least as restrictive as the terms herein;
or

                          4.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	 is required to be disclosed by law or court order,
provided that prior written notice is promptly delivered to the
disclosing Party in order to provide an opportunity to challenge or limit the
disclosure obligations, and provided further that such disclosure may be
only to the extent reasonably necessary to comply with the applicable law or
court order.

3.3 Press Releases. Neither Party shall issue a press
release relating to this Agreement without the prior review and written consent
of the other Party.

3.4 Confidential Treatment. In the event that either
Party is required to include this Agreement and any agreements or letters
referred to herein in any report, statement or other document filed by such
Party with the United States Securities and Exchange Commission (the
"SEC"), then the disclosing Party shall [*] to obtain, to the maximum
extent permitted by law, confidential treatment from the SEC of any trade
secrets and commercial or financial information of a privileged or confidential
nature, including without limitation all information relating to the number or
identity of the Designated Proteins under this Agreement and the specific
financial terms and the nature of the milestones herein, and shall notify the
other Party as to such efforts and all related communications with the SEC.
Notwithstanding the foregoing, neither Party shall submit a confidentiality
request for this Agreement or the agreements or letters referred to herein
without providing the other Party with an opportunity for prior review and
comment on such confidentiality request.  The filing Party shall reasonably
consider and attempt to accommodate any such comments by such other Party in
submitting such confidentiality request.  

ARTICLE IV

PAYMENTS

4.1 Upfront Fees. In consideration for the license
granted to TKT by CELL GENESYS under Section 2.1 and CELL GENESYS's fulfillment
of its other obligations and undertakings under this Agreement, within [*] of
the Effective Date, TKT shall make a cash payment to CELL GENESYS of Eleven
Million U.S. Dollars (US $11,000,000), less the credit provided for in Section
4.2, and shall issue and transfer to CELL GENESYS three hundred sixty six
thousand nine hundred twenty eight (366,928) shares of TKT common stock (the
"Shares") pursuant to an executed Stock Purchase Agreement in the form
attached hereto as Exhibit A (the "Stock Purchase
Agreement").

4.2 Cash Payment Credit. The cash payment of Three
Million U.S. Dollars (US $3,000,000) paid by TKT pursuant to the LOI between the
Parties effective April 21, 2002 shall be credited towards the upfront cash fee
of Eleven Million U.S. Dollars (US $11,000,000), thereby reducing the payment
required pursuant to Section 4.1 to Eight Million U.S. Dollars (US $8,000,000).

4.3 Milestone Payments. In further consideration for
the license granted to TKT by CELL GENESYS under Section 2.1 and CELL GENESYS's
fulfillment of its other obligations and undertakings under this Agreement, TKT
shall make cash and stock payments to CELL GENESYS in the respective amounts set
forth below upon attainment of the respective milestone events set forth
below:

                          5.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	
EVENT
	AMOUNT

	
(a)  [*].

	[*].

	
(b)  [*].

	[*].   

	
(c)  [*].

	[*].  

4.4 Milestone Conditions. No milestone shall be
payable more than once by TKT. Further, milestones 4.3(b) and 4.3(c) are
exclusive of each other and only one can be achieved.  For clarity, TKT shall
pay the amount designated above upon the achievement of milestone 4.3(a) and
upon the achievement of either of milestone 4.3(b) or 4.3(c).  CELL GENESYS
shall notify TKT in writing upon the achievement of any of the milestone events
set forth above and TKT shall have [*] to make the corresponding cash payment or
stock grant, as the case may be. 

4.5 Late Payments. For each payment or stock transfer
not received by CELL GENESYS when due pursuant to Section 4.4, TKT must pay to
CELL GENESYS a simple interest charge of [*] per annum, or such lower amount as
is required by law, on the cash value of such payment or stock transfer to be
calculated from the date such payment or stock transfer is due until it is
actually received by CELL GENESYS. 

ARTICLE V

REPRESENTATIONS, WARRANTIES AND INDEMNIFICATION

5.1 CELL GENESYS Representations and Warranties. CELL
GENESYS hereby represents and warrants to TKT that:

	it has the full corporate power and authority to enter
into this Agreement, to perform its obligations under this Agreement and to
grant the license granted under Article 2 hereof, and this Agreement has been
duly executed and constitutes a legal, valid and binding obligation of it
enforceable against it in accordance with its terms and conditions, except as
such enforceability may be limited by applicable bankruptcy, insolvency,
moratorium, reorganization or similar laws, from time to time in effect,
affecting creditor's rights generally or by principles of equity;

	as of the Effective Date, there are no claims,
oppositions, litigations, interferences, judgments or settlements against or
owed by CELL GENESYS related to the Patent Rights that would materially affect
the license granted hereunder, and there are no pending, and CELL GENESYS is not
aware of any threatened, claims, oppositions, litigations, or interferences
relating to the Patent Rights, other than those claims of record as of the
Effective Date [*];   

                          6.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	(i) CELL GENESYS is not aware that any of the data and
information made available to TKT for the diligence conducted by TKT pursuant to
the LOI are untrue or inaccurate to any material degree, and (ii) CELL GENESYS
has made available to TKT all Materials (as defined in Section 1(a) of the LOI)
in CELL GENESYS's or any of its agents' control or possession; 

	to CELL GENESYS' knowledge, it does not have any license
rights granted to it by Third Party under any patent that claims the activation
of genes to produce specific desired proteins, which license rights cannot be
sublicensed to TKT for the purposes of the license rights granted under Section
2.1; and 

	the execution and performance by it of its obligations
hereunder does not and will not constitute a material breach of, or materially
conflict with, any agreement or arrangement by which it is bound, and that it
will not enter into any agreement which materially conflicts with the terms
hereof.

5.2 TKT Representations and Warranties. TKT hereby
represents and warrants to CELL GENESYS that:

	it has the full corporate power and authority to enter
into this Agreement and to perform its obligations under this Agreement, and
this Agreement has been duly executed and constitutes a valid, binding and
enforceable agreement in accordance with its terms and conditions, except as
such enforceability may be limited by applicable bankruptcy, insolvency,
moratorium, reorganization or similar laws, from time to time in effect,
affecting creditor's rights generally or by principles of equity; and

	the execution and performance by it of its obligations
hereunder does not and will not constitute a material breach of, or materially
conflict with, any agreement or arrangement by which it is bound, and that it
will not enter into any agreement which materially conflicts with the terms
hereof.

5.3 CELL GENESYS Indemnification. CELL GENESYS hereby
agrees to defend, hold harmless and indemnify TKT, its Affiliates, agents,
directors, officers and employees from and against any and all Third Party
suits, claims, actions, demands, liabilities, costs, damages, fees, expenses
and/or losses, including without limitation reasonable legal expenses and
attorneys' fees (collectively "Third Party Claims") resulting from a
material breach by CELL GENESYS of any of the provisions of this Agreement,
including without limitation, the representations and warranties made in Section
5.1 by CELL GENESYS; except to the extent such Third Party Claims result from
the negligence or intentional misconduct of, or breach hereof by, TKT.

                          7.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

5.4 TKT Indemnification. TKT hereby agrees to defend,
hold harmless and indemnify CELL GENESYS, its Affiliates, agents, directors,
officers and employees from and against any and all Third Party Claims (except
to the extent such Third Party Claims result from the negligence or intentional
misconduct of, or breach hereof by, CELL GENESYS), including any product
liability claims, resulting:

	from a material breach by TKT of any of the provisions of
this Agreement; or

	directly or indirectly from the manufacture, use,
handling, storage, sale or other disposition of Licensed Products by TKT, or its
Affiliates, agents or sublicensees.  

5.5 Indemnification Procedure. The Party seeking
indemnification under this Section 5 (the "Indemnified Party") shall
(i) give prompt written notice to the other Party (the "Indemnifying
Party") of any Third Party Claim for which indemnification is sought, (ii)
permit the Indemnifying Party to assume full responsibility to investigate,
prepare for, defend against and/or compromise or settle (subject to the
provisions set forth below) the Third Party Claim and (iii) if the Indemnifying
Party assumes responsibility under clause (ii), reasonably assist the
Indemnifying Party, at the Indemnifying Party's reasonable expense, in the
investigation of, preparation for and defense of such Third Party Claim.  If the
Indemnifying Party assumes control of the defense of a Third Party Claim, the
Indemnifying Party shall not be liable for any litigation costs or expenses
incurred by the Indemnified Party.  If the Indemnifying Party does not assume
control of the defense of the Third Party Claim, the Indemnified Party shall
control such defense. The Indemnifying Party shall not compromise or settle such
Third Party Claim without the Indemnified Party's prior written consent if the
compromise or settlement does not include a complete and unconditional release
of the Indemnified Party from all liability with respect thereto or if the
compromise or settlement imposes any liability or obligation on the Indemnified
Party.  The Indemnified Party shall not agree to any settlement of such action,
suit, proceeding or claim without the prior written consent of the Indemnifying
Party.  The Indemnified Party shall have the right to participate, at its own
expense and with counsel of its choice, in the defense of any claim or suit the
defense of which has been assumed by the Indemnifying Party. 

5.6 LIMITATION OF LIABILITY. EXCEPT FOR OBLIGATIONS
UNDER SECTIONS 5.3 AND 5.4 OR IN CASE OF BREACH OF SECTION 3.1, IN NO EVENT
SHALL EITHER PARTY, ITS DIRECTORS, OFFICERS, EMPLOYEES, AGENTS OR AFFILIATES BE
LIABLE TO THE OTHER PARTY FOR ANY INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
CONSEQUENTIAL DAMAGES, WHETHER BASED UPON A CLAIM OR ACTION OR CONTRACT,
WARRANTY, NEGLIGENCE, STRICT LIABILITY OR OTHER TORT, OR OTHERWISE, ARISING OUT
OF THIS AGREEMENT.

5.7 Limited Warranty. Except for the warranties
provided by CELL GENESYS in Section 5.1 AND TKT IN SECTION 5.2, EACH PARTY
disclaims all other warranties, express or implied, and specifically, without
limitation, any warranty of merchantability, fitness for a particular purpose or
any warranty that exercise by TKT of its rights under the Patent Rights will not
infringe any intellectual property rights of third parties.  

                          8.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

ARTICLE VI

INTELLECTUAL PROPERTY PROVISIONS

6.1 Filing, Prosecution and Maintenance of Patents.

	Subject to Section 6.1(b) below, CELL GENESYS agrees to
file, prosecute and maintain all Patent Rights [*] and shall provide TKT with
copies of all documentation and correspondence related to the Patent Rights.  To
the extent that such documents pertain to Licensed Products, CELL GENESYS shall
give TKT an opportunity to comment on any official communication related to the
Patent Rights before filing, shall [*], and shall consult with TKT thereon.
CELL GENESYS shall comply with any applicable duty to disclose information
relevant to the Patent Rights as required by the United States Patent and
Trademark Office during the prosecution of any of the Patent Rights.   CELL
GENESYS shall [*] related to the scope of appropriate disclosure.

	In the event that CELL GENESYS elects not to file,
prosecute or maintain any patent or patent application involving a particular
patent or application in the Patent Rights, then CELL GENESYS shall give TKT at
least [*] notice thereof and TKT shall have the right to elect to undertake such
prosecution and maintenance.  In such case, CELL GENESYS shall, [*], either (i)
assign such patent or application (including any patent or application to which
such patent or application claims priority) to TKT, or (ii) allow TKT to
prosecute and maintain such patent in CELL GENESYS's name, and grant TKT an [*].
TKT shall, [*], file, prosecute and maintain such patent or application
(including any priority patent or patent application) in the Patent Rights. CELL
GENESYS shall execute such documents and perform such acts as may be reasonably
necessary to effect an assignment or transfer of power of attorney to such
Patent Rights to allow TKT to continue in a timely manner such filing,
prosecution or maintenance.  Upon completion of such assignment or transfer of
power of attorney, TKT shall be deemed automatically to have granted CELL
GENESYS a [*] license under such assigned Patent Rights [*].  Further, TKT
agrees that it shall, upon CELL GENESYS's request, negotiate in good faith with
any Third Party to grant a license [*] under such Patent Right to the extent
needed for such Third Party to practice license rights granted to it by CELL
GENESYS under the Patent Rights prior to CELL GENESYS's election not to file,
prosecute or maintain such Patent Right, provided that (a) such license
rights do not include any right to make, have made, use, sell, offer for sale or
import any Licensed Products, and (b) such Third Party is not, as of the date
such election is made by CELL GENESYS not to file, prosecute or maintain such
Patent Right, [*].  CELL GENESYS shall give notice to TKT as soon as practicable
in advance of the grant, lapse, revocation, surrender, invalidation or
abandonment of any CELL GENESYS Patent Right being prosecuted by CELL GENESYS.

6.2 Opposition, Reexamination and Reissue.

                          9.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	CELL GENESYS shall notify TKT, within [*] of learning of,
and in any case, prior to public disclosure of, any request for, or filing or
declaration of, any interference, opposition, reexamination, or the foreign
equivalent of any of the foregoing involving the Patent Rights. 

	CELL GENESYS shall not initiate any reexamination,
interference or reissue proceeding involving Patent Rights [*].   

	In connection with any interference, opposition, reissue,
or reexamination proceeding (or the foreign equivalent of any of the foregoing)
involving the Patent Rights, CELL GENESYS agrees to give TKT an opportunity to
provide comments on any official communication related to such interference,
opposition, reissue, or reexamination proceeding (or the foreign equivalent of
any of the foregoing) before filing, shall consider TKT's comments in good
faith, and shall consult with TKT thereon.  Additionally, CELL GENESYS shall
[*], and [*], including to the extent permissible by law and contracts, the [*].
Notwithstanding the foregoing, CELL GENESYS shall not be required to make any
disclosure of any communication or other information that would destroy either
the attorney/client privilege or attorney work product privilege protecting such
communication or other information, provided that if such communication or other
information is material to TKT's interests under this Agreement, CELL GENESYS
shall, in good faith, use [*] to facilitate disclosure without destroying any
such privilege.   

6.3 Enforcement and Defense.

	Each Party shall give the other Party notice of any
unauthorized making, using, selling, offering for sale or importing of a
Licensed Product that may come to such Party's attention (a "Field
Infringement").  Each Party shall have the right to bring suit against such
alleged infringer in a Field Infringement, [*], and either Party may [*];
provided, however, that in the event that TKT brings a suit against an alleged
infringer as the result of a possible Field Infringement, but in its sole
reasonable discretion believes that CELL GENESYS is or may be a necessary or
indispensable party under the applicable law, CELL GENESYS agrees to become a
party in such action , and shall execute all documents and take all steps
reasonably necessary to enable TKT to initiate, prosecute and/or maintain such
action, or for TKT to defend any declaratory judgment action brought by the
alleged infringer.  Each Party shall be entitled in any such action [*] to
assert its own claims for damages and to defend it interests in the Patent
Rights.  For any infringement of the Patent Rights other than a Field
Infringement, CELL GENESYS shall have the sole and exclusive right to bring an
action against the infringer.  

	Each Party shall [*] pursuant to an action brought under
subclause (a) above.

	Each Party shall notify the other of any certification
regarding any Patent Rights which it has received pursuant to 21 U.S.C.
  355(b)(2)(A)(iv) or (j)(2)(A)(vii)(IV) or its successor provisions or the
foreign equivalent thereof and shall provide the other with a copy of such
certification within five (5) days of receipt.  CELL GENESYS's and TKT's rights
with respect to the initiation and prosecution of any legal action (or the
defense of any such legal action) as a result of such certification or any
recovery obtained as a result of such legal action shall be as defined above;
provided, however, that if either Party shall exercise its first
right to initiate and prosecute any action or to control the defense of any such
action, such Party shall notify the other Party of such decision within ten (10)
days of receipt of the certification.

                          10.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	If any Licensed Product becomes the subject of a Third
Party claim, or there is the potential for a claim of patent infringement
related to TKT's development, use, sale, offer for sale or import of Licensed
Products, TKT shall defend against such claim [*].  CELL GENESYS and TKT shall
fully cooperate and provide assistance to each other, [*], with respect to any
proceeding related to gene activation patents owned or licensed by a Third Party
and asserted, or threatened to be asserted, by such Third Party, including
without limitation, [*].

6.4 Third Party License; Settlement. Except as
otherwise provided herein, CELL GENESYS shall [*] that [*] under the Patent
Rights [*].   

ARTICLE VII

TERM AND TERMINATION

7.1 Term and Expiration. This Agreement shall be
effective as of the Effective Date and, unless terminated earlier pursuant to
the provisions of Section 7.2              below, shall continue until the
expiration of the last-to-expire Patent Right.

7.2 Termination by Either Party for Cause. This
Agreement may be terminated by written notice by either Party at any time during
the term of this Agreement if the other Party is in breach of a material
obligation hereunder and has not cured such breach within [*] after notice
requesting cure of the breach.  If TKT terminates this Agreement pursuant to
this Section 7.2, then:  (a) the license granted pursuant to Section 2.1 shall
[*], and provided that [*], and (b) in addition, TKT shall have (i) [*] under
the Patent Rights to [*] and (ii) a [*] under the Patent Rights to [*], but
excluding from the foregoing rights any [*] under the Patent Rights that
have been [*] prior to the date of breach.

7.3 Survivals. Termination or expiration of this
Agreement shall not relieve either Party of any obligation of such Party accrued
prior to such termination or expiration.  Any termination or expiration of this
Agreement shall be without prejudice to the rights of either Party against the
other accrued under this Agreement prior to termination or expiration hereof.
The provisions of Articles 1, 3, 5, and 7 shall survive the termination or
expiration of this Agreement.   Notwithstanding the termination of this
Agreement pursuant to this Article VII, any sublicenses of Patent Rights granted
by TKT hereunder prior to such termination shall survive such termination.  In
such event, CELL GENESYS shall have the right to receive directly from the
sublicensee any payments or other consideration otherwise payable to TKT as the
sublicensor under such sublicense, and to otherwise exercise all of the rights
of TKT as the sublicensor under such sublicense; provided however that CELL
GENESYS shall not assume, and shall not be responsible for, any representations,
warranties, promises or obligations of TKT to any sublicensees other than the
licenses under such sublicenses.

                          11.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

ARTICLE VIII

MISCELLANEOUS

8.1 Force Majeure. Neither Party shall be held liable or
responsible to the other Party nor be deemed to have defaulted under or breached
this Agreement for failure or delay in fulfilling or performing any term of this
Agreement when such failure or delay is caused by or results from causes beyond
the reasonable control of the affected Party including, but not limited to,
earthquakes, fire, floods, embargoes, war, acts of war (whether war be declared
or not), insurrections, riots, civil commotions, strikes, lockouts or other
labor disturbances, acts of God or acts, omissions or delays in acting by any
governmental authority or the other Party.  The affected Party shall notify the
other Party of such force majeure circumstances as soon as reasonably practical
and shall make reasonable, diligent efforts to remove the condition constituting
force majeure or to avoid its effects so as to resume performance as soon as
practicable.

8.2 Sublicenses. All sublicenses granted by TKT under
the Patent Rights to Third Parties shall be on such terms as are consistent with
the provisions of this Agreement and shall be subject to the rights of CELL
GENESYS under this Agreement. 

8.3 Assignment. Except as otherwise expressly provided
below, this Agreement may not be assigned or otherwise transferred, nor may any
right or obligation hereunder be assigned or transferred, by either Party
without the prior written consent of the other Party; except that either Party
may, without the prior written consent of the other Party, assign this Agreement
and such Party's rights and obligations hereunder to its successor in interest
in connection with the transfer or sale of all or substantially all of such
Party's assets or the business to which this Agreement relates, or in the event
of its merger, acquisition, consolidation, change in control or similar
transaction.  Any permitted assignee shall assume all obligations of its
assignor under this Agreement except as otherwise provided herein.

8.4 Severability. If any provision hereof should be
held invalid, illegal or unenforceable in any respect, then, to the fullest
extent permitted by law, (a) all other provisions hereof shall remain in full
force and effect and shall be liberally construed in order to carry out the
intentions of the Parties as nearly as may be possible and (b) the Parties shall
use their best efforts to replace the invalid, illegal or unenforceable
provision(s) with valid, legal and enforceable provision(s) which, insofar as
practical, implement the purposes of such provision(s) in this
Agreement.

8.5 Notices. All notices or other communications which
are required or permitted hereunder, including any notices required pursuant to
Section 2.2 hereof, shall be in writing and addressed as follows:

                          12.

[*]  Certain information on this page has been omitted and filed separately with the
Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	
if to CELL GENESYS:
	
Cell Genesys, Inc.

	 	
342 Lakeside Drive

	 	
Foster City, CA 94404 

	
Attention:
	
Robert Tidwell

	 	
Vice President, Corporate Development

	
With a copy to:
	
Cooley Godward LLP

	

	
3000 El Camino Real

	 	
Palo Alto, CA 94306-2155

	
Attention:
	
Barclay J. Kamb, Esq.

	
if to the Company:
	
Transkaryotic Therapies, Inc.

	

	
195 Albany Street

	 	
Cambridge, MA  02139

	
Attention:
	
Michael J. Astrue 

	 	
Senior Vice President, Administration 

	
With a copy to:
	
Kerry A. Flynn

	

	
Senior Director, Business Development

or to such other address as the Party to whom notice is to be
given may have furnished to the other Party in writing in accordance herewith.
Any such communication shall be deemed to have been given when delivered if
personally delivered or sent by facsimile (provided that the Party providing
such notice promptly confirms receipt of such transmission with the other Party
by telephone), on the business day after dispatch if sent by a nationally-recognized
overnight courier and on the third business day following the date of
mailing if sent by mail, postage prepaid, return receipt requested.

8.6 Applicable Law. This Agreement shall be governed
by and construed in accordance with the laws of the State of Delaware, without
reference to any rules regarding conflict of laws.  The Parties hereby consent
to the jurisdiction of the state and federal courts of the State of Delaware and
agree that any disputes related hereto shall be adjudicated therein.

8.7 Entire Agreement. This Agreement, together with
the Side Letter and the Stock Purchase Agreement, contains the entire
understanding of the Parties with respect to the subject matter hereof and
supercedes all prior agreements with respect to the subject matter hereof,
including without limitation the LOI.  All express or implied agreements and
understandings, either oral or written, heretofore made are expressly merged in
and made a part of this Agreement.  Except as expressly set forth in this
Agreement, this Agreement may be amended, or any term hereof modified, only by a
written instrument duly executed by both Parties.

                          13.

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

8.8 Headings. The captions to the several Articles and
Sections hereof are not a part of this Agreement, but are merely guides or
labels to assist in locating and reading the several Articles and Sections
hereof.

8.9 Independent Contractors. It is expressly agreed
that CELL GENESYS and TKT shall be independent contractors and that the
relationship between the two Parties shall not constitute a partnership, joint
venture or agency.  Neither CELL GENESYS nor TKT shall have the authority to
make any statements, representations or commitments of any kind, or to take any
action, which shall be binding on the other, without the prior written consent
of the other Party.

8.10 Waiver. The waiver by either Party hereto of any
right hereunder or the failure to perform or of a breach by the other Party
shall not be deemed a waiver of any other right hereunder or of any other breach
or failure by said other Party whether of a similar nature or
otherwise.

8.11 Counterparts. This Agreement may be executed in
two (2) or more counterparts, each of which shall be deemed an original, but all
of which together shall constitute one and the same instrument.

8.12 Waiver of Rule of Construction. Each Party has had
the opportunity to consult with counsel in connection with the review, drafting
and negotiation of this Agreement.  Accordingly, the rule of construction that
any ambiguity in this Agreement shall be construed against the drafting Party
shall not apply.

8.13 Section 365(n) of the Bankruptcy Code. All
licenses granted under this Agreement are, and shall otherwise be, deemed to be,
for purposes of Section 365(n) of the Bankruptcy Code, licenses of rights to
"intellectual property" as defined under Section 101(35A) of the
Bankruptcy Code.  The Parties shall retain and may fully exercise all of their
respective rights and elections under the Bankruptcy Code.

8.14 Third Party Beneficiaries. Except as otherwise
expressly provided in this Agreement, nothing herein expressed or implied is
intended or shall be construed to confer upon or to give to any Third Party any
rights or remedies by reason of this Agreement.  Except as otherwise expressly
provided in this Agreement, there are no intended Third Party beneficiaries
under or by reason of this Agreement.

   *    *   *    

                          14.

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

In Witness Whereof, the Parties have executed this
License Agreement as of the date first set forth above.

	
Transkaryotic Therapies, Inc.
	
Cell
Genesys, Inc.

	
By:_______________________

	
By:_______________________

	
Name: Michael J. Astrue
	
Name:

	
Title:   Senior Vice President

Administration & General Counsel
	
Title:

                          15.

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

NOTE:  Information in this document marked with an "[*]" has been omitted and filed separately
with the Commission.  Confidential treatment has been requested with respect to the omitted
portions.

Exhibit A

FORM OF STOCK PURCHASE AGREEMENT

STOCK PURCHASE AGREEMENT

This Agreement ("Agreement") is
made and entered into as of June 7, 2002 (the "Effective
Date"), by and among Transkaryotic Therapies, Inc., a Delaware
corporation (the "Company"), and Cell Genesys, Inc., a
Delaware corporation ("CELL GENESYS").

A.Concurrently with the execution of this
Agreement, the Company is entering into an Exclusive License Agreement with CELL
GENESYS, (the "License Agreement"), pursuant to which CELL
GENESYS is licensing the rights to certain technology to the Company; and

B.In partial consideration for the execution and
delivery of the License Agreement by CELL GENESYS, the Company has agreed to
issue to CELL GENESYS certain shares of the Company's common stock in accordance
with the terms and conditions of this Agreement.

Agreement

In consideration of the mutual covenants contained in
this Agreement and for other good and valuable consideration, the receipt of
which is hereby acknowledged, the Company and CELL GENESYS hereby agree as
follows:

 SECTION 1  Authorization of Sale of Shares.

1.1 Authorization. Subject to the terms and conditions
of this Agreement, the Company has or before the Closings (as defined below)
will have authorized the sale and issuance of up to the following number of
shares of the Company's Common Stock, par value $0.01 per share (the
"Shares"):

	Three-Hundred Sixty Six Thousand Nine Hundred Twenty
Eight (366,928) Shares (the "Up Front Shares"), subject to
adjustment pursuant to Section 1.2;

	that number of Shares equal to [*], divided by the
average closing price of the Company's Shares as reported on the Nasdaq National
Market for the ten (10) consecutive trading days ending two days prior to the
date of the Second Tranche Closing, as hereinafter defined (the
"First Milestone Shares"), which shares shall only be issued if
the milestone in Section 4.3(a) of the License Agreement is
achieved; and 

	that number of Shares equal to [*], divided by the
average closing price of the Company's Shares as reported on the Nasdaq National
Market for the ten (10) consecutive trading days ending two days prior to the
date of the Third Tranche Closing, as hereinafter defined (the
"Second Milestone Shares"), which shares shall only be issued
if the milestone in Section 4.3(b) of the License Agreement is
achieved.

                          A-1

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

If after the date hereof (i) the outstanding shares of the
Company's Common Stock shall be subdivided or split into a greater number of
shares or a dividend in Common Stock shall be paid in respect of such Common
Stock, (ii) the outstanding shares of Common Stock are combined, (iii) the
Company shall pay a dividend in securities of the Company (other than Common
Stock) or of other property (including cash) on the Common Stock, or (iv) there
shall occur any merger, consolidation, capital reorganization or
reclassification in which the Common Stock is converted or exchanged for
securities, cash or other property, all share quantities in this Agreement, as
well as the class or series of stock constituting the Common Stock for purposes
of this Agreement, shall be appropriately adjusted to reflect such stock splits,
stock dividend, combination, other dividend, merger, consolidation, capital
reorganization or reclassification.  After any event referenced in clauses (i)
through (iv) is consummated, if applicable, all references herein to the
Company's Common Stock shall be deemed to refer to the capital stock or property
(including cash) into or for which the Common Stock was converted or exchanged,
with the necessary changes in detail.

1.2 Adjustment of Up Front Shares. The number of Up
Front Shares shall be adjusted as follows:

	if the product of the average closing price of the
Company's Shares as reported on the Nasdaq National Market for the ten (10)
trading days ending two days prior to the effective date of the Registration
Statement (as hereinafter defined) with respect to the Up Front Shares
multiplied by 366,928 is less than Fifteen Million Dollars ($15,000,000), then
the number of Up Front Shares shall be increased by the number of Shares equal
to (x) the difference between $15,000,000 and such product divided by (y) such
average closing price; and the Company shall issue to CELL GENESYS such
additional Up Front Shares promptly following the end of such ten (10) day
period for a price per share equal to the par value of each such share, which
price shall be deemed paid in partial consideration for the execution and
delivery by CELL GENESYS of the License Agreement; provided further that the
Company shall promptly cause any such additional shares to be registered,
through supplement or amendment, on the Registration Statement with respect to
the Up Front Shares; and

	if the product of the average closing price of the
Company's Shares as reported on the Nasdaq National Market for the ten (10)
trading days ending two days prior to the effective date of the Registration
Statement (as hereinafter defined) with respect to the Up Front Shares
multiplied by 366,928 is greater than Fifteen Million Dollars ($15,000,000),
then the number of Up Front Shares shall be decreased by a number of Shares
equal to (x) the difference between such product and $15,000,000 divided by (y)
such average closing price; and CELL GENESYS shall surrender to the Company the
number of Shares by which the Up Front Shares are decreased, promptly following
the end of such ten (10) day period, at a price per share equal to the par value
of each such share, which price shall be deemed paid in partial consideration
for the execution and delivery by Company of the License Agreement.

                          A-2

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

 SECTION 2 
 Closing and Delivery

2.1 Sale of Shares. Subject to the terms and
conditions of this Agreement and in reliance upon the representations,
warranties and agreements contained herein, the Company will issue and sell to
CELL GENESYS, and CELL GENESYS will purchase from the Company, at each of the
Closings, the applicable number of Shares, at a price per share equal to the par
value of each such share at such time, which price shall be deemed paid in
partial consideration for the execution and delivery by CELL GENESYS of the
License Agreement.

2.2 Closings. The closings of the purchase and sale of
the Shares to be issued pursuant to this Agreement shall be held at the offices
of Hale and Dorr LLP, 60 State Street, Boston, Massachusetts, 02109 in three
tranches, as follows:

	Within ten days of execution of the License Agreement, at
a date mutually agreed by the parties hereto, the closing of the purchase and
sale of the Up Front Shares will occur (the "First Tranche
Closing");

	Within [*] of the achievement of the milestone in Section
4.3(a) of the License Agreement, if achieved, at a date mutually agreed by the
parties hereto, the closing of the purchase and sale of the First Milestone
Shares will occur (the "Second Tranche Closing"); and

	Within [*] of the achievement of the milestone in Section
4.3(b) of the License Agreement, if achieved, at a date mutually agreed by the
parties hereto, the closing of the purchase and sale of the Second Milestone
Shares will occur (the "Third Tranche Closing").

Each of the First Tranche Closing, Second Tranche Closing and
Third Tranche Closing are collectively hereinafter referred to as the
"Closings" and individually as a
"Closing".

2.3 Delivery of the Shares. Promptly following a
Closing, the Company shall deliver to CELL GENESYS a certificate representing
the number of Shares purchased at such Closing, registered in the name of CELL
GENESYS.

 SECTION 3 
Representations, Warranties and Covenants of the Company.

Subject to and except as set forth on the Schedule of
Exceptions which is arranged in sections corresponding to the sub-section
numbered provisions contained below in this Section and except as described in
the SEC Reports (as defined below), the Company hereby represents and warrants
to, and covenants with, CELL GENESYS as of the date hereof as follows:

3.1 Organization and Qualification. The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of Delaware and has all requisite corporate power and authority to
perform its obligations hereunder and to consummate the transactions
contemplated hereby. 

                          A-3

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

3.2 Due Execution, Delivery and Performance. The
Company's execution, delivery and performance of this Agreement and the issuance
and sale of the Shares have been duly authorized by all requisite corporate
action by the Company.  Upon the execution and delivery by the Company, and
assuming the valid execution and delivery of this Agreement by CELL GENESYS,
this Agreement will constitute the valid and binding obligation of the Company,
enforceable in accordance with its terms, except as enforceability may be
limited by applicable bankruptcy, insolvency, reorganization, moratorium or
similar laws affecting creditors' and contracting parties' rights generally and
except as enforceability may be subject to general principles of equity
(regardless of whether such enforceability is considered in a proceeding in
equity or at law), including specific performance, and except as the
indemnification provisions contained in Section 8.3 hereof may be legally
unenforceable.

3.3 No Conflicts. The Company's execution, delivery
and performance of this Agreement and the issuance of the Shares will not
violate, conflict with, result in a breach of or constitute (upon notice or
lapse of time or both) a default under, or result in the creation or imposition
of any lien, security interest, mortgage, pledge, charge or other encumbrance,
of any material nature, upon any properties or assets of the Company under any
(a) law, regulation, rule, injunction, judgment, order, decree, ruling, charge
or other restriction of any government, governmental agency, court or arbitrator
to which the Company is subject, (b) the Company's Amended Restated Certificate
of Incorporation or Bylaws or (c) any provision of any material indenture,
mortgage, agreement, contract or other material instrument to which the Company
is a party or by which the Company or any of its properties or assets is bound
as of the date hereof, except in the case of clause (a) and clause (c), where
such violation, conflict, breach, default, or imposition would not have a
Material Adverse Effect.  As used in this Agreement, a "Material Adverse
Effect" means (a) a material adverse effect upon the business, operations,
properties, assets or condition (financial or otherwise) of the Company or, as
the case may be, the Company and any of its subsidiaries, taken as a whole or
(b) a material impairment of the ability of the Company to perform its
obligations under this Agreement.

3.4 Governmental Consents. Except for applicable
filings with The Nasdaq Stock Market, Inc. (the "Nasdaq Market"),
under the Securities Act of 1933, as amended (the "Securities Act"),
or the Securities Exchange Act of 1934, as amended (the "Exchange
Act"), or state securities laws, no consent, approval, qualification, order
or authorization of, or filing with, any local, state, or federal governmental
authority is required on the part of the Company in connection with the
Company's valid execution, delivery, or performance of this Agreement, or the
offer, sale or issuance of the Up Front Shares by the Company, other than any
post-closing filings as may be required under applicable federal or state
securities laws, which will be timely filed within the applicable periods
therefor.

3.5 Issuance and Sale of the Securities. When issued
and paid for in accordance with this Agreement, the Shares to be sold hereunder
by the Company will be validly issued and outstanding, fully paid and non-
assessable.

3.6 SEC Reports. 

                          A-4

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	Since January 1, 2001, the Company has filed in a timely
manner with the Securities and Exchange Commission (the "SEC")
all reports ("SEC Reports") required to be filed by it under
the Exchange Act.  All of the SEC Reports filed by the Company comply in all
material respects with the requirements of the Exchange Act.  To the knowledge
of the Company, none of the SEC Reports contain, as of the respective dates
thereof, any untrue statement of a material fact or omit to state any material
fact required to be stated therein or necessary to make the statements therein
not misleading in light of the circumstances under which they were made.

	No event has occurred since January 1, 2002, requiring
the filing of an SEC Report that has not heretofore been filed and furnished to
CELL GENESYS (including, without limitation, any amendment to any such SEC
Report).

3.7 Capitalization. The authorized capital stock of
the Company consists of (i) 100,000,000 shares of Common Stock, par value
$0.01 per share, of which 34,383,308 were issued and outstanding as of April 10,
2002 and (ii) 10,000,000 shares of preferred stock, par value $0.01 per share,
of which no shares are issued and outstanding on the date hereof.  As of the
date hereof, the Company has no intention, obligation or commitment, fixed or
contingent, to issue any shares of such Preferred Stock, other than pursuant to
its Stockholders' Rights Plan.

3.8 Nasdaq Compliance. The Company's Common Stock is
registered pursuant to Section 12(g) of the Exchange Act, and is listed on the
Nasdaq National Market (the "Nasdaq Stock Market"), and the Company
has taken no action designed to, or which to its knowledge is likely to have the
effect of, terminating the registration of the Common Stock under the Exchange
Act or delisting the Common Stock from the Nasdaq Stock Market. The issuance of
the shares does not require stockholder approval, including, without limitation,
pursuant to the Nasdaq Marketplace Rules.

3.9 Form S-3 Eligibility. The Company qualifies as a
registrant whose securities may be resold pursuant to Form S-3 promulgated by
the SEC pursuant to the 1933 Securities Act, as amended.

3.10 Absence of Litigation. There is no action, suit,
proceeding or investigation pending or, to the Company's best knowledge, that
has been filed, commenced or threatened, by or before any governmental agency,
court or arbitrator against the Company which might have, either individually or
in the aggregate, a Material Adverse Effect (including, without limitation, any
such action, suit, proceeding or investigation that questions the validity of
this Agreement or the issuance of the Shares hereunder).

3.11 Legal Compliance. To the Company's knowledge, the
Company is not in default or violation of its Amended  Restated Certificate of
Incorporation or Bylaws and has not violated any applicable laws (including,
without limitation, rules, regulations, codes, plans, injunctions, judgments,
orders, decrees, rulings and charges thereunder) of federal, state, local and
foreign governments (and all agencies thereof) in respect of the conduct of its
business or the ownership of its properties which default violation would
(either individually or in the aggregate) have a Material Adverse Effect.  To
the knowledge of the Company, there exists no condition, event or act which
constitutes, or which after notice, lapse of time or both, would constitute,
such a default or violation under any of the foregoing except where such a
default is not reasonably expected to have a Material Adverse Effect.

                          A-5

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

3.12 Securities Act Exemption. Assuming and relying in
part on the truth and accuracy of CELL GENESYS' representations and warranties
in Section 4 of this Agreement, the offer, sale and issuance of the Shares is
exempt from registration under the Securities Act.

3.13 No Manipulation of Stock. The Company has not
taken and will not, in violation of applicable law, take, any action outside the
ordinary course of business designed to or that might reasonably be expected to
cause or result in unlawful manipulation of the price of the Common Stock to
facilitate the sale or resale of the Shares by CELL GENESYS.

3.14 Brokers. Neither the Company nor any of the
officers, directors or employees of the Company has employed any broker or
finder in connection with the transaction contemplated by this Agreement.  The
Company shall indemnify CELL GENESYS from and against any broker's, finder's or
agent's fees for which the Company is responsible.

 SECTION 4 
Representations, Warranties and Covenants of Cell Genesys.

4.1 Authority, Approval and Enforceability.

CELL GENESYS represents and warrants to and covenants with
the Company that:  

	CELL GENESYS is a corporation duly organized, validly
existing and in good standing under the laws of the State of Delaware and has
all requisite corporate power and authority to perform its obligations hereunder
and to consummate the transactions contemplated hereby..

	CELL GENESYS' execution, delivery and performance of this
Agreement have been duly authorized by all requisite corporate action by CELL
GENESYS.  Upon the execution and delivery by CELL GENESYS, and assuming the
valid execution and delivery of this Agreement by the Company, this Agreement
will constitute a valid and binding obligation of CELL GENESYS, enforceable in
accordance with its terms, except as enforceability may be limited by applicable
bankruptcy, insolvency, reorganization, moratorium or similar laws affecting
creditors' and contracting parties' rights generally and except as
enforceability may be subject to general principles of equity (regardless of
whether such enforceability is considered in a proceeding in equity or at law),
including specific performance, and except as the indemnification provisions
contained in Section 8.3 hereof may be legally unenforceable.

4.2 Investment Representations. CELL GENESYS
understands that the Shares have not been registered under the Securities Act.
CELL GENESYS also understands that the Shares are being offered and sold
pursuant to an exemption from registration contained in the Securities Act based
in part upon CELL GENESYS' representations contained in the Agreement, including
this Section 4.2.  CELL GENESYS represents and warrants to, and covenants with,
the Company that as of the date hereof and as of the date of any issuance of
Shares hereunder:

                          A-6

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	CELL GENESYS has substantial experience in evaluating and
investing in private purchases of securities in companies similar to the Company
so that it is capable of evaluating the merits and risks of its investment in
the Company and has the capacity to protect its own interests.  CELL GENESYS
must bear the economic risk of this investment indefinitely unless the Shares
are registered pursuant to the Securities Act, or an exemption from registration
is available.  

	CELL GENESYS has been advised or is aware of the
provisions of Rule 144 promulgated under the Securities Act, which permits
limited resale of shares purchased in a private purchase subject to the
satisfaction of certain conditions.

	CELL GENESYS agrees that it will not sell, assign or
transfer, (collectively, "Transfer") any of the Shares unless
the Transfer will be made pursuant to an exemption from the registration
requirements of the Securities Act or pursuant to an effective registration
statement under the Securities Act and pursuant to an exemption from any
applicable state securities laws or an effective registration or other
qualification under any applicable state securities laws. CELL GENESYS
understands that exemptions from such registration requirements are limited. The
Company is under no obligation to register the Shares except as provided in
Section 8.

	CELL GENESYS acknowledges and agrees that the Shares are
subject to certain restrictions as to resale under the federal and state
securities laws. CELL GENESYS agrees and understands that stop transfer
instructions will be given to the transfer agent for the Shares, and each share
certificate and each certificate delivered on transfer of or in substitution for
any such certificate, shall have affixed a legend in substantially the following
form:

"The shares represented by this certificate have not
been registered under the Securities Act of 1933, as amended (the
"Act"), and may not be offered, sold or otherwise transferred,
assigned, pledged or hypothecated unless and until registered under the Act or
unless the Company has received an opinion of counsel satisfactory to the
Company and its counsel that such registration is not required."

	CELL GENESYS is acquiring the Shares for its own account
for investment only, and not with a view towards their distribution or sale, nor
with any present intention of distributing or selling the same (except in
compliance with securities laws), and except as set forth herein, CELL GENESYS
has no present or contemplated agreement, understanding, obligation, or
commitment providing for the disposition thereof.

	CELL GENESYS represents that by reason of its, or of its
management's, business or financial experience, it has the capacity to protect
its own interests in connection with the transactions contemplated in this
Agreement.  Further, CELL GENESYS is aware of no publication of any
advertisement in connection with the transactions contemplated in the
Agreement.

                          A-7

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

	CELL GENESYS represents that it is an accredited investor
within the meaning of Regulation D under the Securities Act.

	CELL GENESYS has had an opportunity to discuss the
Company's business, management and financial affairs with directors, officers
and management of the Company and has had the opportunity to review the
Company's operations and facilities.  CELL GENESYS has also had the opportunity
to ask questions of and receive answers from, the Company and its management
regarding the terms and conditions of this investment.

4.3 No Manipulation of Stock. CELL GENESYS has not
taken and will not, in violation of applicable law, take, any action outside the
ordinary course of business designed to or that might reasonably be expected to
cause or result in unlawful manipulation of the price of the Company's Common
Stock to facilitate the sale or resale of the Shares. 

 SECTION 5 
Survival of Representations, Warranties and Agreements.

Notwithstanding any investigation made by any party to this
Agreement, all covenants, agreements, representations and warranties made by the
Company and CELL GENESYS herein shall survive the execution of this Agreement
and the issuance and sale to CELL GENESYS of the Shares and shall terminate one
year after the achievement of, or failure to achieve, as the case may be, the
last remaining milestone under the License Agreement; provided, however,
that the representations and warranties in Section 3.6 and Section 3.10 shall
survive the execution of this Agreement for 18 months from the date of the First
Tranche Closing.  

 SECTION 6 
Conditions to Company's Obligations at the Closing.

The Company's obligation to complete the sale and issuance of
the Shares at each Closing shall be subject to the following conditions to the
extent not waived by the Company:

6.1 Representations and Warranties Correct. The
representations and warranties made by CELL GENESYS in Sections 4.1, 4.2  and
4.3 hereof shall be true and correct when made, and shall be true and correct on
the date of the First Tranche Closing.  Moreover, with respect to the Second
Tranche Closing and Third Tranche Closing, the representations and warranties
made by CELL GENESYS in Section 4.2  hereof shall be true and correct on the
respective dates of such Closings. 

6.2 Compliance Certificate; Certified Resolutions. At
the First Tranche Closing, CELL GENESYS shall have delivered to the Company a
compliance certificate, executed by the Chief Executive Officer of CELL GENESYS,
dated as of the date of the First Tranche Closing, to the effect that the
conditions, specified in Sections 6.1 have been satisfied. 

 SECTION 7 
Conditions to Cell Genesys' Obligations At the Closing.

CELL GENESYS' obligation to purchase the Shares at each
Closing shall be subject to the following conditions to the extent not waived by
CELL GENESYS: 

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7.1 Representations and Warranties Correct. The
representations and warranties made by the Company in Section 3 hereof shall be
true and correct when made, and shall be true and correct as of the date of the
First Tranche Closing.

7.2 Legal Opinion. CELL GENESYS shall have received
from counsel to the Company, an opinion letter addressed to CELL GENESYS, with
respect to the issuance of the Shares,  dated as of the date of each Closing, in
a form customarily and usually delivered by law firms of publicly traded
companies in stock issuances of the nature contemplated by this
Agreement.

7.3 Covenants Performed. All covenants, agreements and
conditions contained herein to be performed by the Company on or prior to the
date of each Closing shall have been performed or complied with in all material
respects.

7.4 Qualifications. All authorizations, approvals, or
permits, if any, of any governmental authority or regulatory body of the United
States or of any state that are binding upon the Company and that are required
in connection with the lawful sale and issuance of the Shares at such Closing
pursuant to this Agreement shall have been duly obtained and shall be effective
on and as of the date of each Closing.  No stop order or other order enjoining
the sale of the Shares shall have been issued and no proceedings for such
purpose shall be pending or, to the knowledge of the Company, threatened by the
SEC, or any commissioner of corporations or similar officer of any state having
jurisdiction over this transaction.

7.5 Legal Investment. At the time of each Closing, the
sale and issuance of the Shares shall be legally permitted by all laws and
regulations to which CELL GENESYS and the Company are subject.

7.6 Compliance Certificate; Certified Resolutions. At
the First Tranche Closing, the Company shall have delivered to CELL GENESYS (i)
a compliance certificate, executed by the Chief Executive Officer of the
Company, dated as of the First Tranche Closing, to the effect that the
conditions, specified in Sections 7.1, 7.3 and 7.4 have been satisfied; and (ii)
a certificate of the Secretary of the Company evidencing the effective
authorization of the issuance of the Stock by the Company's Board of Directors
as of the date of each Closing.

 SECTION 8
Registration of the Shares; Compliance with the Securities Act.

8.1 Definitions. As used in this Section 8 the
following terms shall have the following respective meanings:

	"Registrable Shares" shall mean all
Shares issued pursuant to this Agreement (which, for greater certainty, shall
include the Up Front Shares, the First Milestone Shares and the Second Milestone
Shares, when issued) and any other shares of Common Stock issued or issuable in
respect to the Shares (because of stock splits, stock dividends,
reclassifications, recapitalizations, adjustments pursuant to this Agreement or
similar events);

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	"Untrue Statement" shall mean any untrue
statement or alleged untrue statement, or any omission or alleged omission to
state in the Registration Statement a material fact required to be stated
therein or necessary to make the statements therein, in the light of the
circumstances under which they were made, not misleading.

8.2 Registration Procedures and Expenses. The Company
shall:

	within fourteen (14) days immediately following each
Closing, such actual date being referred to as the "Registration
Date," prepare and file with the SEC a registration statement on Form
S-3 in order to register with the SEC under the Securities Act a sale by CELL
GENESYS on a delayed or continuous basis pursuant to Rule 415 under the
Securities Act any or all of the Registrable Shares then issued at such Closing
through the automated quotation system of the Nasdaq National Market System or
the facilities of any national securities exchange on which the Company's Common
Stock is then traded, or in privately-negotiated transactions (a
"Registration Statement") (notwithstanding anything to the
contrary expressed or implied herein, if a registration statement on Form S-3,
or any substitute form, is not then available for registration of the
Registrable Shares, the Company shall be obligated instead to prepare and file
with the SEC a registration statement on Form S-1 in order to register the
Registrable Shares under the Securities Act and such registration statement will
be a "Registration Statement" for the purposes of this
Agreement);

	subject to receipt of necessary information from CELL
GENESYS, use its commercially reasonably efforts to cause such Registration
Statement to become effective as soon as possible after the Registration Date,
and take all other reasonable actions necessary under any federal law or
regulation to permit all Registrable Shares to be sold or otherwise disposed of
thereunder;

	promptly notify CELL GENESYS, at any time when a
prospectus relating to such Registration Statement is required to be delivered
under the Securities Act, of the happening of any event as a result of which the
prospectus included in or relating to such Registration Statement contains an
Untrue Statement;

	promptly prepare and file with the SEC, and deliver to
CELL GENESYS, such amendments and supplements to such Registration Statement and
the prospectus used in connection therewith as may be necessary to keep such
Registration Statement effective until termination of such obligation as
provided in Section 8.7 below;

	furnish to CELL GENESYS such number of copies of
prospectuses in conformity with the requirements of the Securities Act, in order
to facilitate the public sale or other disposition of all or any of the
Registrable Shares by CELL GENESYS;

	file such documents as may be required of the Company for
normal state securities law clearance for the resale of the Registrable Shares
in which states of the United States as may be reasonably requested by CELL
GENESYS provided, however, that the Company shall not be required in connection
with this paragraph (f) to qualify as a foreign corporation or execute a general
consent to service of process in any jurisdiction;

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	no later than the Registration Date, use its best efforts
to cause all Registrable Shares to be listed on each securities exchange, if
any, on which equity securities by the Company are then listed; and

	bear all expenses in connection with the procedures in
Section 8.2, other than (i) fees and expenses, if any, of counsel or other
advisers to CELL GENESYS, and (ii) any expenses relating to the sale of the
Registrable Shares by CELL GENESYS, including broker's commission, discounts or
fees and transfer taxes.

8.3 Indemnification.

	The Company agrees to indemnify and hold harmless CELL
GENESYS (and each person, if any, who controls CELL GENESYS within the meaning
of Section 15 of the Securities Act) from and against any losses, claims,
damages or liabilities to which CELL GENESYS may become subject (under the
Securities Act or otherwise) insofar as such losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) arise out of, or are
based upon, any Untrue Statement contained in the Registration Statement on the
effective date thereof, or arise out of any failure by the Company to fulfill
any undertaking included in the Registration Statement and the Company will
reimburse CELL GENESYS for any reasonable legal or other expenses reasonably
incurred in investigating, defending or preparing to defend any such action,
proceeding or claim; provided, however, that the Company shall not be
liable in any such case to the extent that such loss, claim, damage or liability
arises out of, or is based upon, an Untrue Statement made in such Registration
Statement in reliance upon and in conformity with written information furnished
to the Company by or on behalf of CELL GENESYS specifically for use in
preparation of the Registration Statement, or the failure of CELL GENESYS to
comply with the covenants and agreements contained in Section 8.4 hereof
respecting the sale of the Registrable Shares or any statement or omission in
any prospectus that is corrected in any subsequent prospectus that was delivered
to CELL GENESYS prior to the pertinent sale or sales by CELL GENESYS.

	CELL GENESYS agrees to indemnify and hold harmless the
Company and underwriter (and each person, if any, who controls the Company
within the meaning of Section 15 of the Securities Act, each officer of the
Company who signs the Registration Statement and each director of the Company)
from and against any losses, claims, damages or liabilities to which the Company
(or any such underwriter, officer, director or controlling person) may become
subject (under the Securities Act or otherwise), insofar as such losses, claims,
damages or liabilities (or actions or proceedings in respect thereof) arise out
of, or are based upon, any failure to comply with the covenants and agreements
contained in Section 8.4 hereof respecting sale of the Registrable Shares, or
any Untrue Statement made in the Registration Statement on the effective date
thereof if such Untrue Statement was made in reliance upon and in conformity
with written information furnished by or on behalf of CELL GENESYS specifically
for use in preparation of the Registration Statement, and CELL GENESYS will
reimburse the Company (or such underwriter, officer, director or controlling
person), as the case may be, for any legal or other expenses reasonably incurred
in investigating, defending or preparing to defend any such action, proceeding
or claim; provided that in no event shall any indemnity by CELL GENESYS
under this Section 8.3 exceed the net proceeds received by CELL GENESYS
from the sale of the Registrable Shares covered by such Registration
Statement.

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	Promptly after receipt by any indemnified person of a
written notice of a claim or the beginning of any action in respect of which
indemnity is to be sought against an indemnifying person pursuant to this
Section 8.3, such indemnified person shall notify the indemnifying person in
writing of such claim or of the commencement of such action, and, subject to the
provisions hereinafter stated, in case any such action shall be brought against
an indemnified person and such indemnifying person shall have been notified
thereof, such indemnifying person shall be entitled to participate therein, and,
to the extent it shall wish, to assume the defense thereof, with counsel
reasonably satisfactory to such indemnified person.  After notice from the
indemnifying person to such indemnified person of its election to assume the
defense thereof, such indemnifying person shall not be liable to such
indemnified person for any legal expenses subsequently incurred by such
indemnified person in connection with the defense thereof; provided,
however, that if there exists or shall exist a conflict of interest that
would make it inappropriate, in the opinion of counsel to the indemnified
person, for the same counsel to represent both the indemnified person and such
indemnifying person or any affiliate or associate thereof, the indemnified
person shall be entitled to retain its own counsel at the expense of such
indemnifying person; provided, however, that no indemnifying person shall
be responsible for the fees and expenses of more than one separate counsel for
all indemnified parties; provided, however, that no person guilty of
fraudulent misrepresentation (within the meaning of Section 11(f) of the
Securities Act) shall be entitled to contribution from any person who was not
guilty of such fraudulent misrepresentation.  No indemnifying party, in the
defense of any such claim or litigation shall, except with the consent of each
indemnified party, consent to entry of any judgment or enter into any settlement
which does not include as an unconditional term thereof the giving by the
claimant or plaintiff to such indemnified party of a release from all liability
in respect to such claim or litigation.  No indemnified party may settle or
agree to settle any claim or litigation as to which indemnification may be
sought hereunder without the prior written consent of the indemnifying party.
Each indemnified party shall furnish such information regarding itself or the
claim in question as an indemnifying party may reasonably request in writing as
shall be reasonably required in connection with the defense of such claim and
litigation resulting therefrom.

	If the indemnification provided for in this Section 8.3
is held by a court of competent jurisdiction to be unavailable to an indemnified
party with respect to any loss, liability, claim, damage, or expense referred to
therein, then the indemnifying party, in lieu of indemnifying such indemnified
party hereunder, shall contribute to the amount paid or payable by such
indemnified party as a result of such loss, liability, claim, damage, or expense
in such proportion as is appropriate to reflect the relative fault of the
indemnifying party on the one hand and of the indemnified party on the other in
connection with the statements or omissions that resulted in such loss,
liability, claim, damage, or expense as well as any other relevant equitable
considerations.  The relative fault of the indemnifying party and of the
indemnified party shall be determined by reference to, among other things,
whether the untrue or alleged untrue statement of material fact or the omission
to state a material fact relates to information supplied by the indemnifying
party or by the indemnified party and the parties' relative intent, knowledge,
access to information, and opportunity to correct or prevent such statement or
omission; provided, that in no event shall any contribution by CELL
GENESYS hereunder exceed the net proceeds received by CELL GENESYS from the sale
of the Shares covered by the Registration Statement.

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8.4 Transfer of Shares After Registration; Notice.
CELL GENESYS hereby covenants with the Company not to make any sale of the
Registrable Shares after registration without effectively causing the prospectus
delivery requirement under the Securities Act to be satisfied, and not to make
sale of the Registrable Shares under the Registration Statement if the Company
has notified it that the Registration Statement contains an Untrue
Statement.

8.5 Reporting Requirements. The Company agrees to use
its best efforts to:

	make and keep public information available, as those
terms are understood and defined in Rule 144 under the Securities Act;

	file with the SEC in a timely manner all reports and
other documents required of the Company under the Securities Act and the
Securities Exchange Act of 1934; and

	so long as CELL GENESYS own Registrable Shares, to
furnish to CELL GENESYS forthwith upon request (1) a written statement by the
Company as to whether it complies with the reporting requirements of said Rule
144, the Securities Act and Securities Exchange Act of 1934, and whether it
qualifies as a registrant whose securities may be resold pursuant to SEC Form S-
3, and (2) such other information as may be reasonably requested in availing
CELL GENESYS of any rule or regulation of the SEC that would permit the selling
of the Registrable Shares without registration.

8.6 Limitations on Registration Rights. 

	Notwithstanding the provisions of this Section 8, the
Company may be written notice to CELL GENESYS require that CELL GENESYS
immediately cease sales of shares under the Registration Statement
("Suspended Registration Statement"), in any period during which the
Company is engaged in  any activity or transaction or preparations or
negotiations for any activity or transaction ("Company Activity") that
the Company desires to keep confidential for business reasons, if the Company
determines in good faith that the public disclosure requirements imposed on the
Company under the Securities Act in connection with any such Registration
Statement would require disclosure of the Company Activity; provided, that (i)
that the Company shall not suspend the use of said prospectus more than two
times in any twelve month period and the duration of any one such suspension
shall not be more than thirty (30) days and (ii) the Company shall cause any
Suspended Registration Statement to remain effective for one additional day for
each day, or any portion of a day, that CELL GENESYS was required to cease sales
of shares thereunder, and provided further, that the Company shall use
its commercially reasonable efforts to minimize the duration of any such
suspension; and

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	If the Company requires CELL GENESYS to cease sales of
shares pursuant to Section 8.6(a) above, the Company shall, as promptly as
practicable following the termination of the circumstance which entitled the
Company to do so, give prompt written notice to CELL GENESYS that such
circumstance has terminated and that it may resume sales pursuant to the
Suspended Registration Statement has been amended to comply with the
requirements of the Securities Act, the Company shall enclose such revised
prospectus with the notice to CELL GENESYS given pursuant to this section 8.6(b)
and CELL GENESYS shall make no offers or sales of shares pursuant to such
Suspended Registration Statement other than by means of such revised prospectus.
The foregoing provisions of this Section 8.6 shall in no manner diminish or
otherwise impair the Company's obligations under Section 8.2 and Section 8.3
hereof.

8.7 Termination of Obligations. The obligations of the
Company pursuant to Sections 8.2 through 8.5 hereof with respect to Registrable
Shares issued at a Closing and the Registration Statement covering such Shares
shall cease and terminate at such time as all of such Registrable Shares (i)
have been resold or (ii) such time as all of such Registrable Shares held by
such Purchaser may be sold during any 90 day period pursuant to Rule 144,
including Rule 144 (k), without being restricted by the volume limitations of
Rule 144(e).

 SECTION 9
Notices.

All notices or other communications which are required or
permitted hereunder shall be in writing and addressed as follows:

	
if to CELL GENESYS:
	
Cell Genesys, Inc.

	 	
342 Lakeside Drive

	 	
Foster City, CA 94404 

	
Attention:
	
Robert Tidwell

	 	
Vice President, Corporate Development

	
With a copy to:
	
Cooley Godward LLP

	

	
3000 El Camino Real

	 	
Palo Alto, CA 94306-2155

	
Attention:
	
Barclay J. Kamb, Esq.

	
if to the Company:
	
Transkaryotic Therapies, Inc.

	

	
195 Albany Street

	 	
Cambridge, MA  02139

	
Attention:
	
Richard F Selden

	 	
President and Chief Executive Officer

	
With a copy to:
	
Kerry A. Flynn

	

	
Senior Director, Business Development

or to such other address as the party to whom notice is to be
given may have furnished to the other party in writing in accordance herewith.
Any such communication shall be deemed to have been given when delivered if
personally delivered or sent by facsimile (provided that the party providing
such notice promptly confirms receipt of such transmission with the other party
by telephone), on the business day after dispatch if sent by a nationally-recognized
overnight courier and on the third business day following the date of
mailing if sent by mail, postage prepaid, return receipt requested.

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 SECTION 10
 Miscellaneous.

10.1 Waivers and Amendments. Neither this Agreement nor
any provision hereof may be changed, waived, discharged, terminated, modified or
amended except upon the written consent of the parties hereto.

10.2 Headings. The headings of the various sections of
this Agreement have been inserted for convenience of reference only and shall
not be deemed to be part of this Agreement.

10.3 Severability. If any provision hereof should be
held invalid, illegal or unenforceable in any respect, then, to the fullest
extent permitted by law, (a) all other provisions hereof shall remain in full
force and effect and shall be liberally construed in order to carry out the
intentions of the Parties as nearly as may be possible and (b) the parties shall
use their best efforts to replace the invalid, illegal or unenforceable
provision(s) with valid, legal and enforceable provision(s) which, insofar as
practical, implement the purposes of such provision(s) in this
Agreement.

10.4 Governing Law. This Agreement shall be governed by
and construed in accordance with the laws of the State of Delaware as applied to
contracts entered into and performed entirely in the State of Delaware by
Delaware residents, without regard to conflicts of law principles.

10.5 Counterparts. This Agreement may be executed in
two or more counterparts, each of which shall constitute an original, but all of
which, when taken together, shall constitute but one instrument, and shall
become effective when one or more counterparts have been signed by each party
hereto and delivered to the other parties.

10.6 Successors and Assigns. Except as otherwise
expressly provided herein, the provisions hereof shall inure to the benefit of,
and be binding upon, the successors, assigns, heirs, executors and
administrators of the parties hereto, provided that CELL GENESYS shall not
assign its rights or obligations hereunder unless CELL GENESYS assigns such
rights in whole and not in part to an assignee of such rights and obligations
which shall agree in writing with the Company to be bound by this Agreement.
The Company shall not, directly or indirectly, enter into any merger,
consolidation or reorganization in which the Company shall not be the surviving
corporation unless the proposed surviving corporation shall, prior to such
merger, consolidation or reorganization, agree in writing to assume the
obligations of the Company under this Agreement; provided, however, that the
provisions of this Section 10.6 shall not apply in the event of any merger,
consolidation or reorganization in which the Company is not the surviving
corporation if CELL GENESYS is entitled to receive in exchange for their
Registrable Shares consideration consisting solely of (i) cash, or (ii)
securities of the acquiring corporation which may be immediately sold to the
public without registration under the Securities Act.

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10.7 Expenses. Each party shall pay all costs and
expenses that it incurs with respect to the negotiation, execution, delivery and
performance of this Agreement.  

10.8 Entire Agreement. This Agreement, the License
Agreements and other documents delivered pursuant hereto and thereto, including
the exhibits, constitute the full and entire understanding and agreement between
the parties with regard to the subjects hereof and thereof.

10.9 Publicity. No party shall issue any press releases
or otherwise make any public statement with respect to the transactions
contemplated by this Agreement without the prior written consent of the other
party, except as may be required by applicable law or regulations, in which case
such party shall provide the other parties with reasonable notice of such
publicity and/or opportunity to review such disclosure.

10.10 Waiver of Rule of Construction.  Each Party has
had the opportunity to consult with counsel in connection with the review,
drafting and negotiation of this Agreement.  Accordingly, the rule of
construction that any ambiguity in this Agreement shall be construed against the
drafting Party shall not apply.

10.11 Termination. This Agreement shall terminate when
the Company's obligation to issue additional shares under the License Agreement
terminates. 

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Commission.  Confidential treatment has been requested with respect to the omitted portions. 

In Witness Whereof, the parties hereto have caused
this Stock Purchase Agreement to be executed by their duly authorized
representatives as of the day and year first above written.

	
Transkaryotic Therapies, Inc.
	
Cell
Genesys, Inc.

	
By:__________________________

	
By:__________________________

	
Name: 
	
Name:

	
Title: 
	
Title:

                          i

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Commission.  Confidential treatment has been requested with respect to the omitted portions.

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