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EXHIBIT 10.35



April 20, 2000

Walter Gilbert, Ph.D.
Carl M. Loeb University Professor
Department of Molecular and Cellular Biology
Harvard University
15 Gray Gardens West
Cambridge, MA  02138

Dear Wally:

We are pleased to offer you a position on Transkaryotic Therapies, Inc. ("TKT"
or the "Company") Board of Directors effective as of April 20, 2000, upon the
following terms and conditions:

1.  TERM. Unless you earlier resign or are removed, you will serve until the
    next annual meeting of the Company's stockholders and until your successor
    is duly elected and qualified.

2.  DIRECTORS' FEES. As compensation for your services, you will receive a
    Director's fee of $1,000 for attendance at each meeting of the Board
    including Annual Meeting, other than telephonic meetings. The Company will
    also reimburse you for reasonable out-of-pocket expenses incurred by you in
    the performance of your duties as a Director of the Company upon
    presentation to the Company of an itemized and appropriately documented
    invoice for such expenses.

3.  STOCK. Upon your election to the Board of Directors at this year's annual
    meeting, the Company will grant you an option to purchase 2,500 shares of
    Common Stock at an exercise price per share equal to the fair market value
    on that date. The options will be granted under the terms and conditions of
    the 1993 Directors' Plan. Thereafter, at each subsequent annual meeting,
    contingent upon your continued election, you will receive an annual option
    grant under the 1993 Directors' Plan to purchase 6,750 shares of Common
    Stock at an exercise price per share equal to the fair market value on that
    date.

4.  CONFIDENTIALITY.

         (a)  You recognize and acknowledge that the Company's trade secrets,
              know-how and proprietary processes as they may exist



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              from time to time (including, without limitation, information
              regarding methods, cultures, vectors, plasmids, synthesis
              techniques, nucleic acid sequences, purification techniques and
              assay procedures) as well as the Company's confidential business
              plans and financial data are valuable, special and unique assets
              of the Company's business, access to and knowledge of which are
              essential to the performance of your duties as a Director of the
              Company. You agree that you will not, while you are serving as a
              Director of the Company or thereafter, in whole or in part,
              disclose such secrets, know-how, processes, business plans or
              financial data to any person, firm, corporation, association or
              other entity for any reason or purpose whatsoever, nor shall you
              make use of any such property for your own purposes or for the
              benefit of any person, firm, corporation or other entity (except
              the Company) under any circumstances while you are serving as a
              Director of the Company or thereafter, provided that these
              restrictions shall not apply to such secrets, know-how and
              processes which you can establish by competent proof:

                   (i)  were known to you, as evidenced by written documents,
                        other than under binder of secrecy, prior to your
                        serving as a Director of the Company;

                   (ii) have passed into the public domain prior to or after
                        their development by or for the Company, other than
                        through acts or omissions attributable to you; or

                   (iii) were subsequently obtained by you, other than under
                        binder of secrecy, from a third party not acquiring the
                        information under an obligation of confidentiality from
                        the disclosing party.

         (b)  Upon your resignation, removal or completion of your term as a
              Director of the Company, you shall promptly turn over to the
              Company all originals and copies of the Company's confidential
              information described in this Section 4 then in your possession or
              under your control.

5.  COVENANTS NOT TO COMPETE.

         (a)  While you serve as a Director of the Company, you shall not engage
              in any business (whether as an officer, director, owner, employee,
              partner, consultant, advisor or other direct or


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              indirect participant) engaged in the development of gene
              activation methods, and/or the sale of products or rendering of
              services related to gene activation, and/or to any other
              activities which directly compete with the Company's business
              activities.

         (b)  During the period in which this covenant not to compete is in
              effect you also shall not interfere with, disrupt or attempt to
              disrupt the relationship, contractual or otherwise, between the
              Company and any customer, supplier, lessor, lessee, employee,
              consultant, research partner or investor of the Company.

         (c)  It is the desire and intent of the parties that the provisions of
              this Section 5 shall be enforced to the fullest extent permissible
              under the laws and public policies applied in each jurisdiction in
              which enforcement is sought. Accordingly, if any particular
              subsection or portion of this Section 5 shall be deemed amended to
              delete therefrom the portion thus adjudicated to be invalid or
              unenforceable, such deletion shall apply only with respect to the
              operation of this Section in the particular jurisdiction in which
              such adjudication is made.

If you agree to accept this offer, please so indicate by countersigning the
enclosed duplicate of this letter and returning it to me.

                                           Very truly yours,

                                           TRANSKARYOTIC THERAPIES, INC.

                                           By: /s/ RICHARD F SELDEN
                                           ---------------------
                                           Richard F Selden
                                           President & Chief Executive Officer

Accepted and agreed to:

/s/ WALTER GILBERT
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Walter Gilbert

4/20/00
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Date