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                                                                   EXHIBIT 10.23


March 30, 2000

Incyte Pharmaceuticals, Inc.
SmithKline Beecham plc
SmithKline Beecham Corporation

Re:  Collaboration and License Agreement between and among SmithKline Beecham
     plc and SmithKline Beecham Corporation (collectively, "SB"), Incyte
     Pharmaceuticals, Inc. ("Incyte"), and diaDexus, LLC ("diaDexus") dated
     September 2, 1997 as amended (the "Agreement")

Ladies and Gentlemen:

     This letter is intended to clarify and confirm the understandings of the
parties to the Agreement in respect of certain change of control transactions
involving diaDexus. In particular, the parties hereby confirm that,
notwithstanding Section 19.2 or any other provision of the Agreement, the
Agreement shall continue in full force and effect and be binding upon and
assignable (either expressly or by operation of law) to any successor in
interest of diaDexus upon any merger, consolidation, reorganization or
acquisition of stock or assets or similar transaction affecting all or
substantially all of the assets or voting control of diaDexus (collectively
"Merger"), without the need for any consent of the parties to the Agreement;
provided, however, that in the event of a Merger (i) in which the stockholders
of record as constituted immediately prior to such Merger will, immediately
after such Merger (by virtue of securities issued in connection with such
Merger), hold less than 50% of the voting power of the surviving entity or other
successor, (ii) occurring prior to September 2, 2001, and (iii) in which the
surviving entity or other successor in interest to diaDexus is a Pharmaceutical
Company (as defined below) (a "Restricted Merger"), SB's obligations pursuant to
Sections 4.2-4.5, 4.6.1, 4.6.3, 4.8, and 9.1 of the Agreement shall immediately
terminate; and further provided that as of the closing of Restricted Merger (the
"Merger Date") and until September 2, 2001:

     (1)  SB grants to diaDexus an exclusive, world-wide license under SB
          Diagnostic Patents with the right to sublicense pursuant to Section
          4.7, to Make, Have Made, Use, and Sell cDx Homebrews and cDx Kits in
          the field of Class I Dx Tests, Class II Dx Tests, all for which
          diaDexus has filed a Research Plan pursuant to Section 1.1, subject to
          all of the terms and conditions of the Agreement; and

     (2)  SB grants to diaDexus a non-exclusive, world-wide license under SB
          Diagnostic Patents with the right to sublicense pursuant to Section
          4.7, to Make, Have Made, Use, and Sell rDx Services and rDx Kits in
          the field of Class I Dx, all for which diaDexus has filed a Research
          Plan pursuant to Section 1.1, subject to all of the terms and
          conditions of the Agreement; and



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        (iii) after the Merger Date, SB shall not be required to disclose any
              licensed SB Diagnostic Patents to diaDexus unless requested to do
              so by diaDexus pursuant to Section 14.2 and the text of such SB
              Diagnostic Patent has become published.

        For purposes of this letter agreement, "Pharmaceutical Company" shall
mean any third party that performs ethical pharmaceuticals research and
development activities or an affiliate (as defined in Rule 405 under the
Securities Act of 1933, as amended) of such a third party.

        For each cDx Kit, rDx Service and rDx Kits that is based on or derived
from SB Diagnostic IP, diaDexus or its licenses, as appropriate, shall provide
SB with non-exclusive access to such Kits or Services on Commercially reasonable
terms.

        The parties additionally agree to delete (i) Sections 7.1 and 7.2 of the
Agreement and (ii) to terminate all of Incyte's obligations with respect to any
Incyte Technology IP, Incyte Technology Know-How and Incyte Technology Patents
to diaDexus.

        Capitalized terms used but not otherwise defined herein shall have the
meanings given them in the Agreement.

        The parties hereby acknowledge and agree that, as of the date of this
letter agreement, this letter agreement is a binding and enforceable agreement
among the parties with respect to the matters set forth herein.

        Except as expressly set forth above, the terms and of the Agreement
shall continue in full force and effect from and after the date hereof.





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        Please indicate your acceptance of and agreement with the foregoing by
countersigning a copy of this letter in the space provided below.


                               DIADEXUS LLC


                               By: /s/ PATRICK PLEWMAN
                                  -----------------------------
                               Name: Patrick Plewman
                               Title: President and Chief Operating Officer


Accepted and Agreed to:


INCYTE PHARMACEUTICALS, INC.

By: /s/ ROY A. WHITFIELD
   ------------------------------
Name: Roy A. Whitfield
Title: C.E.O.


SMITHKLINE BEECHAM, PLC

By: /s/ DONALD F. PARMAN
   ------------------------------
Name: Donald F. Parman
Title: Attorney-in-Fact


SMITHKLINE BEECHAM CORPORATION

By: /s/ DONALD F. PARMAN
   ------------------------------
Name: Donald F. Parman
Title: Secretary