<PAGE>

                                                                 Exhibit 10.64.2

                        CONFIDENTIAL TREATMENT REQUESTED.
 CONFIDENTIAL PORTIONS OF THIS DOCUMENT HAVE BEEN OMITTED PURSUANT TO A REQUEST
 FOR CONFIDENTIAL TREATMENT AND HAVE BEEN FILED SEPARATELY WITH THE COMMISSION.

                              SUBLICENSE AGREEMENT

        This Sublicense Agreement (this "Agreement") is made and entered into
this 19th day of September, 2001 (the "Effective Date") by and between Lexicon
Genetics Incorporated, a Delaware corporation having an office at 4000 Research
Forest Drive, The Woodlands, Texas 77381 ("Lexicon"), and Deltagen, Inc., a
Delaware corporation having an office at 740 Bay Road, Redwood City, California
94063-2469 ("Deltagen").

                                    RECITALS:

        WHEREAS, Lexicon holds a license from GenPharm International Inc.
("GenPharm") under certain Patent Rights (as defined herein) relating to the use
of "positive negative selection" and "isogenic DNA constructs" in gene targeting
for the generation, of transgenic and knock out mice, and has the right to grant
sublicenses under said Patent Rights; and

        WHEREAS, under the terms of a Settlement Agreement between the parties
dated of even date herewith (the "Settlement Agreement"), Lexicon and Deltagen
have agreed to the settlement and dismissal of litigation brought by Lexicon and
the University of Utah Research Foundation (the "Foundation") against Deltagen
in the United States District Court for the Northern District of California,
Civil Action No. 00-21060-PVT, and the litigation brought by Lexicon in the
United States District Court for the District of Delaware, Civil Action No.
00-516-JJF, and all claims and counterclaims asserted therein;

        WHEREAS, the Settlement Agreement provides for (i) the grant by Deltagen
to Lexicon of certain rights of access and licenses to Deltagen's DeltaBase(TM)
Database and Materials (as defined herein) under that certain DeltaBase
Collaboration Agreement between the parties of even date herewith (the
"DeltaBase Agreement") and (ii) the grant by Lexicon to Deltagen of a sublicense
under the Patent Rights in the Field of Use and certain agreements not to sue on
behalf of both parties upon the terms and conditions hereinafter set forth;

NOW, THEREFORE, in consideration of the premises and the mutual covenants
contained herein, the parties hereto agree as follows:

        1.      Definitions. For the purposes of this Agreement, the following
words and phrases shall have the following meanings:

                1.1     "Academic Collaborator" shall mean a principal
        investigator, employed by a university or other not for profit research
        institution, who is performing collaborative research with Deltagen, a
        Deltagen Corporate Partner or a Deltagen Fee-For-Service Customer, as
        applicable, involving use of a Rodent Model or Progeny.

                1.2     "Affiliate" shall mean, with respect to any corporation
        or other entity, any other corporation or other entity that directly or
        indirectly controls, is controlled by or is under common control with
        such corporation or entity, where "control" is defined as the ownership
        of [*] percent ([*]%) of the equity or beneficial ownership interests of
        such

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        corporation or entity; provided, however, that where local laws require
        a minimum percentage of local ownership, the ownership of the maximum
        percentage of the equity or beneficial ownership interests of a
        corporation or entity that may, under such local laws, be owned or
        controlled by foreign interests shall be deemed to constitute "control"
        of such corporation or entity.

                1.3     "Corporate Partnership Agreement" shall mean:

                        (a)     in the case of Deltagen or its Affiliates, any
                agreement, other than a Fee-For-Service Agreement, between
                Deltagen or its Affiliates and a Third Party involving [*]);
                and

                        (b)     in the case of any Deltagen Corporate Partner or
                Deltagen Fee-For-Service Customer to which Deltagen transfers a
                Rodent Model or Progeny, any agreement between such Deltagen
                Corporate Partner, Deltagen Fee For-Service-Customer or its
                Affiliates and another Third Party involving [*].

                1.4     "DeltaBase Database and Materials" shall mean DeltaBase
        Information and Knockout Mice Materials, as such terms are defined in
        the DeltaBase Agreement.

                1.5     "Deltagen Corporate Partner" means a Third Party that
        enters into a Corporate Partnership Agreement with Deltagen or its
        Affiliates.

                1.6     "Deltagen Fee-For-Service Customer" means any Third
        Patty that enters into a Fee-For-Service Agreement with Deltagen or its
        Affiliates.

                1.7     "Fee-For-Service Agreement" shall mean any agreement
        entered into between Deltagen or its Affiliates and a Third Party under
        which Deltagen or its Affiliates performs contract research services on
        a work for hire basis to generate or develop Rodent Model(s) for such
        Third Party. For purposes of the foregoing, "work for hire" shall mean
        and include [*].

                1.8     "Field of Use" shall mean (i) the generation and
        development of Rodent Models by Deltagen and its Affiliates (including
        the right to make but not have made), (ii) the use of Rodent Models and
        Progeny by Deltagen and its Affiliates, at the internal research
        facilities of Deltagen or such Affiliates, (A) for research purposes,
        including research directed toward the discovery, development or
        commercialization of Products, and (B) for purposes of commercially
        using, developing, selling and otherwise malting available (e.g.,
        through the DeltaBase Database and Materials) to Third Parties
        Phenotypic Data generated by Deltagen or its Affiliates with respect
        thereto, including, without limitation, rights to discover, develop and
        commercialize Products derived from such Rodent Models, Progeny or
        Phenotypic Data, and (iii) the use of Rodent Models and

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        Progeny by any Academic Collaborators, Deltagen Corporate Partners,
        Deltagen Fee-For-Service Customers and Product Collaborators that have
        entered into an agreement with respect thereto containing the provisions
        specified in Section 2.2, Academic Collaborators, Deltagen Corporate
        Partners, Deltagen Fee-For-Service Customers or Product Collaborators,
        in each such case for research purposes only, including research
        directed toward the discovery, development or commercialization of
        Products.

                1.9     "Rodent Model" shall mean any rodent cell or rodent
        generated or developed by Deltagen or its Affiliates through use of any
        product or process covered by a Valid Claim of the Patent Rights, and
        any materials derived therefrom. A "line of Rodent Models" shall mean
        any Rodent Model with the same mutation in the same gene.

                1.10    "Patent Rights" shall mean all of Lexicon's rights and
        interests in and to (i) the United States patents listed in Exhibit A,
        and any patent applications that claim the benefit of priority to any of
        the patents listed in Exhibit A, any patent applications from which the
        patents listed in Exhibit A claim priority, and all foreign counterparts
        thereof, (ii) any patents issuing from such patent applications and
        (iii) any continuations, continuations-in-part, divisionals, reissues,
        reexaminations, substitutions, supplementary protection certificates or
        extensions of any of the foregoing.

                1.11    "Phenotypic Data" means genetic, phenotypic and related
        data generated or developed from a Rodent Model or Progeny and
        intellectual property rights therein.

                1.12    "Product" shall mean any human therapeutic or diagnostic
        product which is discovered, identified, selected, characterized or
        determined to have utility, or which interacts with a gene and/or the
        protein it encodes (including allelic variants thereof and post
        translationally modified variants thereof) which is discovered,
        identified, selected, characterized or determined to have utility,
        through use by Deltagen, its Affiliates, Academic Collaborators or
        Corporate Partners of a Rodent Model or Progeny, or Phenotypic Data with
        respect to a Rodent Model or Progeny. For purposes of this Agreement, a
        Product is referred to as having been "derived from" the Rodent Model,
        Progeny or Phenotypic Data used in discovering, identifying, selecting,
        characterizing or determining the utility of such Product.

                1.13    "Product Collaborators" means any Third Party that
        enters into a Corporate Partnership Agreement with a Deltagen Corporate
        Partner or its Affiliates, or with a Deltagen Fee-For-Service Customer
        or its Affiliates.

                1.14    "Progeny" shall mean any rodent cells or rodents,
        including successive generations thereof, that are produced or developed
        by Deltagen, its Affiliates, Academic Collaborators or Corporate
        Partners by breeding a Rodent Model with any other rodent (including,
        without limitation, any other Rodent Model), and any materials derived
        therefrom; provided, however, that Progeny shall not include, and the
        rights and licenses granted under this Agreement shall not extend to,
        any rodent cell or rodent that is produced or developed by breeding a
        Rodent Model with any rodent, other than another Rodent Model, that
        contains a mutation in its genome which was generated or developed, or
        whose progenitors include a rodent containing such mutation which was
        generated or

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        developed, through use of any product or process covered by a Valid
        Claim of the Patent Rights.

                1.15    "Third Party" shall mean any person or entity other than
        Lexicon, Lexicon's Affiliates, Deltagen and Deltagen's Affiliates.

                1.16    "Valid Claim" shall mean a claim of an issued and
        unexpired patent which has not been [*].

        2.      Grant of Sublicense and Covenants Not to Sue.

                2.1     Subject to the terms and conditions of this Agreement,
        Lexicon hereby grants to Deltagen and its Affiliates, during the term of
        this Agreement, a worldwide, nonexclusive right and sublicense under the
        Patent Rights in the Field of Use to make, breed and use Rodent Models
        and Progeny. Deltagen and its Affiliates shall have no right to grant
        any sublicense under the Patent Rights. Nothing in this Agreement shall
        be construed to confer upon Deltagen or its Affiliates any rights under
        the Patent Rights outside the Field of Use. Notwithstanding the
        foregoing, the rights and sublicense granted under this Section 2.1
        shall not extend to the following (the "Excluded Fields"):

                        (a)     the sale of any Rodent Model or Progeny (for
                purposes of which "sale" means the transfer of a Rodent Model or
                Progeny for consideration under an agreement entered into after
                the use of the methods claimed by the Patent Rights to create
                such Rodent Model or Progeny; [*];

                        (b)     the use of any Rodent Model or Progeny for
                purposes of contract testing services (for purposes of which
                "contract testing services" means [*]; and

                        (c)     the generation, development and use of (i) [*].

                [*].

                2.2     Deltagen and its Affiliates shall have the right to
        transfer Rodent Models and Progeny to (i) Academic Collaborators, (ii)
        Deltagen Corporate Partners and (iii) Deltagen Fee-For-Service
        Customers, in each case subject to the terms of this Section 2.2. Any
        such transfer shall be made pursuant to a material transfer agreement or
        other agreement containing terms relating to the transfer of such
        material that expressly (i) prohibits the use of such Rodent Models and
        Progeny thereof for any purpose other than use in the Field of Use and
        (ii) prohibits the transfer of such Rodent Models or Progeny thereof by
        such Academic Collaborator, Deltagen Corporate Partner or Deltagen
        Fee-For-Service Customer to any Third Party; provided that [*].

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

                2.3     Subject to the terms of this Agreement, Lexicon and its
        Affiliates shall not assert or enforce against Deltagen or its
        Affiliates any claims of an issued patent (other than the patents
        included in the Patent Rights) now owned or hereafter acquired or
        obtained by Lexicon or its Affiliates, to [*] to the extent, but only to
        the extent, any such assertion or enforcement would, absent a license
        from Lexicon or its Affiliates, prevent Deltagen or its Affiliates from
        [*]. Nothing herein shall be deemed to restrict the right of Lexicon and
        its Affiliates to assert or enforce against Deltagen or its Affiliates
        any claims of an issued patent to any product or process for generating
        or developing transgenic rodent cells or rodents using gene trapping.

                2.4     Subject to the terms of this Agreement, Deltagen and its
        Affiliates shall not assert or enforce against Lexicon or its Affiliates
        any claims of an issued patent now owned or hereafter acquired or
        obtained by Deltagen or its Affiliates, to [*], to the extent, but only
        to the extent, any such assertion or enforcement would, absent a license
        from Deltagen or its Affiliates, prevent Lexicon or its Affiliates from
        [*]. Nothing herein shall be deemed to restrict the right of Deltagen
        and its Affiliates to assert or enforce against Lexicon or its
        Affiliates any claims of an issued patent to any product or process for
        generating or developing transgenic rodent cells or rodents using gene
        trapping.

                2.5     Deltagen and its Affiliates shall use reasonable, good
        faith efforts to obtain the agreement of its Academic Collaborators,
        Deltagen Corporate Partners and Deltagen Fee-For-Service Customers not
        to assert or enforce (and to use reasonable, good faith efforts to
        obtain the agreement of their respective Academic Collaborators and
        Product Collaborators not to assert or enforce) against Lexicon or its
        Affiliates any claims of an issued patent, now owned or hereafter
        acquired or obtained by any such party (including, without limitation,
        any issued patent under which Deltagen has granted exclusive rights to
        such party), (i) to any Rodent Model or use thereof or (ii) arising in
        whole or in part from the use of any Rodent Model, Progeny or Phenotypic
        Data, to the extent, but only to the extent, any such assertion or
        enforcement would, absent a license from such party, prevent Lexicon
        and/or its Affiliates from:

                        (a)     using, for research purposes only (including,
                without limitation, research directed toward the discovery,
                development and commercialization of human therapeutic and
                diagnostic products) any [*]; or

                        (b)     discovering, developing or commercializing any
                human therapeutic or diagnostic products derived from such
                research use.

        Nothing herein shall be deemed to restrict the rights of Deltagen's
        Academic Collaborators, Deltagen Corporate Partners or Deltagen
        Fee-For-Service Customers to assert or enforce against Lexicon or its
        Affiliates any claims of an issued patent to the composition of matter
        of any Product or the use of any Product as a human therapeutic or
        diagnostic.

                2.6     Except as expressly provided in this Agreement, under no
        circumstances shall a party hereto, as a result of this Agreement,
        obtain any ownership interest in or other right to or under any
        technology, know how, patents, patent applications, gene or

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        genomic sequence data or information, products, or biological materials
        of the other party, including items owned, controlled or developed by,
        or licensed to, the other party, at any time pursuant to this Agreement.

        3.      Consideration for License; Royalty Obligations.

                3.1     For the rights, privileges and sublicense granted under
        the Patent Rights hereunder, Deltagen shall grant Lexicon the rights and
        licenses specified in the DeltaBase Agreement.

                3.2     Deltagen shall pay to the Foundation and to GenPharm for
        payment to the Netherlands Cancer Institute ("NCI") the royalties
        specified in the agreements attached hereto as Exhibit B and Exhibit C,
        respectively, in accordance with the terms of such agreements.

                3.3     Deltagen shall pay Lexicon a royalty of [*] Dollars
        ($[*]) for each line of Rodent Models made for a Third Party under a
        Fee-For-Service Agreement, payable within [*] days after the end of each
        calendar quarter for each such line of Rodent Models completed during
        such quarter. For purposes of the foregoing, a line of Rodent Models
        shall be deemed to be completed upon the earlier of (i) the development
        of chimeras for such line of Rodent Models or (ii) the first transfer to
        such Third Party of Rodent Models or materials related to such Rodent
        Model (e.g., targeting vectors).

                3.4     Within [*] days after each calendar quarter, Deltagen
        shall furnish to Lexicon a written quarterly report showing: (i) the
        number of lines of Rodent Models made by Deltagen and its Affiliates
        during the reporting period, separately specifying the number of lines
        of Rodent Models made under Fee-For-Service Agreements, and the
        calculation of royalties under Sections 3.2 and 3.3 and (ii) the
        royalties paid or payable with respect thereto. If no royalty or payment
        is due for any royalty period hereunder, Deltagen shall so report.
        Deltagen shall keep complete and accurate records in sufficient detail
        to properly reflect its activities under this Agreement and to enable
        the royalties payable hereunder to be determined. Information disclosed
        by Deltagen to Lexicon pursuant to this Section 3.4 will be treated as
        confidential information.

                3.5     Upon the written request of Lexicon, Deltagen shall
        permit an independent certified public accountant selected by Lexicon
        and acceptable to Deltagen, which acceptance shall not be unreasonably
        withheld, to have access, at reasonable times and during normal business
        hours, to such records of Deltagen as may be reasonably necessary to
        verify the accuracy of the reports described herein, in respect of any
        fiscal year ending not more than [*] months prior to the date of such
        request. Lexicon and Deltagen shall use commercially reasonable efforts
        to schedule all such verifications within [*] days after Lexicon makes
        its written request. All such verifications shall be conducted not more
        than once in, or with respect to, each calendar year. The report of
        Lexicon's independent certified public accountant shall be made
        available to both parties. Subject to Deltagen's rights under Section
        10, in the event Lexicon's independent certified public accountant
        concludes that additional royalties were owed to Lexicon for such
        period, the additional royalty shall be paid by Deltagen within [*] days
        of the date

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        Lexicon delivers to Deltagen such independent certified public
        accountant's written report so concluding, unless such report contains
        manifest error. In the event Lexicon's independent certified public
        accountant concludes that there was an overpayment of royalties to
        Lexicon during such period, the overpayment shall be repaid by Lexicon
        within [*] days of the date Lexicon received such independent certified
        public accountant's written report so concluding, unless such report
        contains manifest error. The fees charged by such independent certified
        public accountant shall be paid by Lexicon unless such audit discloses
        an underpayment of more than [*] percent ([*]%) of the amount due under
        this Agreement for the period in question, in which case Deltagen will
        bear the full cost of such audit. Lexicon agrees that all information
        subject to review under this Section 3.5 is confidential and that
        Lexicon shall cause its independent certified public accountant to
        retain all such information in confidence. Lexicon's independent
        certified public accountant shall only report to Lexicon as to
        information required to be reported under Section 3.4, including the
        computation of the royalties and other payments due to Lexicon under
        this Agreement, and shall not disclose to Lexicon any other information
        of Deltagen.

                3.6     To the extent that payments due under this Agreement are
        not paid within the specified time period and are not timely, such
        outstanding sums shall accrue interest from the date due, to be computed
        for such unpaid amount on the last day of each calendar quarter
        (accruing quarterly) at the prime rate as published in the Wall Street
        Journal for the last day of the calendar quarter plus one and a half
        percent (1.5%) (or the highest interest rate permissible under
        applicable law, whichever is lower), compounded on an annual basis.

                3.7     Payments to be made by Deltagen to Lexicon under this
        Agreement shall be payable in United States dollars and shall be paid by
        check delivered to Lexicon at its principal office at The Woodlands,
        Texas or bank wire transfer in immediately available funds to such bank
        account in the State of Texas as may be designated in writing by Lexicon
        from time to time.

                3.8     Payment obligations under Sections 3.1 shall not apply
        to Rodent Models made by Deltagen for Third Parties under
        Fee-For-Service Agreements in effect as of June 30, 2001; provided that
        any extension or expansion of any such Fee-For-Service Agreement
        (including, without limitation, any amendment that has the effect of
        increasing the number of Rodent Models that may be made under any
        Fee-For-Service Agreement) that is made or entered after June 30, 2001
        shall be deemed to be a new Fee-For-Service Agreement with respect to
        which such payment obligations shall apply. Deltagen represents and
        warrants that, as of the Effective Date, Deltagen Fee-For-Service
        Customers have the right to request no more than 16 lines of Rodent
        Models in addition to the lines of Rodent Models that have already been
        commenced by Deltagen prior to the Effective Date; provided that the
        parties acknowledge and agree that any breach of the foregoing
        representation and warranty shall not be deemed a material breach or
        default for purposes of Section 6.2, and that Lexicon's remedy for any
        such breach shall be the payment by Deltagen of the royalties specified
        in Section 3.3 with respect to the lines of

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        Rodent Models made for such Deltagen Fee-For-Service Customers in excess
        of the number of lines specified in such representation and warranty.

        4.      Infringement of Patent Rights. Lexicon shall have the exclusive
right, but shall not be obligated, to prosecute any infringements of the Patent
Rights. The total cost of any such infringement action commenced or defended by
Lexicon shall be borne by Lexicon, and Lexicon shall keep any recovery or
damages for past infringement derived therefrom.

        5.      Indemnification and Limitation of Liability.

                5.1     Deltagen shall at all times during the term of this
        Agreement and thereafter, indemnify, defend and hold Lexicon and its
        directors, officers, employees and Affiliates harmless from and against
        all claims, proceedings, demands, liabilities and losses of any kind
        whatsoever that are brought by a Third Party, including legal expenses
        and reasonable attorneys' fees, arising out of, based upon or resulting
        from (i) the use of the Patent Rights by Deltagen, its Affiliates,
        Academic Collaborators or Corporate Partners hereunder, (ii) the use of
        Rodent Models or Progeny by Deltagen, its Affiliates, Academic
        Collaborators or Corporate Partners or (iii) the use, testing, marketing
        or sale of Products, except to the extent that such claims, proceedings,
        demands, liabilities and losses result from Lexicon's gross negligence
        or willful misconduct.

                5.2     Lexicon warrants to Deltagen that: (i) Lexicon has the
        lawful right to grant the rights and licenses under the Patent Rights
        set forth in this Agreement; (ii) Lexicon's grant of such license does
        not require the approval or consent of any person or entity which has
        not already been obtained; and (iii) Lexicon's entering into of this
        Agreement will not breach any agreement to which Lexicon is party.
        EXCEPT AS OTHERWISE EXPRESSLY SET FORTH IN THIS AGREEMENT, LEXICON AND
        ITS DIRECTORS, OFFICERS, EMPLOYEES, AND AFFILIATES MAKE NO
        REPRESENTATIONS AND EXTEND NO WARRANTIES OF ANY KIND, EITHER EXPRESS OR
        IMPLIED, INCLUDING BUT NOT LIMITED TO WARRANTIES OF MERCHANTABILITY,
        FITNESS FOR A PARTICULAR PURPOSE, VALIDITY OF PATENT CLAIMS, ISSUED OR
        PENDING, AND THE ABSENCE OF LATENT OR OTHER DEFECTS, WHETHER OR NOT
        DISCOVERABLE. NOTHING IN THIS AGREEMENT SHALL BE CONSTRUED AS A
        REPRESENTATION MADE OR WARRANTY GIVEN BY LEXICON THAT THE PRACTICE BY
        DELTAGEN OF THE SUBLICENSE RIGHTS GRANTED HEREUNDER SHALL NOT INFRINGE
        THE PATENT RIGHTS OF ANY THIRD PARTY. IN NO EVENT SHALL LEXICON OR ITS
        DIRECTORS, OFFICERS, EMPLOYEES AND AFFILIATES BE LIABLE FOR INCIDENTAL
        OR CONSEQUENTIAL DAMAGES OF ANY KIND, INCLUDING ECONOMIC DAMAGE OR
        INJURY TO PROPERTY AND LOST PROFITS, REGARDLESS OF WITHER LEXICON SHALL
        BE ADVISED, SHALL HAVE OTHER REASON TO KNOW, OR IN FACT SHALL KNOW OF
        THE POSSIBILITY OF THE FOREGOING.

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        6.      Term and Termination.

                6.1     This Agreement shall become effective on the Effective
        Date and, unless terminated earlier, shall remain in full force and
        effect until the expiration of the last to expire of any Valid Claim
        included in the Patent Rights.

                6.2     Subject to the provisions of Section 10, upon any
        material breach or default of this Agreement, the Settlement Agreement
        or the DeltaBase Agreement by Deltagen or any of its Affiliates, or by
        Lexicon or any of its Affiliates, the party not in default shall have
        the right to terminate this Agreement and the rights, privileges and
        licenses granted hereunder, effective on [*] days' written notice,
        unless the other party shall have cured any such material breach or
        default prior to the expiration of such [*] day period. Notwithstanding
        the foregoing, the rights and licenses granted by Lexicon to Deltagen
        pursuant to Sections 2.1 and 2.2 to breed and use Rodent Models and
        Progeny shall survive any such termination of this Agreement with
        respect to lines of Rodent Models made by Deltagen prior to such
        termination and Progeny thereof, subject to Deltagen's continued
        compliance with its obligations under this Agreement with respect
        thereto; provided that, if this Agreement is terminated by Lexicon, such
        licenses shall terminate with respect to any lines of Rodent Models to
        which a breach or, default giving rise to such right of termination
        directly relate; and provided, further, that, following such
        termination, Deltagen shall have no further right or license under the
        Patent Rights to make Rodent Models.

                6.3     If either party shall liquidate, dissolve, file a
        voluntary petition in bankruptcy, be adjudicated a bankrupt, make a
        general assignment for the benefit of creditors, admit in writing that
        it is insolvent or fail to discharge within [*] days an involuntary
        petition in bankruptcy filed against it, this Agreement shall terminate
        upon written notice by the other party.

                6.4     No termination of this Agreement shall be construed to
        release either party from any obligation that matured prior to the
        effective date of such termination. The provisions of Sections 2.6, 4,
        5, 6.4, 8, 9, 10 and 12 shall survive the termination or expiration of
        this Agreement. The provisions of Section 2.3 shall survive the
        termination or expiration of this Agreement unless this Agreement is
        terminated by Lexicon pursuant to Section 6.2 or 6.3. The provisions of
        Sections 2.4 and 2.5 shall survive the termination or expiration of this
        Agreement unless this Agreement is terminated by Deltagen pursuant to
        Section 6.2 or 6.3.

        7.      Export Controls. Deltagen acknowledges that it is subject to
United States laws and regulations controlling the export of technical data,
computer software, laboratory prototypes and other commodities (including the
Arms Export Control Act, as amended and the United States Department of Commerce
Export Administration Regulations). The transfer of such items may require a
license from the relevant agency of the United States Government and/or written
assurances by Deltagen that Deltagen shall not export data or commodities to
certain foreign countries without prior approval of such agency. Lexicon does
not represent that a license shall not be required nor that, if required, it
shall be issued.

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        8.      Confidentiality of Terms; Publicity. The terms of this Agreement
shall be treated as confidential and shall not be disclosed to anyone except for
the parties' respective employees, consultants, agents and attorneys assisting
in the review and negotiation of this Agreement who have a need to know the
terms of this Agreement and have an obligation to keep such terms confidential,
or such other attorneys or agents who are performing due diligence on either
party and who are under an implied obligation of confidentiality, without the
written permission of the other party; provided that each party may disclose
that Deltagen has obtained a sublicense under the Patent Rights hereunder.
Notwithstanding the foregoing, each party may disclose such terms as may be
requested or required pursuant to interrogatories, requests for information or
documents; subpoena, civil investigative demand issued by a court or
governmental agency or as otherwise required by law; provided, however, that
such party shall provide prompt notice to the other party of such request or
requirement, giving (where practicable) the other party sufficient advance
notice to permit it to oppose, limit or seek confidential treatment for such
disclosure. The parties have agreed upon a public announcement with respect to
the execution of this Agreement. If either party desires to release any other
public announcement relating to this Agreement, it shall first allow the other
party to approve in writing such proposed announcement; provided that such
approval shall not be unreasonably withheld or delayed.

        9.      Assignment. This Agreement may not be assigned or otherwise
transferred by either party without the consent of the other party; provided
that either party may, without such consent, assign its rights and obligations
under this Agreement (i) to any Affiliate or (ii) in connection with a merger,
consolidation or sale of substantially all of its assets to a Third Party;
provided, however, that such party's rights and obligations under this Agreement
shall be assumed by its successor in interest in any such transaction. Any
purported assignment in violation of the preceding sentence shall be void. Any
permitted assignee shall assume all obligations of its assignor under this
Agreement.

        10.     Dispute Resolution.

                10.1    If any controversy or claim should arise under this
        Agreement, the matter shall be referred to an individual designated by
        the Chief Executive Officer (or the equivalent position) of Lexicon and
        an individual designated by the Chief Executive Officer (or the
        equivalent position) of Deltagen (the "Representatives"), who will
        attempt in good faith to resolve such controversy or, claim promptly by
        negotiations. If the matter has not been resolved within [*] days of the
        first meeting of the Representatives (which period may be extended by
        mutual agreement) concerning such matter, such matter shall be resolved
        by binding arbitration in accordance with Section 10.2.

                10.2    Any and all claims, disputes or controversies arising
        under, out of, or in connection with this Agreement that are not
        resolved by the parties under Section 10.1 shall be resolved by final
        and binding arbitration in [*] under the Commercial Arbitration Rules of
        the American Arbitration Association as then in effect. The arbitrators
        shall have no power to add to, subtract from or modify any of the terms
        or conditions of this Agreement or to award punitive damages. Any award
        rendered in such arbitration may be enforced by either party in any
        court of competent jurisdiction.

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

                10.3    Notwithstanding the foregoing, nothing in this Section
        shall be construed to, waive any rights or timely performance of any
        obligations existing under this Agreement.

        11.     Notices and Other Communications. Any notice or other
communication pursuant to this Agreement shall be sufficiently made or given on
the date of mailing if sent to such party by certified first class mail, return
receipt requested, postage prepaid, addressed to it at its address below or as
it shall designate by written notice given to the other party:

        In the case of Deltagen:

        Deltagen, Inc.
        740 Bay Road
        Redwood City, CA 94063-2469
        Attention: President
        Copy to: General Counsel

        In the case of Lexicon:

        Lexicon Genetics Incorporated
        4000 Research Forest Drive
        The Woodlands, Texas 77381
        Attention: President
        Copy to: Vice President, Intellectual Property

        12.     Miscellaneous.

                12.1    Governing Law. All disputes arising out of or related to
        this Agreement, or the performance, enforcement, breach or termination
        hereof, and any remedies relating thereto, shall be construed, governed,
        interpreted and applied in accordance with the laws of the State of
        Delaware, U.S.A., except that questions affecting the construction and
        effect of any patent shall be determined by the law of the country in
        which the patent shall have been granted.

                12.2    Entire Agreement. The parties hereto acknowledge that
        this Agreement sets forth the entire Agreement and understanding of the
        parties hereto as to the subject matter hereof, and shall not be subject
        to any change or modification except by the execution of a written
        instrument signed by the parties.

                12.3    Severability. The provisions of this Agreement are
        severable, and in the event that any provisions of this Agreement shall
        be determined to be invalid or unenforceable under any controlling body
        of the law, such invalidity or unenforceability shall not in any way
        affect the validity or enforceability of the remaining provisions
        hereof. The parties shall thereafter in good faith amend this Agreement
        to provide for an acceptable provision to replace such invalid or
        unenforceable provision.

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

                12.4    No Waiver. The failure of either party to assert a right
        hereunder or to insist upon compliance with any term or condition of
        this Agreement shall not constitute a waiver of that right or excuse a
        similar subsequent failure to perform any such term or condition by the
        other party.

                12.5    No Trademark Rights. Except as otherwise provided herein
        or agreed to in advance in writing, no right, express or implied, is
        granted by this Agreement to a party to use in any manner the names
        "Lexicon" or "Deltagen," or any other trade name or trademark of a party
        or the names of any employees thereof, for any purpose.

                12.6    Captions. The captions to this Agreement are for
        convenience only, and are to be of no force or effect in construing or
        interpreting any of the provisions of this Agreement.

                12.7    Counterparts. This Agreement may be executed in two or
        more counterparts, each of which shall be deemed an original, but all of
        which together shall constitute one and the same instrument.

                12.8    Independent Contractors. The relationship between
        Lexicon and Deltagen is that of independent contractors. Lexicon and
        Deltagen are not joint venturers, partners, principal and agent, master
        and servant, employer or employee, and have no other relationship other
        than independent contracting parties. Lexicon shall have no power to
        bind or obligate Deltagen in any manner, other than as is expressly set
        forth in this Agreement. Likewise Deltagen shall have no power to bind
        or obligate Lexicon in any manner other than as is expressly set forth
        in this Agreement.

                    [THIS SPACE IS INTENTIONALLY LEFT BLANK]

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

        IN WITNESS WHEREOF, the parties have duly executed this Agreement as of
the Effective Date.

                                 "DELTAGEN"

                                 DELTAGEN, INC.

                                 By:
                                      ------------------------------------------
                                 For: William Matthews, Ph.D.
                                      President and Chief Executive Officer

                                 "LEXICON"

                                 LEXICON GENETICS INCORPORATED

                                 By:
                                      ------------------------------------------
                                      Arthur T. Sands, M.D., Ph.D.
                                      President and Chief Executive Officer

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.

<PAGE>

                                    EXHIBIT A

                                  PATENT RIGHTS

POSITIVE-NEGATIVE SELECTION

1.      U.S. Patent No. 5,464,764 issued November 7, 1995 entitled
        "Positive-Negative Selection Methods and Vectors"

2.      U.S. Patent No. 5,487,992 issued January 30, 1996 entitled "Cells and
        Non-Human Organisms Containing Predetermined Genomic Modifications and
        Positive-Negative Selection Methods and Vectors for Making Same"

3.      U.S. Patent No. 5,627,059 issued May 6, 1997 entitled "Cells and
        Non-Human Organisms Containing Predetermined Genomic Modifications and
        Positive-Negative Selection Methods and Vectors for Making Same"

4.      U.S. Patent No. 5,631,153 issued May 20, 1997 entitled "Cells and
        Non-Human Organisms Containing Predetermined Genomic Modifications and
        Positive-Negative Selection Methods and Vectors for Making Same"

5.      U.S. Patent No. 6,204,061 issued March 20, 2001 entitled "Cells and
        Non-Human Organisms Containing Predetermined Genomic Modifications and
        Positive-Negative Selection Methods and Vectors for Making Same"

ISOGENIC DNA

U.S. Patent No. 5,789,215 issued August 4, 1998 entitled "Gene Targeting in
Animal Cells Using Isogenic DNA Constructs"

Certain confidential information contained in this document, marked by brackets,
has been omitted and filed separately with the Securities and Exchange
Commission pursuant to Rule 24b-2 of the Securities Exchange Act of 1934, as
amended.