Exhibit 99.4


To:   Lynx Therapeutics, Inc.,
      25861 Industrial Boulevard
      Hayward
      California 94545
      (the "OFFEROR")

                                                                 _________, 2004

Dear Sirs,

1.       All defined terms herein shall have the meanings given in the Company
         Support Agreement between Offeror and Offeree dated 28 September 2004
         ("CSA") unless otherwise defined herein.

2.       In consideration of the Offeror agreeing to make the Offer we hereby
         irrevocably undertake, represent and warrant to the Offeror as
         follows:-

         (A)      we shall accept or procure acceptance of the Offer in
                  accordance with its terms in respect of all the Shares held by
                  the Offeree by not later than 5.00 p.m. California time on the
                  tenth Business Day after the despatch of the Offer Documents,
                  and shall forward or procure that there is forwarded, with
                  such acceptance, the share certificates or other documents of
                  title in respect of the Shares in accordance with the terms of
                  the Offer;

         (B)      we shall not withdraw our acceptance(s) and shall procure that
                  our acceptance(s) is not withdrawn in respect of all or any of
                  the Shares;

         (C)      until the earlier of the First Closing Date or the Expiration
                  Date the Offeree shall not accept or solicit any exchange
                  offer commenced by a third party other than Parent or any
                  Subsidiary of Parent with respect to the Shares.

3.       We agree and consent to the deletion of Clause 2 of the CSA
         ("Irrevocable Undertaking to Accept Offer"), and such Clause 2 shall
         forthwith cease to have effect. All other terms of the CSA shall remain
         valid and binding.

4.       This undertaking shall not be binding upon Offeree in the event that
         the Acquisition Agreement is amended by the parties thereto to lower or
         change the form of consideration set forth in the Acquisition
         Agreement.

5.       This undertaking shall be governed by, and construed in accordance
         with, English law and the English courts shall have exclusive
         jurisdiction to determine all disputes in relation to it. We agree that
         if we should fail to accept, or procure the acceptance of, the Offer in
         accordance with our obligations under this undertaking or should
         otherwise be in breach of any of our obligations under this
         undertaking, an order of specific performance will be the only adequate
         remedy.

6.       In the case where the Shares are registered in the name of a nominee,
         we shall direct the nominee to act as if the nominee were bound by the
         terms of this irrevocable undertaking and we shall use our best
         endeavours to do all acts and things necessary to carry the terms
         hereof into effect as if we had been the registered holder of the
         Offeree Shares registered in the name of such nominee.

7.       In the event that the First Closing shall not have taken place before
         31 March 2005 this irrevocable undertaking shall automatically lapse
         and be of no further force or effect and no party to it shall have any
         claim against the other save in respect of any antecedent breach of its
         terms.

8.       The benefit of this undertaking may not be assigned by you or your
         successors.

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9.       This undertaking may be executed in more than one part.

This undertaking is entered into on the date appearing at the top of page 1.

We agree and accept the terms of this undertaking.

--------------------------
Director
For and on behalf of
Lynx Therapeutics, Inc.

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