<PAGE>

                                                                    Exhibit 10.6
                                                                    ------------

                      COLLABORATION AND LICENSE AGREEMENT

     This Collaboration and License Agreement (the "Agreement") dated as of
March 29/th/, 1999, is by and between NEXSTAR PHARMACEUTICALS, INC. ("NeXstar"),
having an address of 2860 Wilderness Place, Boulder Colorado 80301 and MEDAREX,
INC. ("Medarex") having an address of 1545 Route 22 East, Annandale, New Jersey
08801.

     WHEREAS NeXstar entered into the Regents Option which included an option to
negotiate an exclusive license for the Invention, namely Blockade of T
Lymphocyte Down-Regulation Associated with CTLA-4 Signaling;

     WHEREAS exercise of the Regents Option requires NeXstar to engage in
certain activities, including:

     (1)  Notice to Regents of its election to exercise the Regents Option
          before March 31, 1999;

     (2)  Submission of an acceptable Commercialization Plan to Regents before
          March 31, 1999;

     (3)  Reimbursement for outstanding past patent costs to Regents; and

     (4)  Execution of the Regents License no later than four months following
          NeXstar's exercise of the Regents Option;

     WHEREAS Medarex possesses expertise in developing drugs for therapeutic
uses, particularly the development of antibody drugs;

     WHEREAS NeXstar and Medarex wish to collaborate on the development and
commercialization of the Invention with therapeutic agents comprised of
antibodies, and possibly Aptamers and small molecules;

     WHEREAS Medarex wishes to obtain and NeXstar wishes to grant to Medarex a
sublicense under the Regents License to the Invention for exclusive use with
antibodies for all human therapeutic uses permitted by the Regents License;

     WHEREAS NeXstar may pursue development of a CTLA-4 Aptamer and Medarex may
wish to obtain and NeXstar wishes to grant to Medarex an option to negotiate an
option to license the CTLA-4 Aptamer from NeXstar; and

                                       1
<PAGE>

     WHEREAS NeXstar may initiate or Medarex may request NeXstar to initiate a
Small Molecule Program, and Medarex and NeXstar may then negotiate terms to
license the small molecule coming out of the Small Molecule Program.

     NOW THEREFORE, in consideration of the mutual covenants contained in this
Agreement and other good and valuable consideration, the receipt and sufficiency
of which are hereby acknowledged, the parties agree as follows:

1.   DEFINITIONS
     -----------

1.1  Defined Terms - In this Agreement, unless the context otherwise requires:
     -------------

     (a)  "Antibody Sublicense" means the sublicense granted under subparagraph
          3.1(a).

     (b)  "Affiliate" means any corporation, company, partnership, joint venture
          and/or firm which controls, is controlled by or is under common
          control with NeXstar or Medarex. For purpose of this subparagraph
          1.1(b), "control" means:

          (i)  In the case of corporate entities, direct or indirect ownership
               of at least 50% of the stock or shares entitled to vote for the
               election of directors, and

          (ii) In the case of non-corporate entities, direct or indirect
               ownership of at least 50% of the equity interest with the power
               to direct the management and policies of such non-corporate
               entities.

     (c)  "Aptamer License Option" means the exclusive option which the parties
          may negotiate pursuant to the Aptamer Option. If negotiated between
          the parties pursuant to the Aptamer Option, the Aptamer License Option
          will be an exclusive option by NeXstar to Medarex to license a CTLA-4
          Aptamer successfully developed by NeXstar.

     (d)  "Aptamer Option" means the exclusive option granted by NeXstar to
          Medarex pursuant to paragraph 4.1 to negotiate the terms of the
          Aptamer License Option.

     (e)  "Aptamers" means non-naturally occurring nucleic acid ligands having a
          specific binding affinity for a target molecule. For the purpose of
          this Agreement, the target molecule is CTLA-4 and the Aptamers are
          capable of the Blockade of T Lymphocyte Down-Regulation Associated
          with CTLA-4 Signaling.

     (f)  "Commercialization Plan" means a written plan prepared by Medarex with
          assistance from NeXstar pertaining to the development and
          commercialization of a CTLA-4 antibody containing at least the
          following criteria:

                                       2
<PAGE>

          [*****]

          A copy of the Commercialization Plan is attached as Exhibit A to this
          Agreement.

     (g)  "Commercially Reasonable Efforts"[*****]

     (h)  "Confidential Information" shall mean:

          (i)  Any proprietary or confidential information or material in
               tangible form, including but not limited to scientific, technical
               and/or commercial information and materials and trade secrets,
               disclosed under this Agreement by NeXstar and/or Medarex that is
               marked as "Confidential" at the time it is delivered to the
               Recipient, or

          (ii) Proprietary or confidential information disclosed orally under
               this Agreement by NeXstar and/or Medarex which is identified as
               confidential or proprietary when disclosed.

     (i)  "CTLA-4 Aptamer" means an Aptamer to CTLA-4 (including without
          limitation, truncates, point mutations and chimeras of the Aptamers)
          developed by NeXstar.

     (j)  "Disclosing Party" means either NeXstar or Medarex, whichever is
          disclosing Confidential Information in the particular circumstance.

     (k)  "Effective Date" means the effective date of this Agreement, set forth
          in the first paragraph of this Agreement.

     (l)  "Evolutionary Chemistry Technology" includes all NeXstar technology
          necessary or useful for the generation of starting libraries for the
          Evolutionary Chemistry process or necessary or useful for the
          generation, development and commercialization of compounds generated
          and identified through the Evolutionary Chemistry process in
          accordance with the invention described in U.S. Patent No. 5,732,289.
          Evolutionary Chemistry Technology includes, but is not limited to:

          (i)  The patents and patent applications listed in Exhibit B to this
               Agreement,

          (ii) Any patents and patent applications which may be filed and which
               relate to the generation of starting libraries for the
               Evolutionary Chemistry process or which relate to the generation,
               development and commercialization of

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURTIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       3
<PAGE>

                compounds generated and identified through the Evolutionary
                Chemistry process,

          (iii) Any and all continuations, continuations-in-part, divisionals,
                reissues or re-examinations of or to the patents and patent
                applications listed at clauses (i) and (ii) which have been or
                may be filed and which relate to the generation of starting
                libraries for the Evolutionary Chemistry process or which relate
                to the generation, development and commercialization of
                compounds generated and identified through the Evolutionary
                Chemistry process, and

          (iv)  All corresponding foreign applications and patents which have
                been or may be filed and which relate to the generation of
                starting libraries for the Evolutionary Chemistry process or
                which relate to the generation, development and
                commercialization of compounds generated and identified through
                the Evolutionary Chemistry process.

     (m)  "Infringement" means any patent infringement of the Licensed Patents
          by a product in the Sublicensed Field of Use.

     (n)  "Infringement Notice" means the prompt written notice from one party
          to the other if one party becomes aware of an Infringement.

     (o)  "Invention" means the inventions described in "Blockade of T-
          Lymphocyte Down-Regulation Associated with CTLA-4 Signaling" as
          described in Regents' Case No. B96-002.

     (p)  [*****]

     (q)  "Licensed Method" means any method or process that is covered by the
          Licensed Patent Rights, or any method or process the use or practice
          of which would constitute an infringement of any pending claims (were
          they to issue) to issued claims within the Licensed Patent Rights.

     (r)  "Licensed Patent Rights" shall have the same meaning as Regents Patent
          Rights under the Regents Option, namely:

          (i)   The U.S. Continuation-in-Part application entitled "Blockade of
                T Lymphocyte Down-Regulation Associated with CTLA-4 Signaling,"
                serial number 08/566,853 (U.C. Case No.: B96-002-2) filed on
                December 4, 1995,

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURTIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       4
<PAGE>

                by Drs. James P. Allison, Dana R. Leach and Matthew Krummel of
                the University of California at Berkeley and assigned to
                Regents, that claims priority to Serial Number 08/506,666 (U.C.
                Case No.: B96-002-1, now abandoned) filed on July 25, 1995,

          (ii)  The U.S.. Continuation-in-Part application entitled, "Blockade
                of T Lymphocyte Down-Regulation Associated with CTLA-4
                Signaling," serial number 08/646,605 (U.C. Case No.: B96-002-3)
                filed May 8, 1996, by Drs. James P. Allison, Dana R. Leach and
                Matthew Krummel of the University of California at Berkeley and
                assigned to Regents and issued as U.S. Patent No. 5,811,097,
                that is a CIP of Serial Number 08/566,853, which is a U.S. CIP
                of 08/506,666,

          (iii) The U.S. Continuation-in-Part application entitled, "Blockade of
                T Lymphocyte Down-Regulation Associated with CTLA-4 Signaling,"
                serial number 08/760,288 (U.C. Case No.: B96-002-4) filed
                December 4, 1996, by Drs. James P. Allison, Dana R. Leach and
                Matthew Krummel of the University of California at Berkeley and
                assigned to Regents,

          (iv)  PCT International Patent application, Serial Number US96/19375
                filed on December 4, 1996, by Drs. James P. Allison, Dana R.
                Leach and Matthew Krummel and assigned to Regents (U.C. Case No.
                B93-002/PCT) that claims the convention priority benefits of:
                U.S. Patent Application Serial No. 08/566,853 dating from the
                U.S. filing date of December 4, 1995, and U.S. Patent
                Application Serial No. 08/646,605 dating from the U.S. filing
                date of May 8, 1996, and

          (v)   All other international patents and patent applications, and/or
                any divisions, continuations, reissues and claims in
                continuations-in-part that are (a) entitled to the priority
                filing date of Serial Numbers 08/506,666, 08,566,853, 08/646,605
                or 08/760,288 and (b) are claimed or described in Serial Numbers
                08/506,666, 08/566,853, 08/646,605 or 08/760,288.

     (s)  "Litigation" means a proceeding in respect of an Infringement,
          including, without limitation, a proceeding before the United States
          Patent and Trademark Office, the European Patent Office or other
          regulatory body or litigation in a court of law.

     (t)  "Medarex Confidential Information" means Confidential Information,
          whether belonging to Regents or Medarex, disclosed to NeXstar by or on
          behalf of Medarex.

     (u)  "NDA" shall have the same definition as is included in the Regents
          License. If no such definition is provided, NDA shall mean a New Drug
          Application filed with the United States Food and Drug Administration.

                                       5
<PAGE>

     (v)  "Net Sales" shall have the same definition with regard to Sublicensed
          Products under this Agreement as with regard to Licensed Products
          under the Regents License. If no such definition is provided, "Net
          Sales" shall mean the invoiced sales price of Sublicensed Products
          billed to unaffiliated customers, less:

          (i)   Actual credited allowances and/or charge-backs for returned
                product,

          (ii)  Quantity and other trade discounts actually allowed and taken,

          (iii) Sales, value-added and other direct taxes incurred,

          (iv)  Customs duties and surcharges and other governmental charges
                incurred in connection with the exportation or importation of
                any Sublicensed Product, and

          (v)   Legally mandated rebates.

     (w)  "NeXstar Confidential Information" means Confidential Information,
          whether belonging to Regents or NeXstar, disclosed to Medarex by or on
          behalf of NeXstar.

     (x)  "NeXstar's Option Legal Fees" means fees paid to outside legal counsel
          by NeXstar of [*****] related to the execution of the Regents Option
          and evaluation of the Licensed Patent Rights under the Regents Option.

     (y)  "Option Fee" means the [*****] fee paid by NeXstar to Regents upon
          signing of the Regents Option.

     (z)  "Publication" means any proposed publication, press release or other
          such public disclosure, written or oral:

          (i)   Of the terms of this Agreement, or

          (ii)  Concerning, relating to or derived from Confidential Information
                of the other party or use of any Confidential Information of the
                other party.

     (aa) "Recipient" means either NeXstar or Medarex, whichever is receiving
          Confidential Information in the particular circumstance.


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURTIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       6
<PAGE>

     (bb) "Regents" means the Regents of the University of California, a
          California corporation whose legal address is 300 Lakeside Drive,
          Oakland, California 94612-3550, acting through its Office of
          Technology Licensing, of the University of California, Berkeley, 2150
          Shattuck Avenue, Suite 510, Berkeley, California 97420-1620.

     (cc) "Regents Foreign Patent Costs" means the costs incurred by Regents in
          pursuing PCT applications for the Invention and patent prosecution in
          those countries outside the United States designated by NeXstar under
          the Regents License, and after the date of execution of the Regents
          License, as requested in writing by Medarex.

     (dd) "Regents License" means a license agreement to be negotiated from
          Regents to NeXstar under the Licensed Patent Rights, pursuant to the
          Regent's Option, granting to NeXstar the right to sublicense to
          Medarex the rights set forth in this Agreement, including, without
          limitation, the grant of the Antibody Sublicense in subparagraph
          3.1(a).

     (ee) "Regents Option" means the agreement titled Option to Negotiate an
          Exclusive License between NeXstar Pharmaceuticals, Inc. and the
          Regents of the University of California for Blockade of T Lymphocyte
          Down-Regulation Associated with CTLA-4 Signaling effective April 1,
          1998 and expiring March 31, 1999.

     (ff) "Regents Past Patent Costs" means costs incurred by Regents and paid
          by NeXstar to Regents under the Regents Option for filing, prosecution
          and maintenance of the Licensed Patent Rights.

     (gg) "Related Patent Costs" means all costs, excluding previously
          reimbursed Regents Past Patent Costs, or Regents Foreign Patent Costs,
          incurred by Regents prior to or during the term of the Regents License
          associated with preparing, filing, protecting and maintaining the
          Licensed Patent Rights.

     (hh) [*****]

     (ii) "Sublicensed Field of Use" means use of the Sublicensed Products that
          are antibodies (including without limitation, bispecific antibodies,
          single chain antibodies and/or immunoconjugated antibodies) for all
          human therapeutic uses permitted by the Regents License.

     (jj) "Sublicensed Products" means within the Sublicensed Field of Use:

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURTIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       7
<PAGE>

          (i)   Any product produced using a Licensed Method, or for use in the
                practice of a Licensed Method,

          (ii)  Any method, process, material or product the manufacture, use or
                sale of which would be an infringement of any pending claim
                (were it to issue) or issued claim of the Licensed Patent
                Rights, or

          (iii) Any material, method, process or product, or part thereof, that:

                (A) Is covered by a valid claim of any issued, unexpired patent
                    within Licensed Patent Rights. A claim within the Licensed
                    Patent Rights shall be presumed to be valid unless and until
                    it has been held to be invalid by a final judgment of a
                    court of competent jurisdiction from which no appeal can be
                    or is taken, or

                (B) Is covered by any claim being prosecuted in a pending
                    application directed to the Invention (were it to issue).

     (kk) "Sublicensed Territory" means wherever Regents has Licensed Patent
          Rights.

     (ll) "Sub-licensee" means a third party, including any Medarex Affiliate,
          to which Medarex has granted a further sublicense to make, have made,
          use, offer for sale and import under the rights granted under
          paragraph 3.1 of this Agreement.

     (mm) "Term" shall have the meaning set forth in paragraph 9.1.

1.2  Additional Definitions - Any words defined elsewhere in this Argument shall
     ----------------------
     have the particular meaning assigned thereto.

2.   CONTINGENCY - Except as specified in this Section 2, the terms, conditions,
     -----------
     and obligations set forth in this Agreement are contingent upon NeXstar and
     Regents executing the Regents License. In conjunction with negotiating the
     Regents License (unless NeXstar is prohibited in the course of the
     negotiations), NeXstar shall provide Medarex with copies of proposed drafts
     of the Regents License so that Medarex may comment upon and assist NeXstar
     in the negotiation of the Regents License. In the event that NeXstar and
     Regents fail to enter into the Regents License on or before August 1, 1999
     (or such later date as may be acceptable to Medarex) or in the event that
     the executed Regents License is not reasonably acceptable to Medarex, this
     Agreement shall be null and void except with regard to the obligations of
     confidentiality set forth in Section 10. Provided that the form of the
     Regents License is reasonably acceptable to Medarex, Medarex will
     acknowledge this acceptability in writing prior to execution by NeXstar of
     the Regents License.

                                       8
<PAGE>

3.   ANTIBODY SUBLICENSE
     -------------------

3.1  Grant Of Sublicense and License
     -------------------------------

     (a)  Sublicense under Regents License - Subject to the government rights
          described in the Regents License and the provisions of subparagraph
          3.1(d), NeXstar grants to Medarex a worldwide, exclusive sublicense
          under the Regents License to make, have made, use, sell, offer for
          sale and import the Sublicensed Products in the Sublicensed Field of
          Use and the Sublicensed Territory for the life of the last to expire
          patent under the Licensed Patent Rights.

     (b)  Mouse Hybridoma License - Subject to the provisions of subparagraph
          3.1(d), NeXstar grants to Medarex:

          (i)   A worldwide, exclusive license to use and propagate NeXstar's
                Mouse Hybridoma No. 147.1.33 to make antibodies, and

          (ii)  A worldwide, exclusive license to make, have made, use, sell,
                offer for sale and import Sublicensed Products containing
                antibodies produced from NeXstar's Mouse Hybridoma No. 147.1.33
                in the Sublicensed Field of Use and the Sublicensed Territory.

     (c)  Delivery - If NeXstar has not already done so prior to the Effective
          Date, within five days of the Effective Date, NeXstar shall deliver to
          Medarex the NeXstar Mouse Hybridoma No. 147.1.33 and all NeXstar
          technical information relating to the NeXstar Mouse Hybridoma No.
          147.1.33.

     (d)  Retention of Research Rights - Notwithstanding subparagraphs 3.1(a)
          and (b) above, the exclusive worldwide sublicenses granted by NeXstar
          to Medarex under subparagraphs 3.1(a) and (b) shall be subject to a
          right (non-transferable except pursuant to paragraphs 14.4 and 14.5)
          retained by NeXstar to use Mouse Hybridoma No. 147.1.33 and the murine
          antibody produced by Mouse Hybridoma No. 147.1.33 in the Sublicensed
          Field of Use in the Sublicensed Territory solely for internal non-
          commercial research purposes.

3.2  Further Sublicenses  - Provided further sublicensing is permitted by the
     -------------------
     Regents License, Medarex shall have the right to grant further sublicenses
     of any of the rights granted under subparagraphs 3.1(a) and (b), subject to
     the following terms and conditions:

     (a)  Medarex Still Liable - Notwithstanding any sublicense granted by
          Medarex under this Agreement, Medarex shall remain liable for the
          performance of all duties and obligations contained in this Agreement.

                                       9
<PAGE>

     (b)  Notice  - Medarex shall promptly provide NeXstar and the Regents with
          a copy of each sublicense issued (such copy shall be treated as
          Medarex Confidential Information and shall not be disclosed to any
          third party except as permitted by the provisions of Section 10 of
          this Agreement).

     (c)  Payments - Medarex shall collect and guarantee payment of all payments
          due NeXstar and Regents from Sub-licensees.

     (d)  Reports - Medarex shall summarize and deliver all reports due NeXstar
          and Regents from Sub-licensees.

     (e)  Termination

          (i)   Termination of sublicenses under this Agreement shall be
                governed by the provisions of the Regents License with respect
                to termination of sublicenses. In the absence of provisions in
                the Regents License with respect to termination of sublicenses,
                the provisions of clause (ii) shall apply.

          (ii)  Upon termination of this Agreement for any reason, all
                sublicenses granted by Medarex in accordance with this Agreement
                will remain subject to the terms of such sublicenses in effect,
                and shall be assigned to and assumed by NeXstar except that
                sublicenses which are in a state of material breach as yet
                uncured by the Sub-licensee shall not be assigned to and assumed
                by NeXstar. NeXstar shall not be bound by any duties and
                obligations beyond the duties and obligations assumed by NeXstar
                in this Agreement.

3.3  Payments and Royalties
     ----------------------

     (a)  Reimbursement to NeXstar - Within ten days following the execution of
          the Regents License in a form reasonably acceptable to Medarex,
          Medarex shall reimburse NeXstar's out of pocket costs associated with
          the performance of the terms of the Regents Option in an amount not to
          exceed [*****]. These out of pocket costs shall be limited to:

          (i)   The [*****] Option Fee under the Regents Option,

          (ii)  Up to [*****] in Regents Past Patent Costs,

          (iii) [*****]  in Regents Foreign Patent Costs, and


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURTIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       10
<PAGE>

          (iv)  [*****] in NeXstar's Option Legal Fees.

     (b)  Flow Through License Payments - Medarex shall pay to NeXstar the
          amounts set out below in this subparagraph (b) required to be paid by
          NeXstar to Regents under the Regents License. Provided that NeXstar
          has given Medarex at least ten days advance notice of the date that
          each payment is due, Medarex shall make the payment to NeXstar at the
          time these payments are to be paid by NeXstar to Regents (so that
          there is no out of pocket expense to NeXstar for these payments).

          (i)   All Related Patent Costs,

          (ii)  A non-refundable license fee of [*****] at the time of execution
                of the Regents License in a form reasonably acceptable to
                Medarex,

          (iii) Milestone payments as follows:

                [******]

     (c)  [*****]

     (d)  Royalty Rates - Medarex shall pay NeXstar a [*****] royalty on Net
          Sales of all Sublicensed Products.  Royalties shall be calculated and
          paid quarterly and shall be due within 60 days following the end of
          each calendar quarter (or such shorter period of time as is set out in
          the Regents License). NeXstar shall be responsible for payment to
          Regents of royalties owing under the Regents License.

     (e)  Offsets and Deductions - With respect to running royalties due NeXstar
          under this Agreement, to the extent that the Regents License contains
          provisions entitling NeXstar to credit any amounts toward royalties or
          other payment due Regents or allows any offset against amounts due
          Regents with respect to payments made to third parties, Medarex shall
          be entitled to equivalent credits and offsets against amounts due to
          NeXstar, provided, regardless of any credits or offsets available to
          Medarex, in no event shall Medarex pay to NeXstar a royalty of  less
          than a [*****] royalty on Net Sales of all Sublicensed Products.

     (f)  Penalties - Failure of Medarex to make the payments set out in
          subparagraphs (b), (c) and (d) above  may subject Medarex to penalties
          specified under the Regents License for failure to make these
          payments.


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURTIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       11
<PAGE>

3.4  Commercially Reasonable Efforts - Medarex shall use Commercially Reasonable
     -------------------------------
     Efforts in meeting the criteria set forth in the Commercialization Plan
     attached as Exhibit A, the terms of which are incorporated into this
     Agreement.

4.   CTLA-4 APTAMER OPTION
     ---------------------

4.1  Aptamer Option - NeXstar grants the Aptamer Option to Medarex. The Aptamer
     --------------
     Option will expire at 11:59 p.m. Mountain Standard Time on that date which
     is four months after the date of signing of the Regents License. Medarex
     may exercise the Aptamer Option by written notice to NeXstar, specifying
     that Medarex wishes to exercise the Aptamer Option and the terms proposed
     by Medarex for the Aptamer License Option. If Medarex gives notice of
     exercise of the Aptamer Option, the parties shall immediately meet and use
     good faith efforts to mutually agree within the four month period of the
     Aptamer Option to the terms of the Aptamer License Option. The parties
     agree that any license to the CTLA-4 Aptamer will include, but not be
     limited to, an up-front sublicense fee, milestones, royalties, reporting
     and record keeping provisions and due diligence development and
     commercialization provisions.

4.2  Failure to License
     ------------------

     (a)  Third Party Negotiations - If Medarex exercises the Aptamer Option but
          the parties (both acting reasonably) do not agree before expiry of the
          four month Aptamer Option period on reasonable terms for a license to
          the CTLA-4 Aptamer, then NeXstar shall be free (subject to the
          provisions of subparagraph (b) below) to negotiate with a third party
          for a license to the CTLA-4 Aptamer.

     (b)  Second Right In Favor of Medarex - NeXstar agrees that it will not
          license the CTLA-4 Aptamer to a third party on terms substantially
          more favorable than the terms discussed with Medarex under paragraph
          4.1 without giving Medarex an opportunity to license the CTLA-4
          Aptamer on the more favorable terms.   NeXstar shall give written
          notice of the more favorable terms for a license to the CTLA-4 Aptamer
          to Medarex. Medarex shall have ten days from this written notice to
          exercise this second right (Medarex must give written notice to
          NeXstar within this ten day period in order to exercise this right).
          This second right shall only be a "yes/no" right, with no right to
          negotiate terms. If Medarex does agree to license the CTLA-4 Aptamer
          pursuant to the second right contained in this subparagraph (b), then
          NeXstar and Medarex shall promptly enter into a license for the CTLA-4
          Aptamer on the more favorable terms. If Medarex does not agree to
          license the CTLA-4 Aptamer pursuant to the second right contained in
          this subparagraph (b), then NeXstar shall be free to license or
          otherwise exploit the CTLA-4 Aptamer as

                                       12
<PAGE>

          NeXstar determines in its sole discretion, without further
          consultation with or obligation to Medarex.

5.   SMALL MOLECULE OPTION
     ---------------------

5.1  NeXstar Undertaking - If during the Term NeXstar undertakes a Small
     -------------------
     Molecule Pro-ram, NeXstar shall promptly notify Medarex in writing.
     Medarex shall then have an option for a period of six months from the date
     of such notice from NeXstar to participate in the Small Molecule Program
     under terms to be agreed by the parties under paragraph 5.3 below. Medarex
     may exercise its option by written notice to NeXstar.

5.2  Medarex Request
     ---------------

     (a)  Right to Request a Small Molecule Program - Subject to the provisions
          of subparagraph (b) below, Medarex shall have the right, beginning one
          year from the Effective Date and extending for a period of two years,
          to request that NeXstar undertake the Small Molecule Program. In this
          situation, upon the parties reaching agreement on the terms of
          participation in the Small Molecule Program under paragraph 5.3 below,
          NeXstar shall undertake the Small Molecule Program.

     (b)  Provisio to Request - NeXstar's obligation to undertake a Small
          Molecule Program upon request by Medarex under subparagraph (a) is
          contingent upon NeXstar still owning and exploiting at least part of
          the Evolutionary Chemistry Technology at the time of the request from
          Medarex. If NeXstar does not own or is not exploiting at least part of
          the Evolutionary Chemistry Technology at the time of the request from
          Medarex, NeXstar shall have no obligation to undertake the Small
          Molecule Program.

     (c)  Acknowledgment/Covenant - Medarex acknowledges that NeXstar shall have
          an absolute right to use, dispose of or otherwise deal with its
          Evolutionary Chemistry Technology as it sees fit, in its sole
          discretion. NeXstar covenants and agrees, however, that if NeXstar
          sells or otherwise disposes of its Evolutionary Chemistry Technology,
          NeXstar will not also grant a sublicense under the Regents License to
          Sublicensed Products that are small molecule therapeutics identified
          using the Evolutionary Chemistry Technology to the acquiring party
          without Medarex's prior written consent, such consent not to be
          unreasonably withheld or delayed.

5.3  Terms of Participation - If Medarex exercises its option under paragraph
     ----------------------
     5.1 or if Medarex requests that NeXstar undertake the Small Molecule
     Program under paragraph 5.2 (subject to the provisions of subparagraph
     5.2(b)), the parties shall meet and use good faith efforts to mutually
     agree to the terms under which NeXstar and Medarex will participate in the
     Small Molecule Program, such terms to include, but not being limited to,
     research funding, an up-front sublicense fee, milestones, royalties,
     reporting, and record keeping provisions

                                       13
<PAGE>

     and due diligence development and commercialization provisions. If the
     parties can not agree within a six month period (commencing at the time of
     exercise by Medarex of its option under paragraph 5.1 or at the time
     Medarex requests NeXstar to undertake the Small Molecule Program under
     paragraph 5.2 (subject to the provisions of subparagraph 5.2 (b)), as the
     case may be) on reasonable terms under which NeXstar and Medarex will
     participate in the Small Molecule Program, the unresolved matters or
     matters in dispute shall be submitted to mediation pursuant to paragraph
     13.1.

5.4  Failure to License - If:
     ------------------

     (a)  No Program - Medarex does not exercise its option under paragraph 5.1
          and Medarex does not request that NeXstar undertake the Small Molecule
          Program under paragraph 5.2; or

     (b)  Failure to Reach Agreement on Terms - Medarex exercises its option
          under paragraph 5.1 or Medarex requests that NeXstar undertake the
          Small Molecule Program under paragraph 5.2, but the parties can not
          agree within the six month period on reasonable terms under which
          NeXstar and Medarex will participate in the Small Molecule Program
          (and mediation does not assist the parties to reach terms upon which
          they will work together);

     then, subject to the provisions of paragraph 5.5, NeXstar shall be free to
     negotiate with a third party to collaborate on the Small Molecule Program.

5.5  Second Right In Favor of Medarex - NeXstar agrees that it will not
     --------------------------------
     collaborate with a third party on the Small Molecule Program on terms
     substantially more favorable than the terms discussed with Medarex under
     paragraph 5.3 without giving Medarex an opportunity to participate in the
     Small Molecule Program on the more favorable terms. NeXstar shall give
     written notice of the more favorable terms for the Small Molecule Program
     to Medarex. Medarex shall have ten days from this written notice to
     exercise this second right (Medarex must give written notice to NeXstar
     within this ten day period in order to exercise this right). This second
     right shall only be a "yes/no" right, with no right to negotiate terms. If
     Medarex does agree to participate in the Small Molecule Program pursuant to
     the second right contained in this paragraph 5.5, then NeXstar and Medarex
     shall promptly enter into an agreement for the Small Molecule Program on
     the more favorable terms. If Medarex does not agree to participate in the
     Small Molecule Program pursuant to the second right contained in this
     paragraph 5.5, then NeXstar shall be free to collaborate, license or
     otherwise pursue and exploit the Small Molecule Program as NeXstar
     determines in its sole discretion, without further consultation with or
     obligation to Medarex.

5.6  Additional Right to Sublicense - Subject to the provisions of paragraphs
     ------------------------------
     5.7 and 5.8, if:

                                       14
<PAGE>

     (a)  No Access to Evolutionary Chemistry Technology - NeXstar has sold or
          otherwise disposed of the Evolutionary Chemistry Technology, or is no
          longer exploiting at least part of the Evolutionary Chemistry
          Technology;

     (b)  Medarex Has Small Molecule Opportunity - Medarex has an opportunity to
          pursue commercial development of a small molecule therapeutic to CTLA-
          4 using the Invention (as evidenced by agreed upon terms in writing);
          and

     (c)  Medarex Given Notice - Medarex has given written notice to NeXstar of
          Medarex's small molecule opportunity and that Medarex wishes to obtain
          an additional sublicense under the Regents License for Sublicensed
          Products that are small molecule therapeutics;

     then the parties shall meet and use good faith efforts to negotiate the
     terms of an additional sublicense under the Regents License for Sublicensed
     Products that are small molecule therapeutics. If the parties (both acting
     reasonably) do not reach agreement on the terms of this additional
     sublicense within three months of the notice from Medarex under
     subparagraph (c), the parties may submit the unresolved matters or matters
     in dispute to mediation pursuant to paragraph 13.1.  If the parties (both
     acting reasonably):

          (i)   Do not reach agreement on the terms of this additional
                sublicense within three months of the notice from Medarex under
                subparagraph (c), or

          (ii)  Do not reach agreement as set out in clause (i) and proceed to
                mediation, but still do not reach agreement on the terms of this
                additional sublicense, the parties shall have no further rights
                or obligations to each other under this paragraph 5.6.

5.7  First Exception to the Additional Right to Sublicense - The provisions of
     -----------------------------------------------------
     paragraph 5.6 are subject to the following provisions. If:

     (a)  No Access to Evolutionary Chemistry Technology - NeXstar has sold or
          otherwise disposed of the Evolutionary Chemistry Technology, or is no
          longer exploiting at least part of the Evolutionary Chemistry
          Technology;

     (b)  NeXstar Has Small Molecule Opportunity - NeXstar has an opportunity to
          grant a sublicense under the Regents License to a third party for
          Sublicensed Products that are small molecule therapeutics (as
          evidenced by agreed upon terms in writing); and

     (c)  NeXstar Given Notice - NeXstar has given written notice to Medarex of
          NeXstar's small molecule therapeutic sublicense opportunity and that
          NeXstar wishes to grant this sublicense;

                                       15
<PAGE>

     then, unless within three months from the date of the written notice from
     NeXstar to Medarex as set out in subparagraph (c), Medarex gives written
     notice to NeXstar that:

          (i)   Medarex has an opportunity to pursue commercial development of a
                small molecule therapeutic to CTLA-4 using the Invention (as
                evidenced by agreed upon terms in writing), and

          (ii)  Medarex agrees to the same sublicense terms as NeXstar has
                negotiated with a third party (as evidenced by the agreed upon
                terms in writing as set out at subparagraph (b)),

     NeXstar shall be free to enter into the sublicense under the Regents
     License (as described in subparagraph (b) above) with the third party for
     Sublicensed Products that are small molecule therapeutics and Medarex shall
     have no further rights under paragraph 5.6. If Medarex gives written notice
     to NeXstar as set out in clauses (i) and (ii) above, NeXstar and Medarex
     shall enter into a sublicense under the Regents License for Sublicensed
     Products that are small molecule therapeutics. NeXstar covenants and agrees
     that it will not actively seek out sub-licensees under the Regents License
     for Sublicensed Products that are small molecule therapeutics.

5.8  Second Exception to the Additional Right to Sublicense - The provisions of
     ------------------------------------------------------
     paragraph 5.6 are subject to the following provisions. If:

     (a)  No Access to Evolutionary Chemistry Technology - NeXstar has sold or
          otherwise disposed of the Evolutionary Chemistry Technology, or is no
          longer exploiting the Evolutionary Chemistry Technology; and

     (b)  NeXstar Intends to Pursue its Own Small Molecule Program - NeXstar
          intends to pursue a small molecule program to CTLA-4 using the
          Invention itself;

     NeXstar shall give written notice of its intention to pursue its own small
     molecule program to CTLA-4 using the Invention and NeXstar shall thereafter
     be free to pursue its own small molecule program to CTLA-4 using the
     Invention and Medarex shall have no further rights under paragraph 5.6.

6.   REIMBURSEMENT OF PATENT COSTS
     -----------------------------

     (a)  Aptamer Option - If.-

          (i)   Medarex does not exercise the Aptamer Option under paragraph 4.
                1, or

          (ii)  Medarex exercises the Aptamer Option under paragraph 4.1 but
                does not acquire a license to the CTLA-4 Aptamer,

                                       16
<PAGE>

     then NeXstar shall reimburse Medarex for [*****] of Regents Past Patent
     Costs, Regents Foreign Patent Costs and Related Patent Costs paid by
     Medarex up to such date. NeXstar shall pay this amount to Medarex within 30
     days of the last to occur of:

          (A)  Expiry of the Aptamer Option, or

          (B)  Expiry of the Aptamer License Option.

     (b)  Small Molecule Program - If:

          (i)   Within three and a half years from the Effective Date a Small
                Molecule Program is not undertaken by NeXstar (either on its own
                undertaking as set out in paragraph 5.1 or at Medarex's request
                as set out in paragraph 5.2), or

          (ii)  A Small Molecule Program is undertaken by NeXstar and Medarex
                does not exercise its option under paragraph 5.1,

     then NeXstar shall reimburse Medarex for Regents Past Patent Costs, Regents
     Foreign Patent Costs and Related Patent Costs paid by Medarex up to such
     date in accordance with the following schedule:

     (A)  If Medarex was reimbursed pursuant to subparagraph 6(a), NeXstar shall
          reimburse Medarex for [*****] of Regents Past Patent Costs, Regents
          Foreign Patent Costs and Related Patent Costs paid by Medarex to date
          (for a total reimbursement of [*****], or

     (B)  If Medarex was not reimbursed pursuant to subparagraph 6(a), NeXstar
          shall reimburse Medarex for [*****] of Regents Past Patent Costs,
          Regents Foreign Patent Costs and Related Patent Costs paid by Medarex
          to date.

     Unless the parties have gone to mediation to attempt to resolve matters in
     dispute, NeXstar shall pay this amount to Medarex within 30 days of expiry
     of the six month period (to negotiate the terms of participation in the
     Small Molecule Program).

7.   REPORTS - During the Term, Medarex shall have the following reporting
     -------
     obligations to NeXstar with regard to the Antibody Sublicense under Section
     3:


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       17
<PAGE>

     (a)  Regents Reports - Medarex shall provide to NeXstar all reports
          required under the Regents License as specified in writing by NeXstar.

     (b)  Other Reports - Unless a substantially equivalent report is required
          under subparagraph 7(a), Medarex shall provide the following reports
          to NeXstar:

          (i)  Semiannual Progress Reports - Until the first commercial sale of
               ---------------------------
               any Sublicensed Product(s) under this Agreement, Medarex shall
               submit semiannual progress reports to NeXstar, such reports to
               set out Medarex's progress with development of any Sublicensed
               Product(s) in the past six months. These semiannual reports shall
               be delivered to NeXstar within 60 days of the end of each six-
               month period, the first six-month period commencing on the
               Effective Date.

          (ii) Royalty Reports - After the first commercial sale of any
               ---------------
               Sublicensed Product(s) under this Agreement, and for the
               remainder of the Term, Medarex shall submit quarterly payment
               reports to NeXstar (within 60 days of the end of the calendar
               quarter, at the same time as the royalty payment for the
               quarter). The reports shall include, but not be limited to:

          (A)  An accounting of worldwide Net Sales on a country-by-country
               basis,

          (B)  The sales price for all Sublicensed Product(s) sold in the
               currency of sale and in United States dollars (and the exchange
               rates used in determining the amount of United States dollars),
               and

          (C)  An accounting of the calculation of Net Sales (in United States
               dollars) and the royalty amounts owing to NeXstar pursuant to
               subparagraph 3.3(d) (in United States dollars), including, if
               applicable, the exchange rates used in determining amounts owing
               in United States dollars.

8.   RECORD KEEPING
     --------------

8.1  Written Records - Medarex shall maintain such records as required under the
     ---------------
     Regents License as specified in writing by NeXstar. In the absence of such
     records requirements, Medarex shall keep written records with respect to
     its activities under this Agreement in accordance with generally accepted
     accounting practices, sufficient to ascertain and verify the accuracy and
     completeness of the accounting and the amounts owing pursuant to this
     Agreement. Medarex shall preserve these written records for a period of at
     least three years after their creation.

8.2  Audits - Medarex shall permit audits by Regents as required under the
     ------
     Regents License and as specified in writing by NeXstar. In addition, not
     more than once per year during the Term,

                                       18
<PAGE>

     Medarex shall permit independent auditors (chosen by NeXstar and reasonably
     acceptable to Medarex) to inspect, copy and abstract books, records and
     accounts and to talk to finance, sales and other relevant Medarex employees
     as may be reasonably necessary to determine the completeness and accuracy
     of the reports and payments made or required to be made under this
     Agreement. NeXstar shall give reasonable notice to Medarex of its request
     for an audit pursuant to this provision, and the audit shall be conducted
     during normal business hours. NeXstar shall be responsible for the cost of
     this audit except as set out below. If the audit indicates that Medarex
     underpaid NeXstar by [*****], Medarex will pay the cost of the audit. If
     the audit determines that additional payments were owed during a period,
     Medarex shall pay the additional amount to NeXstar within 45 days of the
     date of the delivery of the audit report. The parties acknowledge and agree
     that the confidentiality provisions of Section 10 of this Agreement shall
     apply to audits conducted pursuant to this provision and NeXstar shall
     ensure that its auditors (including all audit-personnel working on the
     audit) are bound by the confidentiality provisions of Section 10.

9.   TERM AND TERMINATION
     --------------------

9.1  Term - The term of this Agreement shall commence on the Effective Date.
     Unless earlier terminated as provided in this Section 9, this Agreement
     shall continue in full force and effect on a country-by-country and
     Sublicensed Product-by-Sublicensed Product basis until there are no
     remaining royalty payment obligations in a country. Once there are no
     remaining royalty payment obligations in a country, the Agreement shall
     expire in its entirety in such country on that day which is 61 days after
     the end of the quarter in which the royalty obligations expired.

9.2  Early Termination - This Agreement (including the Antibody Sublicense, the
     -----------------
     Aptamer Option and any other option, right of first refusal, sublicense or
     license granted under Sections 4 or 5) may be terminated prior to expiry of
     the Term in any of the circumstances set out in this paragraph 9.2.

     (a)  Bankruptcy, Etc. - If either party becomes bankrupt or insolvent,
          petitions any judicial body under any bankruptcy or insolvency laws,
          or has any proceeding commenced against it for insolvency, bankruptcy
          or liquidation, the other party may terminate this Agreement by
          providing ten days written notice to the bankrupt or insolvent party.

     (b)  Medarex Termination - Medarex shall have the same rights of
          termination under this Agreement as are granted to NeXstar under the
          Regents License.


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       19
<PAGE>

     (c)  NeXstar Termination (Regents License) - If Regents terminates Regents
          License, then, unless the Regents License provides for the survival of
          sublicenses granted by NeXstar, NeXstar shall have the right to
          terminate this Agreement concurrently with the termination of the
          Regents License by Regents.

     (d)  NeXstar Termination (Abandonment) - In addition to NeXstar's right of
          termination set out in subparagraph (c), NeXstar will have the right
          to terminate this Agreement if Medarex abandons its development and/or
          commercialization efforts with respect to the Sublicensed Products.
          For the purposes of this provision, Medarex will be considered to have
          abandoned its development and/or commercialization efforts if it has
          not carried out any development or commercialization activities on the
          Sublicensed Products for 12 consecutive months. If NeXstar wishes to
          terminate this Agreement pursuant to this provision, NeXstar shall
          give 45 days notice in writing to Medarex of NeXstar's intention to
          terminate. Medarex shall have the opportunity to cure the breach
          within the 45 day period to avoid termination of this Agreement.

     (e)  NeXstar Termination (Default) - Subject to subparagraphs (a), (c) and
          (d) above, NeXstar will have the right to terminate this Agreement
          only in the event of material breach by Medarex of any of its material
          obligations under this Agreement by providing 45 days notice in
          writing to Medarex.  Medarex shall have the opportunity to cure the
          breach within the 45 day period to avoid termination of this
          Agreement.

9.3  Effect of Early Termination - In any of the situations set out in paragraph
     ---------------------------
     9.2, this Agreement shall end as of the effective date of the termination
     and, except as set out below, Medarex shall have no further right to
     develop, make, have made, use, sell, have sold, offer for sale, import or
     otherwise dispose of any Sublicensed Product.  Notwithstanding early
     termination of this Agreement, Medarex, its Affiliates and its Sub-
     licensees shall be permitted for a [*****] following the effective date of
     the termination to sell off their existing inventory of Sublicensed
     Products produced pursuant to this Agreement. Early termination of this
     Agreement pursuant to paragraph 9.2 shall not affect any rights or
     obligations accrued prior to the effective date of the termination,
     including, without limitation:

     (a)  Sublicenses - The sublicense provisions of paragraph 3.2 shall survive
          for the term of the sublicenses then in effect;

     (b)  Termination - The termination provisions of paragraphs 9.3 and 9.4
          shall survive for a period of five years after the effective date of
          the termination;


***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       20
<PAGE>

     (c)  Confidentiality and Publication - The confidentiality and publication
          provisions of Section 10 shall survive for a period of five years
          after the effective date of the termination;

     (d)  Liability/Indemnification - The liability and indemnity provisions of
          paragraphs 12.4, 12.5, 12.6 and 12.7 shall survive for a period of
          five years after the effective date of the termination;

     (e)  Dispute Resolution - The dispute resolution provisions of Section 13
          shall survive for a period of five years after the effective date of
          the termination; and

     (f)  Miscellaneous - The miscellaneous provisions of Section 14 shall
          survive for a period of five years after the effective date of the
          termination.

9.4  Expiry of the Term - The following, provisions will survive termination of
     ------------------
     this Agreement at expiry of the Term:

     (a)  Termination - The termination provisions of paragraphs 9.3 and 9.4
          shall survive for a period of five years from the time of expiry of
          the Term;

     (b)  Confidentiality and Publication - The confidentiality and publication
          provisions of Section 10 shall survive for a period of five years from
          the time of expiry of the Term;

     (c)  Liability/Indemnification - The liability and indemnity provisions of
          paragraphs 12.4, 12.5, 12.6 and 12.7 shall survive for a period of
          five years from the time of expiry of the Term;

     (d)  Dispute Resolution - The dispute resolution provisions of Section 13
          shall survive for a period of five years from the time of expiry of
          the Term; and

     (e)  Miscellaneous - The miscellaneous provisions of Section 14 shall
          survive for a period of five years from the time of expiry of the
          Term.

9.5  March In - NeXstar shall use reasonable commercial efforts to meet its
     --------
     obligations under the Regents License. In any event, NeXstar shall not
     terminate or enter into any modification of the Regents License if such
     modification or termination would affect the rights of Medarex under this
     Agreement, without the prior written consent of Medarex, such consent not
     to be unreasonably withheld or delayed. NeXstar shall notify Medarex within
     ten business days if NeXstar receives any notice from Regents that:

                                       21
<PAGE>

     (a)  Default - Regents believes that NeXstar is in default or breach of the
          Regents License; or

     (b)  Modification - Regents intends to modify NeXstar's rights under the
          Regents License (e.g., by converting NeXstar's exclusive rights under
          the Regents License to non-exclusive rights).

     In the event NeXstar fails to meet any obligation under the Regents License
     causing Regents to threaten termination of the Regents License, NeXstar
     shall use reasonable efforts to cure its breach, or in the alternative, to
     assign its rights under the Regents License to Medarex.

10.  CONFIDENTIALITY AND PUBLICATION
     -------------------------------

10.1 Compliance - Medarex shall adhere to the confidentiality and publication
     ----------
     provisions of the Regents License as specified in writing by NeXstar.
     Medarex further agrees to the terms regarding confidentiality and
     publication set forth in paragraphs 10.2 - 10.7 below, except to the extent
     such terms are less restrictive than those of the Regents License.

10.2 Exclusions -The confidentiality obligations of this Section 10 shall not
     ----------
     extend to information which:

     (a)  Public Domain - Is now or later made known to the public through no
          default by the Recipient of its obligations under this Agreement;

     (b)  Prior Knowledge - The Recipient can show (by written records) was in
          its possession prior to disclosure by the Disclosing Party;

     (c)  Received from a Third Party - Is received by the Recipient from a
          third party not under an obligation of secrecy to the Disclosing
          Party; or

     (d)  Independent Development - Was independently developed by the Recipient
          without reference to the Confidential Information of the other as
          demonstrated by documented evidence prepared contemporaneously with
          the independent development.

10.3 Obligation of Confidentiality - The NeXstar Confidential Information is
     -----------------------------
     considered by NeXstar and the Medarex Confidential Information is
     considered by Medarex to be proprietary and confidential. Each of NeXstar
     and Medarex agrees to hold in confidence and not disclose to any third
     parties any of the other's Confidential Information without the prior
     written consent of the other. Each of NeXstar and Medarex agrees to use the
     same degree of care (and in any event not less than reasonable care) to
     safeguard the confidentiality of the other's Confidential Information that
     it uses to protect its own Confidential Information. Each of NeXstar and
     Medarex agrees to limit access to the other's Confidential Information to
     its

                                       22
<PAGE>

     employees and outside professional advisors who have a need to know and who
     are bound by confidentiality obligations, and to advise such persons of the
     Recipient's obligations under this Section.

10.4 Use of Confidential Information - Each of NeXstar and Medarex agrees only
     -------------------------------
     to use the other's Confidential Information for the purposes of this
     Agreement.

10.5 Publication - Any proposed Publication shall be provided by the Recipient
     -----------
     to the Disclosing Party 30 days prior to submission for Publication. The
     Disclosing Party shall review the proposed Publication and shall inform the
     Recipient (within this 30-day period of time) as to whether or not it
     consents to the proposed Publication. Contributions by the Disclosing Party
     shall be acknowledged in any Publication. In the event that the Disclosing
     Party determines that the proposed Publication contains patentable subject
     matter which the Disclosing Party wishes to protect, the Disclosing Party
     may request that the recipient delay publication for an additional 30 days
     for the purpose of allowing the Disclosing Party to pursue patent
     protection. In any event, Publication shall only occur with the consent of
     the Disclosing Party, which consent will not be unreasonably withheld.

10.6 Permitted Disclosures - Nothing in this Section 10 is intended to prohibit
     ---------------------
     either party from:

     (a)  Regulatory - Disclosing information to a government regulatory agency
          as required for securing applicable regulatory approvals;

     (b)  Clinical Trials - Disclosing relevant information to investigators or
          other third parties as required for the conduct of clinical trials;

     (c)  Sublicenses - Disclosing information to a Sub-licensee, provided the
          Sub-licensee is bound by confidentiality provisions substantially
          similar to the confidentiality provisions contained in this Agreement;
          or

     (d)  Required by Law - Disclosing factual information or data relating to
          this Agreement which is required by law to be disclosed. Without
          limiting the generality of the foregoing, it is acknowledged and
          agreed by the parties that NeXstar may be required to file this
          Agreement with the U.S. Securities and Exchange Commission in
          connection with its reporting obligations. If requested in writing by
          Medarex, NeXstar shall seek confidential treatment for provisions of
          this Agreement as requested in writing by Medarex.

10.7 Press Releases - Nothing in this Section 10 (including, without limitation,
     --------------
     the 30 day Publication approval provision) is intended to prohibit either
     party from releasing press releases with respect to this Agreement.
     However, the parties agree to the following provisions with respect to the
     release of press releases in connection with this Agreement:

                                       23
<PAGE>

     (a)  First Press Release - The parties agree to the release of a press
          release immediately after the signing of the Regents License, such
          press release to be either joint or separate, but in any event to be
          approved by both parties prior to release (such approval not to be
          unreasonably withheld or delayed).

     (b)  Additional Press Releases - Either party may mention the fact of this
          Agreement and previously disclosed details of this Agreement in
          further press releases without the prior written approval of the
          other.

11.  PATENT PROSECUTION, MAINTENANCE AND ENFORCEMENT
     -----------------------------------------------

11.1 Prosecution and Maintenance
     ---------------------------

     (a)  Regents' Responsibility - Regents shall be responsible for the
          prosecution and maintenance of the Licensed Patent Rights using
          counsel of Regents' choice (reasonably acceptable to Medarex, provided
          this right of input is included in the Regents License or is otherwise
          authorized by Regents). Regents shall provide NeXstar and NeXstar
          shall provide Medarex with copies of all relevant documentation so
          that Medarex may be informed and apprised of the continuing
          prosecution, and Medarex may provide to NeXstar substantive comment
          and input thereon (if permitted by the Regents License, Regents shall
          provide this documentation directly to Medarex). At Medarex's request,
          NeXstar will forward Medarex's comments and input on to Regents.
          Medarex shall treat this documentation as NeXstar Confidential
          Information under this Agreement.

     (b)  Reasonable Efforts - NeXstar shall use all reasonable efforts to
          compel Regents to amend any patent application to include claims
          reasonably requested by Medarex to protect the products and/or
          processes contemplated to be sold under this Agreement if such claims
          would constitute Licensed Patent Rights. In the event that NeXstar is
          unable to compel Regents to amend a patent application to Medarex's
          reasonable satisfaction, provided a right of direct input from Medarex
          is permitted by the Regents License, Medarex shall have the right to
          provide substantive comment and input directly to Regents.

     (c)  Foreign Patent Protection - Regents is responsible for foreign patent
          protection of the Licensed Patent Rights and Regents shall obtain
          patent protection on any invention sublicensed under this Agreement in
          foreign countries if available and if NeXstar so desires. NeXstar
          shall advise Medarex of all notices from Regents of requirements to
          nationalize PCT applications. At Medarex's written request, NeXstar
          shall promptly notify Regents of Medarex's decision to obtain foreign
          patents, this notice specifying the countries desired. The absence of
          such a notice from Medarex to NeXstar shall be considered an election
          not to secure foreign rights in the countries not included in the
          required notice.

                                       24
<PAGE>

     (d)  Costs of Foreign Patent Protection - The preparation, filing,
          prosecuting and maintenance of all foreign patent applications filed
          at Medarex's request, as well as the maintenance of all resulting
          foreign patents, shall be at the sole expense of Medarex and shall
          constitute Related Patent Costs. Such patents shall be held in the
          name of Regents and shall be obtained using counsel of Regents' choice
          (reasonably acceptable to Medarex, provided this right of input is
          included in the Regents License or is otherwise authorized by
          Regents).

     (e)  Other Countries - NeXstar shall have the right to request that Regents
          file, at NeXstar's expense, or Regents may file patent applications at
          its own expense, in any country in which Medarex has not elected to
          secure patent rights, and such applications and resultant patents
          shall not be subject to this Agreement.

     (f)  Reimbursement Obligations - Medarex's obligation to reimburse NeXstar
          for costs specified in this paragraph 11.1 shall continue for so long
          as this Agreement remains in effect; provided, however, that Medarex
          may elect to terminate its obligations with respect to any given
          patent application or patent in any country upon four months written
          notice to NeXstar.  NeXstar shall have 30 days from receipt of this
          patent termination notice to notify Medarex in writing that NeXstar is
          assuming such obligations or notify Regents in writing of Medarex's
          election. In either event, Medarex's obligation to make such payments
          shall cease and its license with regard to such patent application or
          patent in that country shall terminate at expiry of the four month
          notice period. NeXstar and/or Regents may continue prosecution and/or
          maintenance of such application(s) or patent(s) at its sole discretion
          and expense. Medarex shall have no further right or licenses under any
          patents or applications for which Medarex provides patent termination
          notice and for which NeXstar or Regents assume responsibility for
          costs.

     (g)  Periodic Statements - NeXstar shall provide Medarex with periodic
          statements from Regents showing the amounts of Related Patent Costs to
          be reimbursed to Regents under this paragraph 11.1. Medarex shall
          tender payment to NeXstar within 20 days of the date of such
          statements.

     (h)  Cooperation - In addition to all of the specific provisions set out in
          this paragraph 11. 1, the parties agree that they will work together
          and cooperate with respect to prosecution and maintenance of the
          Licensed Patent Rights to ensure reasonable input of Medarex is
          included in the prosecution and maintenance of the Licensed Patent
          Rights.

11.2 Infringement
     ------------

     (a)  Notice - Each of NeXstar and Medarex agrees that during the Term it
          will promptly give the other an Infringement Notice if it becomes
          aware of any Infringement.

                                       25
<PAGE>

     [*****]

12.  REPRESENTATIONS, WARRANTIES AND INDEMNIFICATION
     -----------------------------------------------

12.1 NeXstar's Representations - NeXstar represents and warrants to Medarex as
     -------------------------
     follows:

     (a)  Incorporation and Organization - NeXstar is a corporation duly
          incorporated and organized, validly existing and in good standing
          under the laws of the State of Delaware.

     (b)  Corporate Power - NeXstar has all requisite corporate power and
          capacity to own its property and assets and to carry on its business
          as now being conducted by it and as presently contemplated to be
          conducted by it.

     (c)  Authority - NeXstar has due and sufficient right and authority and has
          taken all action required to enter into this Agreement on the terms
          and conditions set forth in this Agreement and to carry out the terms
          of this Agreement, including the granting of the Antibody Sublicense
          in Section 3, the granting of the Aptamer Option and Aptamer License
          Option in Section 4 and the granting of the option to participate in
          the Small Molecule Program in Section 5.

     (d)  Valid Obligation - This Agreement constitutes a valid obligation of
          NeXstar, legally binding on it and enforceable in accordance with its
          terms, subject to applicable bankruptcy, insolvency, moratorium,
          reorganization or similar laws from time to time in effect affecting
          creditors' rights generally.

     (e)  No Conflicting Agreements - NeXstar is not under any obligation
          (contractual or otherwise) to any third party that conflicts with or
          is inconsistent with the provisions of this Agreement or impedes
          NeXstar's ability to carry out its obligations under this Agreement.

     (f)  No Blocking Patents/Rights - NeXstar will not enforce or assist any
          third party in enforcing against Medarex or its Affiliates or any Sub-
          licensee or any purchaser of Sublicensed Products any patent or other
          rights owned or controlled by NeXstar which are necessary or useful
          for the practice of the Licensed Patent Rights.

     (g)  Related Patents/Rights - In the event that any patent or other rights
          of NeXstar arise which are dominant or subordinate to any patent
          within the Licensed Patent Rights,

***** REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED
SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT
HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       26
<PAGE>

          NeXstar shall grant Medarex an option to obtain a nonexclusive license
          to such related patent or other intellectual property rights and the
          parties shall immediately meet and use good faith efforts to mutually
          agree to the terms of the Medarex license to the related patent or
          other rights.

12.2 No Representation - NeXstar makes no representation or warranty whatsoever
     -----------------
     that:

     (a)  Validity - Any patents or patent applications licensed or sublicensed
          under this Agreement are valid; or

     (b)  Infringement - The exercise of the rights or sublicense granted to
          Medarex under this Agreement will not infringe any patents of third
          parties.

12.3 Medarex's Representations - Medarex represents and warrants to NeXstar as
     -------------------------
     follows:

     (a)  Incorporation and Organization - Medarex is a corporation duly
          incorporated and organized, validly existing and in good standing
          under the laws of the State of New Jersey.

     (b)  Corporate Power - Medarex has all requisite corporate power and
          capacity to own its property and assets and to carry on its business
          as now being conducted by it and as presently contemplated to be
          conducted by it.

     (c)  Authority - Medarex has due and sufficient right and authority and has
          taken all action required to enter into this Agreement on the terms
          and conditions set forth in this Agreement and to carry out the terms
          of this Agreement.

     (d)  Valid Obligations - This Agreement constitutes a valid obligation of
          Medarex, legally binding on it and enforceable in accordance with its
          term, subject to applicable bankruptcy, insolvency, moratorium,
          reorganization or laws from time to time in effect affecting
          creditors' rights generally.

     (e)  No Conflicting Agreements - Medarex is not under any obligation
          (contractual or otherwise) to any third party that conflicts with or
          is inconsistent with the provisions of this Agreement or impedes
          Medarex's ability to carry out its obligations under this Agreement.

     (f)  Compliance with Laws - The development, making, sale, use or import of
          any products licensed under this Agreement, including, without
          limitation, Sublicensed Products, by Medarex under this Agreement
          shall be in compliance with all laws, regulations and necessary
          governmental permits and approvals of the applicable jurisdiction.

                                       27
<PAGE>

12.4 No Liability for NeXstar - Subject to the provisions of paragraph 12.5,
     Medarex acknowledges and agrees that NeXstar will not be liable to Medarex,
     successors or assigns of Medarex, or to any third party with respect to:

     (a)  Sublicensed Products - Any claim arising from the development, making,
          sale, use or import of Sublicensed Products; or

     (b)  Other Damages - Any claim for loss of profits, loss or interruption of
          business, or for indirect, special or consequential damages of any
          kind.

12.5 NeXstar Indemnification - NeXstar will be liable for the performance of its
     -----------------------
     obligations under this Agreement and for the veracity of its
     representations and warranties contained in this Agreement. If the
     representations and warranties made by NeXstar are not true and accurate,
     NeXstar shall indemnify and hold harmless Medarex and its officers,
     directors, employees and agents against any liability, claim, loss, cost or
     expense (including direct, consequential and punitive damages and
     reasonable attorney's fees) incurred as a result.

12.6 No Liability for Medarex - Subject to the provisions of paragraph 12.7,
     ------------------------
     NeXstar acknowledges and agrees that Medarex will not be liable to NeXstar
     or successors or assigns of NeXstar with respect to any claim by NeXstar
     for loss of profits, loss or interruption of business, or for indirect,
     special or consequential damages of any kind.

12.7 Medarex Indemnification - Medarex agrees to defend, protect, indemnify and
     -----------------------
     hold harmless NeXstar and its officers, directors, employees and agents
     against any liability, claim, loss, cost or expense (including direct,
     consequential and punitive damages and reasonable attorney's fees) arising:

     (a)  Third Party Claims - From any third party claim arising out of the
          development, making, sale, use or import of Sublicensed Products, by
          or on behalf of Medarex; and

     (b)  Breach of Representations - If any of the representations and
          warranties made by Medarex under this Agreement are not true and
          accurate.

     Medarex further agrees to reimburse NeXstar for any costs NeXstar incurs
     for any indemnification obligation to Regents under the Regents License.
     The indemnity set out in this paragraph shall not apply to any liability,
     claim, loss, cost or expense that arises out of the gross negligence or
     willful misconduct of NeXstar or to any liability, claim, loss, cost or
     expense that arises out of a material breach by NeXstar of its
     representations and warranties contained in this Agreement or the Regents
     License.

                                       28
<PAGE>

12.8 Insurance - To the extent required by the Regents License, Medarex, at its
     sole cost and expense, shall insure its activities in connection with the
     work under this Agreement and obtain, keep in force and maintain insurance
     or an equivalent program of self insurance.

13.  DISPUTE RESOLUTION
     ------------------

13.1 Mediation - Before commencing any arbitration with respect to any dispute
     ---------
     arising out of or relating to this Agreement, or any alleged breach of this
     Agreement (such arbitration to be conducted pursuant to paragraph 13.2),
     the parties agree to seek mediation with respect to any unresolved matter
     or matter in dispute arising out of or relating to this Agreement, or any
     alleged breach of this Agreement, such mediation to by conducted pursuant
     to the Mediation Rules of the American Arbitration Association.

13.2 Arbitration - Any dispute arising out of or relating to this Agreement, or
     -----------
     any alleged breach of this Agreement, shall be settled by binding
     arbitration in accordance with the then-current Arbitration Rules of the
     American Arbitration Association by one arbitrator appointed in accordance
     with the Arbitration Rules, except as modified by this Section 13. The
     arbitrator shall determine what discovery will be permitted, based on the
     principle of limiting the cost and time which the parties must expend on
     discovery; provided, the arbitrator shall permit such discovery as he/she
     deems necessary to achieve an equitable resolution of the dispute. The
     arbitration proceeding shall be conducted in the English language in San
     Francisco, California, unless the parties agree to conduct the arbitration
     in another location.

13.3 Arbitration Decision - The arbitration decision shall be binding and not
     --------------------
     appealable to any court in any jurisdiction. The prevailing party may enter
     the arbitration decision in any court having competent jurisdiction.

14.  MISCELLANEOUS

14.1 Regents- License
     ----------------

     (a)  NeXstar to Remain Liable - Certain of the rights sublicensed by
          NeXstar to Medarex under this Agreement are subject to the provisions
          of the Regents License. As required by the Regents License, NeXstar
          hereby acknowledges to Medarex that it remains responsible for its
          obligations under the Regents License, including, without limitation,
          obligations relating to payment of ongoing Related Patent Costs,
          handling of Regents' confidential information, payment of royalties,
          reporting, warranties and indemnity. To the extent not expressly
          addressed in this Agreement, Medarex agrees to use reasonable efforts
          to assist NeXstar in meeting those obligations.

     (b)  Necessary Amendments - Subject to Medarex's right not to approve the
          form of the Regents License (Section 2), the parties agree that if any
          provision of the Regents

                                       29
<PAGE>

          License is substantially inconsistent with the provisions of this
          Agreement, the parties will meet and negotiate and make such
          reasonable amendments to this Agreement as both parties agree are
          necessary to make this Agreement consistent with the provisions of the
          Regents License.

     (c)  Government Rights - If the Regents License is subject to all of the
          terms and conditions of Public Law 96-517, as amended, the parties
          agree that this Agreement will similarly be subject to all of the
          terms and conditions of Public Law 96-517, as amended, and the parties
          agree to take all action necessary on their part as licensee or sub-
          licensee, as the case may be, under the Regents License to enable
          Regents to satisfy Regents' obligations under the Regents License
          relating to the Invention.

14.2 Currency - The parties agree that, unless otherwise indicated, all dollar
     --------
     amounts referred to in this Agreement are in United States funds.

14.3 Further Assurances - The parties agree to execute such further documents
     ------------------
     and do such further acts as may be necessary to implement and carry out the
     intent of this Agreement.

14.4 Inurement - The parties agree that in all situations, including, without
     ---------
     limitation, in the case of a takeover or merger of either party, this
     Agreement shall inure to the benefit of and be binding upon each of them,
     their successors and permitted assigns and all of their employees and
     agents.

14.5 Assignment
     ----------

     (a)  Acquisition of NeXstar by Gilead - Medarex agrees that this Agreement
          and the rights, duties and obligations of NeXstar under this Agreement
          will become an obligation of and agreement with Gilead Sciences, Inc.
          upon completion of the merger of NeXstar and Gilead Sciences, Inc. and
          that the transfer/assumption of this Agreement as a result of the
          NeXstar/Gilead Sciences, Inc. merger does not require the consent of
          Medarex.

     (b)  Research Technologies Acquiring Entity - Medarex agrees that NeXstar
          may assign this Agreement and the rights, duties and obligations of
          NeXstar under this Agreement without Medarex's consent to any
          Affiliate, or to any entity that acquires all or substantially all of
          the business or assets of NeXstar to which this Agreement pertains,
          whether by merger, reorganization, acquisition, sale or otherwise.

     (c)  Medarex Acquiring Entity - NeXstar agrees that Medarex may assign this
          Agreement and the rights, duties and obligations of Medarex under this
          Agreement without NeXstar's consent to any Affiliate, or to any entity
          that acquires all or substantially all of the business or assets of
          Medarex to which this Agreement pertains, whether by merger,
          reorganization, acquisition, sale or otherwise.

                                       30
<PAGE>

      (d)  General - Except the sublicensing provisions of paragraph 3.2 and
           except as set out at paragraph 14.4 and subparagraphs 14.5(a), (b)
           and (c), the parties agree that this Agreement and the rights, duties
           and obligations of the parties under this Agreement shall not be
           assigned by either of them without the prior written consent of the
           other, and any attempt to assign the rights, duties or obligations
           without this consent will be of no effect.

14.6  Notice - The parties agree that any notice required to be given under this
      ------
      Agreement shall be in writing and shall be delivered personally or by
      facsimile transmission to the addresses set forth on page 1 of this
      Agreement to the attention of the following persons:

      (a)  If to NeXstar - Attn.: Dr. Larry Gold, Chief Scientific Officer, Fax
           No. (303) 546-7603, with a copy to Associate General Counsel, Fax No.
           303-546-7856; and

      (b)  If to Medarex - Attn.: Michael Appelbaum, VP and CFO, Fax No. (908)
           713-6002, with a copy to Corporate Counsel, Fax No. (908) 713-6002;

      (C)  or to such other addresses and persons as may from time to time be
           identified in writing by the parties in accordance with this
           paragraph 14.6. Any notice delivered personally shall be deemed to
           have been given and received at the time of delivery. Any notice
           delivered by facsimile transmission shall be deemed to have been
           given and received on the next business day following the date of
           transmission.

14.7  Severance - The parties agree that if a provision of this Agreement is
      ---------
      wholly or partially invalid, the Agreement shall be interpreted as if the
      invalid provision had not been a part.

14.8  Governing Law - The parties agree that this Agreement shall be governed by
      -------------
      and construed in accordance with the laws of the State of California.

14.9  Recitals/Schedules - The parties agree that the recitals and schedules to
      ------------------
      this Agreement and all other instruments supplementary to this Agreement
      are incorporated into this Agreement and form an integral part of this
      Agreement.

14.10 Entire Agreement - The parties agree that the provisions contained in this
      ----------------
      Agreement constitute the entire agreement between the parties with respect
      to the subject matter and supersede all previous communications,
      representations and agreements (whether verbal or written) between the
      parties with respect to the subject matter hereof, including, without
      limitation:

      (a)  CDA - The Confidential Information Agreement between the parties
           dated February, 1998, as amended October 22, 1998;

                                       31
<PAGE>

      (b)  MTA - The Material Transfer Agreement between the parties dated
           September, 1998; and

      (c)  Letter - The letter dated December 17, 1998, from Celia Courchene of
           NeXstar to Michael Appelbaum of Medarex.

      To the extent there is any conflict between the provisions of this
      Agreement and any other document, the provisions of this Agreement shall
      govern the interpretation of this Agreement.

14.11 Captions - The parties agree that the captions appearing in this Agreement
      --------
      have been inserted for reference and as a matter of convenience and in no
      way define, limit or enlarge the scope or meaning, of this Agreement or
      any provision.

14.12 Amendment - Any amendment to this Agreement shall only be effective if the
      ---------
      amendment is in writing and is signed by NeXstar and by Medarex.

14.13 Counterparts - This Agreement may be executed in facsimile counterparts,
      ------------
      each of which shall be deemed to be an original and both of which together
      shall constitute one and the same Agreement.

      IN WITNESS WHEREOF the parties have executed this Agreement as of the day
and year first written above.

NEXSTAR PHAMACEUTICALS, INC.                   MEDAREX, INC.


By: /s/ Larry Gold                             By: /s/ Michael Appelbaum
   ---------------------------------------        ----------------------------
Title: Chairman & Chief Scientific Officer     Title: Executive V.P
       -----------------------------------           -------------------------

                                       32
<PAGE>

                               LIST OF EXHIBITS
                               ----------------


Exhibit A - Commercialization Plan

Exhibit B - Evolutionary Chemistry Patents and Patent Applications



                                       33
<PAGE>

                                   EXHIBIT A
                                   ---------

                            COMMERCIALIZATION PLAN

                                (See Attached)

                                    [*****]



*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED SEPARATELY
WITH THE SECURITIES AND EXCHANGE COMMISSION CONFIDENTIAL TREATMENT HAS BEEN
REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       34
<PAGE>

                                   EXHIBIT B
                                   ---------

            EVOLUTIONARY CHEMISTRY PATENTS AND PATENT APPLICATIONS

<TABLE>
<CAPTION>
Docket No.     Title
               U.S. Serial No.          U.S. Filing Date         U.S. Patent No.      Issue Date
<S>            <C>                      <C>                     <C>                   <C>
NEX22          Parallel SELEX
=====
               08/309,245                    09/20/94               5,723,289          03/03/98

NEX22/D1       Parallel SELEX
========
               08/463,101                    06/05/95               5,789,160          08/04/98

[*****]

NEX22/D3       Parallel SELEX
========
               08/462,389                    06/05/95               5,723,592          03/03/98

[*****]

NEX22/CIP      Parallel SELEX
=========
               08/618,700                    03/20/96               5,858,660          01/12/99

[*****]

NEX52          Method for the Cyclotrimerization of Alkynes in Aqueous Solutions
=====
               08/619,228                    03/20/96               5,659,069          08/19/97

NEX52/D1       Method for the Cyclotrimerization of Alkynes in Aqueous Solutions
========
               08/816,588                    03/13/97               5,760,266          06/02/98

NEX52/D2       Method for the Cyclotrimerization of Alkynes in Aqueous Solutions
========
               08/816,709                    03/13/97

NEX08          Method for Palladium Catalyzed Carbon-Carbon Coupling and Products
=====
               08/076,735                    06/14/93               5,428,149          06/27/95
</TABLE>


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED SEPARATELY
WITH THE SECURITIES AND EXCHANGE COMMISSION CONFIDENTIAL TREATMENT HAS BEEN
REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       35
<PAGE>

                             EXHIBIT B, continued

            EVOLUTIONARY CHEMISTRY PATENTS AND PATENT APPLICATIONS

<TABLE>
<CAPTION>
Docket No.     Title
               U.S. Serial No.          U.S. Filing Date         U.S. Patent No.      Issue Date
<S>            <C>                      <C>                     <C>                   <C>

NEX08/D1       Method for Palladium Catalyzed Carbon-Carbon Coupling and Products
=========
               08/407,893                  03/21/95                5,633,361            05/27/97

NEX08/D2       5-Modified Pyrimidines From Palladium Catalyzed Carbon-Carbon Coupling
========
               08/423,395                  04/19/95                5,591,843            01/07/97

NEX24          Purine Nucleoside Modifications by Palladium Catalyzed Methods
=====
               08/347,600                  12/01/94                5,580,972            12/03/96

NEX24/C        Purine Nucleoside Modifications by Palladium Catalyzed Methods and
=======
               Oligonucleotides Containing Same as amended
               08/441,881                  05/16/95                5,783,679            07/21/98

NEX35-1        Palladium Catalyzed Nucleoside Modifications Methods Using Nucleophiles
=======
               and Carbon Monoxide
               08/458,421                  06/02/95                5,719,273            02/17/98
</TABLE>

[*****]


*****REPRESENTS CONFIDENTIAL PORTION WHICH HAS BEEN OMITTED AND FILED SEPARATELY
WITH THE SECURITIES AND EXCHANGE COMMISSION. CONFIDENTIAL TREATMENT HAS BEEN
REQUESTED WITH RESPECT TO THE OMITTED PORTIONS.

                                       36