Document:

<PAGE>   1
                                                                   EXHIBIT 10.19

                             CONFIDENTIAL TREATMENT
                                   REQUESTED

<TABLE>
<S>                                                   <C>                                      <C>
***** Certain information on this page has been       The asterisked portions of this             REDACTED FOR
      omitted and filed separately with the           document have been omitted and are        CONFIDENTIALITY
      Securities and Exchange Commission.             filed separately with the Securities
      Confidential treatment has been requested       and Exchange Commission.
      with respect to the omitted portions.
</TABLE>

                               AMENDMENT NO. 2 TO
                      COLLABORATION AND LICENSE AGREEMENT

     This Amendment No. 2 to Collaboration and License Agreement is made as of
July __, 1998, by and among SmithKline Beecham Corporation, a Pennsylvania
corporation with offices at One Franklin Plaza, Philadelphia, Pennsylvania
19101; SmithKline Beecham plc. an English corporation with its principal offices
at New Horizons Court, Brantford, Middlesex, TW8 9SEP, England; Incyte
Pharmaceuticals, Inc., a Delaware corporation with offices at 3174 Porter Drive,
Palo Alto, California 94304; and diaDeXus, a Delaware limited liability company
with offices at 3233 Scott Boulevard, Suite 102, Santa Clara, California 95054.

     The parties to this Amendment No. 2 are parties to a Collaboration and
License Agreement, dated as of September 2, 1997, as amended by that certain
Amendment dated as of February 28, 1998 (the "Collaboration Agreement");

     NOW, THEREFORE, the parties hereby agree as follows:

     1.  Appendix C, "Incyte Diagnostic Patents", is hereby amended to delete
the patents and patent applications listed in Attachment 1 to this Amendment
No. 2, which patents and patent applications are solely related to **********

     2.  Except as modified by this Amendment No. 2, the Collaboration Agreement
remains in effect pursuant to its terms.

     IN WITNESS WHEREOF, the parties, through their authorized representatives,
have executed this Amendment No. 2 as of the date first written above.

SmithKline Beecham Corporation

By  /s/ [SIGNATURE ILLEGIBLE]
   -----------------------------

SmithKline Beecham plc

By  /s/ [SIGNATURE ILLEGIBLE]
   -----------------------------

Incyte Pharmaceuticals, Inc.

By  /s/ [SIGNATURE ILLEGIBLE]
   -----------------------------

diaDeXus, LLC

By  /s/ [SIGNATURE ILLEGIBLE]
   -----------------------------
<PAGE>   2

                                  Attachment 1

<TABLE>
<CAPTION>
                              File                                           Patent
    Country          Ref.#    Type   Filing Date   Serial No.   Issue Date    No.     Status
------------------   -----    ----   -----------   ----------   ----------   ------   ------
<S>                  <C>      <C>    <C>           <C>          <C>          <C>      <C>
Australia            *****    DCA    02/14/90      *****        07/19/94     *****    Issued

Australia            *****    DCA    08/13/91      *****        06/29/95     *****    Issued

Australia            *****    DCA    09/28/92      *****        02/27/96     *****    Issued

Australia            *****    DCA    07/01/94      *****                              Pending

Australia            *****    DCA    04/29/94      *****                              Pending

Canada               *****    DCA    02/14/90      *****                              Pending

Canada               *****    DCA    08/13/91      *****                              Pending

Canada               *****    DCA    09/28/92      *****                              Pending

Canada               *****    DCA    07/01/94      *****                              Pending

Canada               *****    DCA    04/29/94      *****                              Pending

European Patent CO   *****    DCA    02/14/90      *****                              Pending

European Patent CO   *****    DCA    08/13/91      *****                              Pending

European Patent CO   *****    DCA    09/28/92      *****                              Pending

European Patent CO   *****    DCA    07/01/94      *****                              Published

European Patent CO   *****    DCA    04/29/94      *****                              Pending

Israel               *****    DCA    02/12/90      *****                              Pending

Japan                *****    DCA    02/14/90      *****                              Pending

Japan                *****    DCA    08/13/91      *****                              Pending

Japan                *****    DCA    09/28/92      *****                              Pending

Japan                *****    DCA    07/11/94      *****                              Pending

Japan                *****    DCA    04/29/94      *****                              Pending

South Korea          *****    DCA    02/14/90      *****                              Pending

South Korea          *****    DCA    08/13/91      *****                              Pending

New Zealand          *****    DCA    02/13/90      *****                              Pending
</TABLE>

***** Certain information on this page has been omitted and filed separately
      with the Securities and Exchange Commission. Confidential treatment has
      been requested with respect to the omitted portions.
<PAGE>   3

                                  Attachment 1

<TABLE>
<CAPTION>
                              File                                           Patent
    Country          Ref.#    Type   Filing Date   Serial No.   Issue Date    No.     Status
------------------   -----    ----   -----------   ----------   ----------   ------   ------
<S>                  <C>      <C>    <C>           <C>          <C>          <C>      <C>
United States        *****    NEW    02/14/89      *****                              Abandoned

United States        *****    CIP    08/13/90      *****                              Abandoned

United States        *****    CIP    04/05/91      *****        12/15/92      ****    Issued

United States        *****    CIP    07/03/91      *****                              Abandoned

United States        *****    DCA    05/21/93      *****                              Abandoned

United States        *****    CIP    07/22/92      *****                              Pending

United States        *****    CIP    07/02/93      *****                              Abandoned

United States        *****    DCA    12/03/93      *****                              Abandoned

United States        *****    CIP    04/30/93      *****                              Abandoned

United States        *****    DCA    05/01/95      *****                              Pending

United States        *****    CIP    01/22/90      *****        02/18/92              Issued

United States        *****    DIV    12/14/92      *****        05/03/94              Issued

United States        *****    DIV    12/14/92      *****        08/02/94              Issued

United States        *****    CIP    09/27/91      *****        08/10/93              Issued

United States        *****    CON    11/13/96      *****                              Pending

United States        *****    CON    11/13/96      *****                              Pending

United States        *****    CON    04/18/97      *****                              Pending

WIPO                 *****    CEQ    02/14/90      *****                              Pending

WIPO                 *****    CEQ    08/13/91      *****                              Pending

WIPO                 *****    CEQ    09/28/92      *****                              Pending

WIPO                 *****    CEQ    07/01/94      *****                              Pending

WIPO                 *****    CEQ    04/29/94      *****                              Pending

WIPO                 *****    CEQ    05/01/95      *****                              Pending
</TABLE>

***** Certain information on this page has been omitted and filed separately
      with the Securities and Exchange Commission. Confidential treatment has
      been requested with respect to the omitted portions.<PAGE>   1
                                                                   Exhibit 10.21

                                                                    REDACTED FOR
                                                                 CONFIDENTIALITY
Execution Copy
                             CONFIDENTIAL TREATMENT
                                   REQUESTED

               The asterisked portions of this document have been
            omitted and are filed separately with the Securities and
                              Exchange Commission

             AMENDMENT NO. 3 TO COLLABORATION AND LICENSE AGREEMENT

          This amendment (the "Amendment") is made this 28 day of July, 1999,
among SmithKline Beecham Corporation, a Pennsylvania corporation with offices
at One Franklin Plaza, Philadelphia, Pennsylvania 19101, USA (hereunder "SB
Corporation"): SmithKline Beecham plc. an English corporation with offices at
New Horizons Court, Brentford, Middlesex, TW8 9EP, England (hereinafter "SB
plc"). Quest Diagnostics Incorporated, a Delaware corporation with offices at
One Malcolm Avenue, Teterboro, NJ 07608 (hereinafter "Quest Diagnostics"),
diaDexus LLC, a Delaware limited liability company with offices at 3303
Octavius Drive, Santa Clara, CA 95054 (hereinafter "diaDexus") and Incyte
Pharmaceuticals, Inc, a Delaware corporation with offices at 3174 Porter Drive,
Palo Alto, CA 94304 (hereinafter "Incyte").

                                  WITNESSETH:

     WHEREAS, SB Corporation, SB plc, diaDexus and Incyte are parties to that
certain Collaboration and License Agreement dated September 2, 1997, as amended
(the "Collaboration Agreement");

     WHEREAS, pursuant to the terms of the Collaboration Agreement, diaDexus
and Incyte granted SB Corporation and SB plc certain beneficial rights (the
"diaDexus Rights") that relate to the conduct of the business of SmithKline
Beechan Clinical Laboratories ("SBCL");

     WHEREAS, SB Corporation and SB plc have entered into that certain Stock
and Asset Purchase Agreement dated February 9, 1999 (hereinafter the "Stock and
Asset Purchase Agreement") pursuant to which SB plc has agreed to sell its
interest in SBCL to Quest;

     WHEREAS, in connection with the Stock and Asset Purchase Agreement, SB plc
has agreed to use its reasonable best efforts to assign the diaDexus Rights to
Quest; and

     WHEREAS, SB Corporation, SB plc, Incyte, diaDexus and Quest desire to
assign the diaDexus Rights to Quest and otherwise amend the Collaboration
Agreement in accordance with the terms and conditions of this Amendment.

     NOW, THEREFORE, in consideration of the covenants and obligations
expressed herein and other good and valuable consideration, the receipt and
adequacy of which is hereby acknowledged, the parties as follows:

1.   DEFINITIONS.

     1.1  All capitalized words used herein shall have the meaning set forth in
the Collaboration Agreement, except as expressly otherwise defined herein.

                                       1
<PAGE>   2

     1.2  The definition of "Affiliate" is hereby deleted in its entirety and
replaced with the following:

           "Affiliate(s)": shall mean individually and collectively
           Incyte Affiliates, DiaDexus Affiliates, SB Affiliates and
           Quest Affiliates, as the case may be. For the purposes of
           this Agreement, DiaDexus shall not be an Affiliate of
           either SB or Incyte.

     1.3  "Quest" shall mean Quest Diagnostics and any Quest Affiliates to
which any rights or obligations shall be assigned and/or delegated pursuant to
the Collaboration Agreement.

     1.4  "Quest Affiliate": shall mean (i) each of those entities listed on
Appendix A hereto for so long as each such entity continues to be owned by
Quest in at least the amounts set forth on Appendix A and (ii) any corporation,
firm, partnership or other legal entity, which is directly or indirectly owned,
by Quest to the extent of which the common stock or other equity ownership
thereof is one hundred percent (100%) owned by Quest; provided however that
where local laws require a minimum percentage of local ownership, the status of
a Quest Affiliate will be established if Quest directly or indirectly owns or
controls one hundred percent (100%) of the maximum ownership percentage that
may, under such local laws, be owned or controlled by foreign interests.

     1.5  "Clinical Laboratory Services": shall mean clinical laboratory,
anatomic pathology or other diagnostics testing services (including, without
limitation, routine and esoteric clinical laboratory services (including
genetics testing), clinical laboratory services involved with clinical trials,
point-of-care testing, clinical laboratory services involving corporate
healthcare and services involved with managing hospital laboratories)
or providing to any unaffiliated person, in any manner, directly or
indirectly, data or information products or services which substantially
consists of laboratory data.

     1.6  The definition of "Clinical Diagnostic(s)" is hereby deleted in its
entirety and replaced with the following:

          "Clinical Diagnostic(s)" ("cDx"): shall mean test(s)
          performed on a human tissue or other human biological
          sample, the results of which are provided to payors,
          providers, or patients for use in the clinical management of
          individual patients, which become part of such patient's
          health record; provided, however, that Clinical Diagnostics
          shall expressly exclude test(s) performed on human tissue or
          other human biological sample in the course of Phase I, II,
          III or IV clinical trials for an ethical pharmaceutical.

     1.7  The definition of "Homebrew Territory," as it related to Quest, shall
mean the countries and territories that would be considered Quest's Homebrew
Territory if

                                       2
<PAGE>   3

Quest was a Party to the Collaboration Agreement (i.e., reading all references
to "a Party" to include "Quest" in such definition).

     1.8  The definition of "Research Diagnostics" is hereby deleted in its
entirety and replaced by the following:

          "Research Diagnostic(s)" ("rDx"): shall mean test(s)
          performed on a commercial basis on a tissue or other
          biological sample, the results of which are provided to any
          party for any purpose, excluding use as Clinical Diagnostic.
          For purposes of clarity, rDx includes test(s) performed on
          human tissue or other human biological sample in the course
          of Phase I, II, III or IV clinical trials for an ethical
          pharmaceutical.

     1.9  "Quest Closing" shall mean the date of the closing of the
transactions contemplated by Stock and Asset Purchase Agreement between SB and
Quest relating to the sale of SBCL to Quest.

2.   SPECIFIC PROVISIONS.

     2.1  Paragraph 3.1.3 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          3.1.3     Notwithstanding anything else to the contrary,
          Incyte does not grant SB or Quest any rights pursuant to
          this Agreement under Incyte IP, Incyte Technology IP, or
          Incyte Diagnostic IP.

     2.2  Paragraph 3.5 of the Collaboration Agreement is hereby deleted in its
entirety.

     2.3  Paragraph 3.8 of the Collaboration Agreement is hereby deleted in its
entirety and replaced with the following:

          3.8  Other: Incyte agrees to negotiate with Quest in good
          faith for access to Incyte Technology IP which is required
          for the commercially reasonable conduct of cDx Homebrews
          licensed to Quest by DiaDexus pursuant to Section 5.

     2.4  Paragraph 4.1.5 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

                                       3
<PAGE>   4
          4.1.5     Notwithstanding anything else to the contrary, SB
          does not grant Incyte or Quest any rights pursuant to this
          Agreement under SB IP, SB Technology IP, or SB Diagnostic
          IP.

     2.5  Paragraph 4.6.5 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          4.6.5     DiaDexus hereby acknowledges that Incyte has
          ongoing research and development programs involving material
          investment by Incyte to produce and develop Incyte IP
          independent of that which would constitute Incyte Diagnostic
          IP, including Incyte IP which is or shall be provided to
          third parties on a non-exclusive commercial basis as Incyte
          database product(s), including Ix Product(s). Incyte shall
          provide SB and DiaDexus with access on commercial terms to
          those Ix Product(s), or other Incyte database products which
          are made generally available by Incyte to other Third
          Parties on a non-exclusive basis. Incyte shall not refuse to
          negotiate in good faith with Quest with respect to access on
          commercial terms to those Ix Product(s), or other Incyte
          database products which are made generally available by
          Incyte to other Third Parties on a non-exclusive basis.

     2.6  Paragraph 5.1.3 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          5.1.3     DiaDexus retains all rights under DiaDexus IP
          unless expressly granted to SB, Incyte and/or Quest pursuant
          to the terms and conditions of this Agreement.

     2.7  Paragraph 5.1.4 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          5.1.4     All rights and licenses granted by DiaDexus to
          Quest and/or Incyte pursuant to Paragraphs 5.2, 5.3, and
          5.4, shall include a non-exclusive sublicense or exclusive
          sublicense, as the case may be, by DiaDexus to any SB
          Diagnostic IP and/or Incyte Diagnostic IP each only as may be
          necessary to exercise such right or license. Such license
          shall only be granted exclusively hereunder to Incyte if
          Incyte had granted the license exclusively to DiaDexus in
          Section 3, and if not, such sublicense shall only be granted
          by DiaDexus to Incyte pursuant to this Paragraph,
          non-exclusively. Such license shall only be granted
          exclusively hereunder to Quest if SB had granted the license
          exclusively to DiaDexus in Section 4,

                                       4
<PAGE>   5
          and if not, such sublicense shall only be granted by DiaDexus to
          Quest pursuant to this Paragraph non-exclusively.

     2.8  Paragraph 5.2, 5.2.1 and 5.2.2 of the Collaboration Agreement are
hereby deleted in their entirety and replaced with the following:

          5.2   DiaDexus Class I Dx Test(s) Grants: Subject to all the terms and
          conditions of this Agreement, DiaDexus hereby grants the following
          rights to Quest in the field of Class I Dx Test(s):

          5.2.1 cDx Homebrew(s)

          (a)  DiaDexus grants Quest a first right to negotiate with DiaDexus
          to obtain an exclusive license (with the right to sublicense) under
          DiaDexus IP, generated during the Extended License Period to Make,
          Have Made, Use, and Sell cDx Homebrew(s) in the field of Class I Dx
          Test(s) in the Homebrew Territory. Such license, if agreed to, would
          include a royalty and license fee as specified in Paragraph 6.3 and
          other terms and conditions mutually acceptable to Quest and DiaDexus
          and customary in such licenses, however, no additional payments
          beyond such royalty and license fee shall be required for such
          license.

          (b)  DiaDexus agrees to notify Quest in writing of the commencement
          of such first right. Quest and DiaDexus shall have a three (3) month
          period in which to negotiate and execute such license for such cDx
          Homebrew if Quest and DiaDexus mutually agree thereto. Quest shall
          within thirty (30) days after receipt of such notice either accept
          in writing or decline such first right (no notice to DiaDexus within
          such thirty (30) day period shall constitute a waiver of such first
          right with respect to such cDx Homebrew). If Quest accepts such first
          right in writing, Quest and DiaDexus agree to negotiate exclusively
          with each other in good faith effort to reach mutually acceptable
          agreement and to execute such agreement within such three (3) month
          period. In the event a license agreement is not entered into during
          such three (3) month period or a mutually agreed extension thereof,
          Quest shall have no further right with respect to such cDx Homebrew
          leaving DiaDexus free to pursue agreements with Third Parties or
          Incyte at its own discretion for such cDx Homebrew.

                                       5

<PAGE>   6

Execution Copy

          5.2.2  rDx Services(s)

          (a)    DiaDexus grants Quest a first right to negotiate with DiaDexus
          to obtain a non-exclusive or exclusive (as negotiated), world-wide
          license (with the right to sublicense) under DiaDexus IP generated
          during the Extended License Period to Make, Have Made, Use, and Sell
          rDx Service(s) in the field of Class I Dx Test(s).

          (b)    DiaDexus agrees to notify Quest in writing of the commencement
          of such first right. Quest and DiaDexus shall have a six (6) month
          period in which to negotiate and execute such license for such rDx
          Service, if Quest and DiaDexus mutually agree thereto. Quest shall
          within thirty (30) days after receipt of such notice either accept in
          writing or decline such first right (no notice to DiaDexus within such
          thirty (30) day period shall constitute a waiver of such first right
          with respect to such rDx Service). If Quest accepts such first right
          in writing, Quest and DiaDexus agree to negotiate exclusively with
          each other in good faith effort to reach mutually acceptable agreement
          and to execute such agreement within such six (6) month period. In the
          event a license agreement is not entered into during such six (6)
          month period or a mutually agreed extension thereof, Quest shall have
          no further right with respect to such rDx Service leaving DiaDexus
          free to pursue agreements with Third Parties or Incyte at its own
          discretion for such rDx Service.

     2.9  Paragraphs 5.3, 5.3.1 and 5.3.2 of the Collaboration Agreement are
hereby deleted in their entirety and replaced with the following:

          5.3    DiaDexus Class II Dx Test(s) Grants. Subject to all the terms
          and conditions of this Agreement, DiaDexus hereby grants the following
          rights to Quest and/or Incyte in the field of Class II Dx Test(s):

          5.3.1  cDx Homebrews

          (a)    DiaDexus grants Quest a first right to negotiate with DiaDexus
          to obtain an exclusive license under DiaDexus IP generated during the
          Extended License Period, with the right to sublicense only to Incyte,
          to Make, Have Made, Use, and Sell cDx Homebrews in the field of Class
          II Dx Test(s) in the Homebrew Territory.

                                       6
<PAGE>   7
          (b)  DiaDexus agrees to notify Quest in writing of the commencement
          of such first right. Quest and DiaDexus shall have a six (6) month
          period in which to negotiate and execute such license for such cDx
          Homebrew if Quest and DiaDexus mutually agree thereto. Quest shall
          within thirty (30) days after receipt of such notice either accept in
          writing or decline such first right (no notice to DiaDexus within
          such thirty (30) day period shall constitute a waiver of such first
          right with respect to such cDx Homebrew). If Quest accepts such first
          right in writing, Quest and DiaDexus agree to negotiate exclusively
          with each other in good faith effort to reach mutually acceptable
          agreement and to execute such agreement within such six (6) month
          period. In the event a license agreement is not entered into during
          such six (6) month period or a mutually agreed extension thereof,
          Quest shall have no further right with respect to such cDx Homebrew.

          (c)  In the event Quest declines such first right with respect to a
          cDx Homebrew, or fails to reach agreement with DiaDexus within the
          six (6) month period, DiaDexus grants Incyte the right to negotiate
          with DiaDexus to obtain an exclusive license under DiaDexus IP, with
          the right to sublicense only to Quest, to Make, Have Made, Use, And
          Sell such cDx Homebrew in the field of Class II Dx Test(s) in the
          Homebrew Territory.

          (d)  DiaDexus agrees to notify Incyte in writing of the commencement
          of such right promptly after the earlier of when Quest declines such
          first right or the expiration of the six (6) month period in which no
          agreement was reached with Quest. Incyte and DiaDexus shall have a
          six (6) month period in which to negotiate and execute such license
          for such cDx Homebrew if Incyte and Quest mutually agree thereto.
          Incyte shall within thirty (30) days after receipt of such notice
          either accept in writing or decline such right (no notice to DiaDexus
          within such thirty (30) day period shall constitute a waiver of such
          right with respect to such cDx Homebrew). If Incyte accepts such
          right in writing, Incyte and DiaDexus agree to negotiate exclusively
          with each other in good faith effort to reach mutually acceptable
          agreement and to execute such agreement within such six (6) month
          period. In the event a license agreement is not entered into during
          such six (6) month period or a mutually agreed extension thereof,

                                       7

<PAGE>   8

          Incyte relinquishes any such right with respect to such cDx Homebrew
          and DiaDexus is free to enter into a license with respect to such cDx
          Homebrew with Third Parties.

          5.3.2  rDx Service(s)

          (a)   DiaDexus grants Quest and Incyte a first right to negotiate with
          DiaDexus to obtain a co-exclusive, world-wide license under DiaDexus
          IP generated during the Extended License Period, each without the
          right to sublicense, to Make, Have Made, Use, and Sell rDx Service(s)
          in the field of Class II Dx Test(s).

          (b)   DiaDexus agrees to notify Quest and Incyte in writing of the
          commencement of such first right. Each of Quest and Incyte shall have
          a six (6) month period in which to negotiate with DiaDexus and execute
          a license for such rDx Service if Quest and/or Incyte, as the case may
          be, and DiaDexus mutually agree thereto. Quest and Incyte shall each
          within thirty (30) days after receipt of such notice either accept in
          writing or decline such first right (no notice to DiaDexus within such
          thirty (30) day period shall constitute a waiver of such first right
          with respect to such rDx Service with respect to the company failing
          to provide notice). If Quest and/or Incyte accept such first right in
          writing, Quest and DiaDexus and/or Incyte and DiaDexus, each agree to
          negotiate exclusively with each other in good faith effort to reach
          mutually acceptable agreement and to execute such agreement within
          such six (6) month period. In the event a license agreement is not
          entered into during such six (6) month period or mutually agreed
          extension(s) thereof, Quest or Incyte, as the case may be, shall have
          no further right with respect to such rDx Service.

    2.10  Paragraphs 5.4, 5.4.1 and 5.4.2 of the Collaboration Agreement are
hereby deleted in their entirety and replaced with the following:

          5.4       DiaDexus Class III Dx Test(s) Grants: Subject to all the
          terms and conditions of this Agreement, DiaDexus hereby grants the
          following rights to Quest and/or Incyte in the field of Class III Dx
          Tests(s):

          5.4.1     cDx Homebrew(s)

          (a)       DiaDexus grants Quest a first right to negotiate with
          DiaDexus to obtain an exclusive, world-wide license under

                                       8

<PAGE>   9

       DiaDexus IP, generated during the Extended License Period, with the
       right to sublicense only to Incyte, to Make, Have Made, Use, and Sell
       cDx Homebrew in the field of Class III Dx Test(s) in the Homebrew
       Territory.

       (b) DiaDexus agrees to notify Quest in writing of the commencement of
       such first right. Quest and DiaDexus shall have a six (6) month period
       in which to negotiate and execute such license for such cDx Homebrew if
       Quest and DiaDexus mutually agree thereto. Quest shall within thirty
       (30) days after receipt of such notice either accept in writing or
       decline such first right (no notice to DiaDexus within such thirty (30)
       day period shall constitute a waiver of such first right with respect to
       such cDx Homebrew). If Quest accepts such first in writing, Quest and
       DiaDexus agree to negotiate exclusively with each other in good faith
       effort to reach mutually acceptable agreement and to execute such
       agreement within such six (6) month period. In the event a license
       agreement is not entered into during such six (6) month period or a
       mutually agreed extension thereof, Quest shall have no further right
       with respect to such cDx Homebrew.

       (c) In the event Quest declines such first right with respect to a cDx
       Homebrew or fails to reach agreement with DiaDexus within a six (6)
       month period, DiaDexus grants Incyte the right to an exclusive license
       under DiaDexus IP, with the right to sublicense only to Quest, to Make,
       Have Made, Use, and Sell Clinical Dx Homebrew(s) in the field of Class
       III Dx Test(s) in the Homebrew Territory.

       (d) DiaDexus agrees to notify Incyte in writing of the commencement of
       such right promptly after the earlier of when Quest declines such right
       or the expiration of the six (6) month period in which no agreement was
       reached with Quest. Incyte and DiaDexus shall have a six (6) month
       period in which to negotiate and execute such license for such cDx
       Homebrew if Incyte and DiaDexus mutually agree thereto. Incyte shall
       within thirty (30) days after receipt of such notice either accept in
       writing or decline such right (no notice to DiaDexus within such thirty
       (30) day period shall constitute a waiver of such right with respect to
       such cDx Homebrew). If Incyte accepts such right in writing, Incyte and
       DiaDexus agree to negotiate exclusively with each other in good faith
       effort to execute

                                       9
<PAGE>   10
          such agreement within such six (6) month period. In the event a
          license agreement is not entered into during such six (6) month
          period or a mutually agreed extension thereof. Incyte shall have no
          further right with respect to such cDx Homebrew and DiaDexus is free
          to enter into a license with respect to such cDx Homebrew with Third
          Parties.

          5.4.2 rDx Service(s)

          (a)  DiaDexus grants Quest and Incyte a first right to negotiate with
          DiaDexus to obtain a co-exclusive, world-wide license under DiaDexus
          IP generated during the Extended License Period, each without the
          right to sublicense, to Make, Have Made, Use, and Sell rDx Service(s)
          in the field of Class III Dx Test(s).

          (b)  DiaDexus agrees to notify Quest and Incyte in writing of the
          commencement of such first right. Each of Quest and Incyte shall have
          a six (6) month period in which to negotiate with DiaDexus and
          execute a license for such rDx Service if Quest and/or Incyte, as the
          case may be, and DiaDexus mutually agree thereto. Quest and Incyte
          shall each within thirty (30) days after receipt of such notice
          either accept in writing or decline such first right (no notice to
          DiaDexus within such thirty (30) day period shall constitute a waiver
          of such first right with respect to such rDx Service with respect to
          the company failing to provide notice). If Quest and/or Incyte accept
          such first right in writing, Quest and DiaDexus and/or Incyte and
          DiaDexus, each agree to negotiate exclusively with each other in good
          faith effort to reach mutually acceptable agreement and to execute
          such agreement within such six (6) month period. In the event a
          license agreement is not entered into during such six (6) month
          period or mutually agreed extension(s) thereof, Quest or Incyte, as
          the case may be, shall have no further right with respect to such rDx
          Service.

     2.11 A New Paragraph 5.6.1(c) is hereby added to the Collaboration
Agreement, as follows:

          (c)  Notwithstanding anything else to the contrary, DiaDexus grants
          to Quest a non-exclusive, paid-up, non-cancellable, royalty-free,
          license in the field of Clinical Laboratory Services under DiaDexus
          IP which is an improvement to SB Technology IP (to the extent and only

                                       10

<PAGE>   11
          to the extent such improvement was conceived and reduced to practice
          as of the Quest Closing). However, diaDexus represents and warrants
          that, as of the Quest Closing, there is no such diaDexus IP which is
          an improvement to SB Technology IP.

     2.12 Paragraph 5.6.4 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          5.6.4 Research

          DiaDexus hereby grants to each of SB and Quest a non-exclusive license
          under DiaDexus IP which is Clinical Non-genomic DiaDexus Data
          generated by or on behalf of DiaDexus during the Extended License
          Period, to use for its own respective internal research purposes.

     2.13 Paragraph 5.6.5(a) of the Collaboration Agreement is hereby deleted
in its entirety and replaced with the following:

          (a)(1) For all Tests to which DiaDexus has lost its licenses to SB
          Diagnostic IP (to the extent such SB Diagnostic IP was conceived and
          reduced to practice as of the Quest Closing) pursuant to Paragraph
          13.5, DiaDexus grants Quest an exclusive, royalty-free,
          non-cancellable, world-wide license under DiaDexus IP, with the right
          to sublicense, to the extent necessary to Make, Have Made, Use, and
          Sell such Tests.

          (a)(2) For all Tests to which DiaDexus has lost its licenses to SB
          Diagnostic IP (to the extent such SB Diagnostic IP was conceived and
          reduced to practice after the Quest Closing) pursuant to Paragraph
          13.5, DiaDexus grants an exclusive, royalty-free, non-cancellable,
          world-wide license to SB under DiaDexus IP, with the right to
          sublicense, to the extent necessary to Make, Have Made, Use, and Sell
          such Tests.

          (a)(3) For all Tests to which DiaDexus has lost its licenses to
          Incyte Diagnostic IP pursuant to Paragraph 13.5, DiaDexus grants an
          exclusive, royalty-free, non-cancellable, world-wide license to
          Incyte under DiaDexus IP, with the right to sublicense, to the
          extent necessary to Make, Have Made, Use, and Sell such Tests.

                                       11
<PAGE>   12
     2.14 Paragraph 5.6.5(c) of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          (c)  During the Extended License Period and for each cDx Kit
               manufactured by DiaDexus, DiaDexus grants Quest most favored
               nation pricing with respect to the price of such cDx Kit sold to
               Third Parties.

     2.15 Paragraph 6.3 of the Collaboration Agreement is hereby deleted in its
entirety and replaced with the following:

          6.3  Tests Field of Use: Payments and Royalties to DiaDexus by Quest.
          Contingent on the following events, and subject to Paragraph 6.5 and
          6.6, the following payments and royalties shall be paid by Quest to
          DiaDexus for Tests under the licenses granted under Paragraph 5.2.1 to
          Quest as applicable:

          (a) Royalties on Net Sales: A royalty will be paid on Net Sales by
          Quest for any Test under the following schedule:

                      Test subject to Paragraph      6.5(a)       6.5(b)

          cDx Homebrew (Class I)                    ******       ******

          (b) License Fee: Promptly upon the execution of each license agreement
          between Quest and DiaDexus for each cDx Homebrew entered into pursuant
          to Paragraph 5.2.1, Quest shall pay to DiaDexus a license fee of
          between ******  Such license fee is fully creditable against future
          royalties for such cDx Homebrew.

          (c) Applicable Royalty Rate/License Fee: The applicable royalty rate
          for each such cDx Homebrew within such range specified in Paragraphs
          (a) shall be calculated as follows:

              (i)  If the Quest profit margin is ******

              (ii) If the Quest profit margin is ******

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

                                       12
<PAGE>   13

               (iii) For profit margins in excess of ******

               (iv) For profit margins in excess of ******

          (d) Other Terms: All other terms of such license agreement entered
          into pursuant to Paragraph 5.2.1 for such cDx Homebrew to be
          negotiated pursuant to the relevant agreement between Quest and
          DiaDexus with respect to such cDx Homebrew.

          (e) Other Tests: Quest and/or Incyte shall pay royalties to DiaDexus
          on Net Sales of Tests to which Quest and/or Incyte has obtained rights
          from DiaDexus pursuant to this Agreement. Such royalty and other
          terms shall be negotiated pursuant to the relevant agreement
          negotiated between Quest and/or Incyte and DiaDexus with respect to
          such Test.

    2.16  Paragraph 6.5 of the Collaboration Agreement is hereby deleted in its
entirety and replaced with the following:

          6.5 Payment Obligation.  The foregoing payment obligations of SB,
          Quest and Incyte to DiaDexus under Sections 6.1 and 6.3 will accrue or
          become due or payable only with respect to Rx Products and Tests:

          (a) which but for the patent licenses granted herein would otherwise
          infringe a Valid Claim(s) of a DiaDexus Patent(s); or

          (b) are covered by a pending claim of a DiaDexus Patent(s) but not a
          Valid Claim(s) of a DiaDexus Patent(s).

    2.17  Paragraph 6.6.2 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          6.6.2  Upon termination of the royalty payment obligation, DiaDexus,
          SB, Quest and Incyte, as the case may be, shall thereafter

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

                                       13
<PAGE>   14

          have in perpetuity a royalty-free license to make, use and sell such
          Product in the specified field(s) of use.

     2.18 Paragraph 9.3.2 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          9.3.2

          (a) During the Extended License Period, DiaDexus shall use reasonable
          and ongoing efforts to disclose and supply to SB, DiaDexus IP which
          are improvements to SB Technology IP which are reasonably available
          and transferable.

          (b) During the Extended License Period, DiaDexus shall use reasonable
          and ongoing efforts to disclose and supply to Quest, DiaDexus IP which
          are improvements to SB Technology IP (to the extent and only to the
          extent such improvements are conceived and reduced to practice as of
          the Quest Closing) which are reasonably available and transferable.

     2.19 Paragraph 9.3.3 of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          9.3.3 cDx Homebrew(s)

          During the Extended License Term and to the extent Quest and/or Incyte
          have a right to a license, DiaDexus shall promptly notify Quest and/or
          Incyte, as the case may be, in writing of any DiaDexus IP which is
          potentially useful as a specific cDx Homebrew and shall specify any
          prior Third Party obligations with respect to such DiaDexus IP.

     2.20 Paragraph 9.3.4(a) of the Collaboration Agreement is hereby deleted in
its entirety and replaced with the following:

          (a) During the Extended License Term and to the extent Quest has a
          right to a license, DiaDexus shall promptly notify Quest in writing of
          any DiaDexus IP which is potentially useful as a rDx Service in the
          field of Class I, Class II, or Class III Dx Test(s) and shall specify
          any prior Third Party obligations with respect to such DiaDexus IP.

     2.21 Paragraphs 9.3.9(a) and (b) of the Collaboration Agreement are hereby
deleted in their entirety and replaced with the following:

                                       14

<PAGE>   15
          (a)  During the Extended License Period and with respect to DiaDexus
          IP which is the subject of any license granted to SB, Quest or Incyte
          under Section 5, DiaDexus shall use reasonable and ongoing efforts to
          disclose and supply to SB, Quest or Incyte, as the case may be, with
          that certain DiaDexus IP which is reasonably available and
          transferable and the subject of such license.

          (b)  Notwithstanding anything else to the contrary, DiaDexus IP which
          is based on the use of or derived by the use of HGS derived SB
          Diagnostic IP as provided to DiaDexus pursuant to Paragraph 9.1 shall
          only be disclosed to SB.

3.   QUEST OBLIGATIONS.

     3.1  Quest hereby assumes all liabilities and agrees to perform all
obligations arising out of or in connection with the exercise by Quest of the
rights granted to it under this Amendment, including, but only to the extent
applicable, the royalty and other payment obligations set forth in Article 6 of
the Collaboration and License Agreement, the diligence obligations set forth in
Article 13 of the Collaboration and License Agreement, the requirements set
forth Paragraph 14.5 of the Collaboration and License Agreement, the reporting
obligations set forth in Article 15 of the Collaboration and License Agreement,
the termination provision set forth in Paragraph 16.2.2 of the Collaboration and
License Agreement, the representation and warranty set forth in Paragraph 17.2
of the Collaboration and License Agreement, and all other obligations due with
respect to the sale by Quest or any of its Affiliates of any Test licensed by
Quest pursuant to the terms of the Collaboration Agreement.

     3.2  Quest acknowledges that in connection with its activities under this
Amendment it may receive Confidential Information of SB, Incyte and/or
DiaDexus. Quest agrees to be subject to the provisions of Article 12 of the
Collaboration Agreement, as if Quest was a Party to the Collaboration
Agreement. In addition, SB, Incyte and DiaDexus acknowledge that each of them
may receive confidential information from Quest in connection with their
respective activities under this Amendment and each agrees to treat such
confidential information as Confidential Information belonging to Quest under
Article 12 of the Collaboration and License Agreement.

     3.3  Quest agrees that with respect to the sale by Quest or any of its
Affiliates of any Test licensed by Quest under this Amendment, it will
undertake the indemnification obligations set forth in Article 18 of the
Collaboration Agreement, as if Quest was a Party to the Collaboration Agreement.

4.   RESEARCH COMMITTEE.

                                       15

<PAGE>   16
The Parties agree that during the Extended License Period Quest shall have
reasonable access to the Research Committee or other multiparty research
governance organizations that are or may be established under the Collaboration
Agreement on a non-voting observer basis, provided, however, that such
participation shall be limited to discussion concerning research projects,
their status, and the conveyance of rights to Quest related thereto, all as
limited to such projects that are subject to the rights and licenses granted to
Quest hereunder.

5.   NO OTHER RIGHTS OR OBLIGATIONS.

Except with respect to the rights and obligations specifically granted to Quest
in this Amendment, Quest shall have no rights or obligations as a Party to the
Collaboration Agreement, including, without limitation, the right to approve
an amendment to the Collaboration Agreement or the right to enforce any rights
or obligations of the Parties to the Agreement.

6.   MISCELLANEOUS.

     6.1  Quest shall be deemed a Party to the Collaboration Agreement with
respect to Paragraphs 16.3, 19.1, 19.3, 19.5, 19.6, 19.7, 19.14, and 19.15 of
the Collaboration Agreement; provided, however, that Quest shall be deemed a
Party to the extent, and only to the extent, such Paragraphs are applicable to
the rights and obligations granted to Quest pursuant to this Amendment.

     6.2  Paragraphs 14.6.1 (subject to the sublicensing restrictions set forth
in Paragraphs 5.3.2 and 5.4.2), 17.3, 19.4, 19.8, 19.9, 19.11, 19.16, and 19.17
are hereby incorporated by reference into this Amendment.

     6.3  The rights and obligations set forth in this Amendment shall be
binding upon and inure to the benefit of the successors in interest of the
respective parties. The rights and obligations set forth in this Amendment
shall not be directly or indirectly assigned or transferred (whether
voluntarily, by operation of law or otherwise) by Quest without the prior
express written consent of the other parties; provided, however, that Quest may
assign or transfer this Amendment or any of its rights or obligations hereunder
(i) to any Quest Affiliate, or (ii) to any Third Party with which it may merge
or consolidate, or to which it may transfer all or substantially all of its
assets in each case without obtaining the consent of the other parties, and if
in any such event (including any merger involving Quest), (i)(A) Quest remains
liable under this Amendment, and (ii)(B) the relevant Quest Affiliate, Third
Party assignee or surviving entity assumes in writing all of Quest's
obligations under this Agreement. Any purported assignment or transfer by Quest
in violation of this Paragraph shall be void.

     6.4  Notices and deliveries to Quest shall be governed by Paragraph 19.10.
If sent to Quest, addressed to:

          Quest Diagnostics Incorporated

                                       16
<PAGE>   17
          One Malcolm Avenue

          Teterboro, NJ 07680

          Attention: Vice President, Business Development

     6.5  This Amendment shall become effective upon the Quest Closing. In the
event the Quest Closing does not occur on or prior to September 30, 1999, this
Amendment shall terminate and be of no further force or effect.

     6.6  The parties acknowledge that the Amendment to the Collaborative
LifeSeq Agreement and the Amendment to the Collaborative PathoSeq Agreement,
both between Incyte and diaDexus and of even date herewith, are supportive of
and constitute partial consideration for this Amendment. Accordingly, during
the Initial License Period, Incyte and diaDexus agree not to subsequently
amend, only with respect to diaDexus rights in Class I rDx Kits and Class I rDx
Services, either the Collaborative LifeSeq Agreement or the Collaborative
PathoSeq Agreement without the prior approval of Quest, which approval shall
not be unreasonably withheld.

                                       17
<PAGE>   18
     IN WITNESS WHEREOF, the parties, through their authorized officers, have
executed this Assignment Agreement as of the date first written above.

SMITHKLINE BEECHAM CORPORATION

By: /s/ DONALD G. PARMAN
   --------------------------
Its: Secretary
    -------------------------

SMITHKLINE BEECHAM PLC

By: /s/ JOSEPH HARRIS
   --------------------------
Its: Attorney-in-Fact
    -------------------------

QUEST DIAGNOSTICS INCORPORATED

By: /s/ [SIGNATURE ILLEGIBLE]
   --------------------------
Its: Vice President, Business Development
    -------------------------

[INITIAL]
PATENT DEPT
28/7/99

DIADEXUS LLC

By: /s/ [SIGNATURE ILLEGIBLE]
   --------------------------
Its: COO
    -------------------------

INCYTE PHARMACEUTICALS, INC.

By: /s/ [SIGNATURE ILLEGIBLE]
   --------------------------
Its: President
    -------------------------

                                       18

<PAGE>   19

                                   APPENDIX A

                            LIST OF QUEST AFFILIATES

          100% SBCL, INC. (DE)(1)
                    100% SmithKline Beecham Clinical Laboratories, Inc. (DE)(2)
                    (33-1/3%)  Compunet Clinical Laboratories (OH)
                    (44%)      Mid America Clinical Laboratories (IN)
                    (49%)      Diagnostic Laboratory of Oklahoma (OK)

          50%  Corporacion MSB, S.A. de C.V. (Mexico)

          100% Quest Holdings Incorporated (DE)
               51% Quest Diagnostics of Missouri LLC (MO)

          100% Quest Holdings Incorporated (MD)
               100% Quest Diagnostics Incorporated (MD)
                    100% Diagnostic Reference Services Inc. (MD)
                         50% Pathology Building Partnership (MD)(gnl pshp)(1)

          100% Quest Holdings Incorporated (MI)
               100% Quest Diagnostics Incorporated (CA)

          100% Quest Diagnostics LLC (IL)
          100% Quest Diagnostics Incorporated (MI)

          100% Laboratory Holdings Incorporated (MA)
               100% Quest Diagnostics Incorporated (CT)
               100% Quest Diagnostics Incorporated (MA)

          100% Quest Diagnostics of Pennsylvania Inc. (DE)
               100% Quest Diagnostics Incorporated (OH)
               100% Medical Management Systems, Inc. (PA)
                    51% Quest Diagnostics Venture LLC (PA)
                    53.5% Associated Clinical Laboratories (PA)(gen pshp)
               50%  Surgical Eye Enterprise L.P. (PA)(ltd pshp)
----------
(1) Will become a subsidiary on the Effective Date and the name will be
    changed to Quest Diagnostics Holdings Inc.

(2) Will become a subsidiary on the Effective Date and the name will be
    changed to Quest Diagnostics Clinical Laboratories, Inc.

(1) Other 50% interest held by Quest Diagnostics Incorporated (MD)

                                       19

<PAGE>   20

                    50% Surgical Eye Institute L.P. (PA)(ltd pshp)

          100% Quest Diagnostics of Arizona Incorporated (DE)
               49% Sonora Quest Laboratories LLC (AZ)

          100% Quest Diagnostics Investments Incorporated (DE)
               100% Quest Diagnostics Finance Incorporated (DE)

          100% Damon Investment Holdings, Inc. (DE)

          100% DPD Holdings, Inc. (DE)
               100% MetWest Inc. (DE)

          100% Quest MRL Inc. (DE)

          100% Nichols Institute Diagnostics (CA)
               100% Nichols Institute Sales Corporation (U.S.V.I.)
          100% Nichols Institute Diagnostics Limited (U.K.)
          100% Nichols Institute Diagnostics Trading S.A. (Switzerland)
          100% Nichols Institute Diagnostica GMBH (Germany)
               100% Nichols Institute Diagnostica GMBH (Austria)
          100% Nichols Institute International Holding B.V. (Netherlands)
               100% Nichols Institute Diagnostics B.V. (Netherlands)
               100% Nichols Institute Diagnostics SARL (France)

          100% Quest Diagnostics do Brasil Ltda. S.C. (Brazil)

          100% Quest Diagnostics Limited (UK)

          100% Nomad-Massachusetts, Inc. (MA)
               100% Quest Laboratorios Clinicos, S.A. de C.V. (Mexico)
               100% Analisis, S.A. (Mexico)
               100% Laboratorios Clinicos de Mexico, S.A. de C.V. (Mexico)
                    100% Servicios de Laboratorio, S.A. de C.V. (Mexico)
               100% Laboratorios de Frontera Polanco, S.A. de C.V. (Mexico)
               100% Laboratorios de Analisis Biomedicos, S.A. (Mexico)

               100% New England Medical Laboratory Inc.*
               100% Stat Toxicology Service of Boston, Inc.*
               100% MetPath Europe Limited (U.K.)*
               100% TransUnited Casualty and Indemnity Insurance Company
                    (Cayman Islands)*
----------
* In process of dissolution

                                       20

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