Exhibit 10.31

Confidential
Materials omitted and filed separately with the
Securities and Exchange Commission. Asterisks denote omissions.

LICENSE AGREEMENT
between
TRANSKARYOTIC THERAPIES, INC.
and
CELL GENESYS, INC.

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LICENSE AGREEMENT

ThisLICENSE AGREEMENT(the "Agreement"), effective as of
June 7, 2002 (the "Effective Date"), is betweenTRANSKARYOTIC 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"), andCELL
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 [**]
proteins (as may be adjusted pursuant to Section 2.3 hereof), [**] thereof that have [**], 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"Gene Therapy" shall mean the treatment or prevention of a disease by
means of Ex Vivo or In Vivo delivery (via viral or nonviral gene transfer systems) of compositions comprising either (a) [**] Designated Protein, wherein such
[**] Designated Protein [**] in the treatment or prevention of such disease, [**] (b) [**] Designated
Protein, wherein such [**] composition [**] Designated Protein [**] Designated Protein) which Designated Protein is used as a
[**] for the composition or (c) [**] Designated Protein that serves [**] in the treatment or prevention of such disease, wherein
such composition [**] Designated Protein [**] Designated Protein) which Designated Protein is used as a [**] for the
composition. As used in this section, (i) "Ex Vivo" delivery shall mean the introduction, outside of the body of a human, of such compositions into a cell, tissue, organoid, or organ, followed
by the administration of cell, tissue, organoid, or organ which contains such introduced compositions into the body of the same (autologous) or different (allogeneic) human, without limitation as to
the formulation, anatomic site, or route of administration or the use of encapsulation or other devices for such administration, and (ii) "In Vivo" delivery shall mean the introduction of such
compositions into

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an individual, without limitation as to the formulation, anatomic site, or route of administration or the use of encapsulation or other devices for such administration.

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

2.1License Grant.Subject to the terms and conditions of this Agreement,
CELL GENESYS grants to TKT (i) an exclusive (even as to CELL GENESYS), worldwide, fully paid license (with the right to sublicense) under the Patent Rights to make, have made, use, sell, offer
for sale and import [**] Licensed Products, and (ii) a non-exclusive, worldwide, fully paid license (with the right to sublicense) under the Patent Rights to make, have
made, use, and import [**] Licensed Products solely for use to produce [**] Licensed Products. For clarity, TKT shall obtain no license or other rights
under the Patent Rights for Gene Therapy purposes.

2.2Identification 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.

2.3Substitution 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 [**].

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ARTICLE III

CONFIDENTIALITY/PRESS RELEASES

3.1Nondisclosure 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:



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



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

3.4Confidential 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

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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:

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.

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ARTICLE V

REPRESENTATIONS, WARRANTIES AND INDEMNIFICATION

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



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



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

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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.

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:



or



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.

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ARTICLE VI

INTELLECTUAL PROPERTY PROVISIONS

6.1Filing, Prosecution and Maintenance of Patents



6.2    Opposition, Reexamination and Reissue.



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6.3    Enforcement and Defense.



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.

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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 ninety (90) days 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 excludingfrom the foregoing rights any [**] under the Patent Rights that have been [**] prior to
the date of breach.

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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.

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.

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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:

if to CELL GENESYS, to:

Cell Genesys, Inc.
342 Lakeside Drive
Foster City, CA 94404Attention:

Robert Tidwell
Vice President, Corporate Developmentwith a copy to:

Cooley Godward LLP
3000 El Camino Real
Palo Alto, CA 94306-2155Attention:

Barclay J. Kamb, Esq.if to TKT, to:

Transkaryotic Therapies, Inc.
195 Albany Street
Cambridge, MA 02139Attention:

Michael J. Astrue
Senior Vice President, Administrationwith 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.

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.

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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.

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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:                           |  | /s/MICHAEL J. ASTRUE
|                              |  |                     
|                              |  | Michael J. Astrue   
|                              |  | Title:              

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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"):



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

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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:



SECTION 2    Closing and Delivery

2.1    Sale of SharesSubject 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:



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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.

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.

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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.



3.7    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.

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.

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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.



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



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:



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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.

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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:

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.

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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:



8.2    Registration Procedures and Expenses.The Company shall:



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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

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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:



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).

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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 94404Attention:

Robert Tidwell
Vice President, Corporate Developmentwith a copy to:

Cooley Godward LLP
3000 El Camino Real
Palo Alto, CA 94306-2155Attention:

Barclay J. Kamb, Esq.if to the Company:

Transkaryotic Therapies, Inc.
195 Albany Street
Cambridge, MA 02139Attention:

Richard F Selden
President and Chief Executive Officerwith 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.

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.

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

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:                           |  |                   
|                              |  | Name:
Title:      

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