Document:

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                                                                    Exhibit 10.5
                                                                    ------------

                  EVALUATION AND COMMERCIALIZATION AGREEMENT

     THIS EVALUATION AND COMMERCIALIZATION AGREEMENT (the "Agreement"),
effective as of February 24, 1999 (the "Effective Date"), is entered by and
between GenPharm International, Inc., a wholly owned subsidiary of Medarex,
Inc., a New Jersey corporation, with a principal place of business at 1545 Route
22 East, Annandale, New Jersey 08801 ("Medarex"), and Leukosite, Inc., with a
principal place of business at 215 First Street, Cambridge, Massachusetts 02142
("Leukosite").

                                  BACKGROUND

     A.   Medarex is the sole and exclusive owner of certain transgenic Mice
          useful for the preparation of fully human monoclonal antibodies;

     B.   Leukosite desires to conduct research with the Mice to evaluate their
          utility for the development of fully human monoclonal antibodies to
          certain Antigens (as defined below), on the terms and conditions
          herein; and

     C.   Leukosite wishes to acquire from Medarex an option to acquire a
          commercial license for the use of monoclonal antibodies with
          specificity for the Antigens to commercialize Products (as defined
          below), on the terms and conditions herein.

     NOW, THEREFORE, Medarex and Leukosite agree as follows:

1.   DEFINITIONS.

     1.1  "Affiliate" means any corporation or other entity which is directly or
           ---------
indirectly controlling, controlled by or under the common control with
Leukosite. For the purpose of this Agreement, "control" shall mean the direct or
indirect ownership of fifty percent (50%) or more of the outstanding shares or
other voting rights of the subject entity to elect directors, or if not meeting
the preceding, any entity owned or controlled by or owning or controlling at the
maximum control or ownership right permitted in the country where such entity
exists.

     1.2  "Antibody" shall mean a human monoclonal antibody with binding
           --------
affinity for an Antigen, which antibody is obtained through the use of nucleic
acid or cells derived from one or more of the Mice.

     1.3  "Antigen" shall mean any antigen used by Leukosite to immunize Mice in
           -------
connection with the Evaluation.
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     1.4  "Approval" shall mean all approvals, licenses, registrations and
           --------
authorizations of all governmental agencies in a country necessary for the
manufacture, use or sale of a Product in the applicable country.

     1.5  "Biological License Application"or "BLA" shall mean Biological License
           ------------------------------     ---
Application as defined in the U.S. Food, Drug and Cosmetic Act and the
regulations promulgated thereunder, and any corresponding foreign application,
registration or certification.

     1.6  "Confidential Information" shall mean (i) any proprietary or
           ------------------------
confidential information or material in tangible form disclosed hereunder that
is marked as "Confidential" at the time it is delivered to the receiving party,
or (ii) proprietary or confidential information disclosed orally hereunder which
is identified as confidential or proprietary when disclosed and such disclosure
of confidential information is confirmed in writing within thirty (30) days by
the disclosing party.

     1.7  "Control" or "Controlled" shall mean possession of the ability to
           -------      ----------
grant the licenses or sublicenses as provided for herein without violating the
terms of any agreement or other arrangement with any third party.

     1.8  "Evaluation" shall mean the immunization of Mice conducted by
           ----------
Leukosite in the Evaluation Period in connection with its assessment of the
usefulness of the Mice to produce Antibodies.

     1.9  "Evaluation Period" shall mean the period commencing on the Effective
           -----------------
Date and continuing until the earlier of (i) [*****] the Effective Date, unless
extended pursuant to Section 2.1.3, in which case the last day of the last
extension period, or (ii) the termination of the Agreement.

     1.10 "IND" shall mean an Investigational New Drug application, as defined
           ---
in the U.S. Food, Drug and Cosmetic Act and the regulations promulgated
thereunder, or any corresponding foreign application, registration or
certification.

     1.11 "Major Country" shall mean [*****].
           -------------

     1.12 "Medarex Technology" shall mean the Patent Rights and Know How.
           ------------------

          1.12.1 "Know How" shall mean the Confidential Information and Mice
                  --------
owned or Controlled by Medarex during the term of the Agreement and transferred
to Leukosite by Medarex necessary for the exercise of the Patent Rights,
including, without limitation, technical data, protocols and methods and
processes. For the avoidance of doubt, the Know How does not include any 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.

                                      -2-
<PAGE>

          1.12.2 "Patent Rights" shall mean all United States and foreign
                  -------------
patents (including all reissues, extensions, substitutions, confirmations, re-
registrations, re-examinations, revalidations and patents of addition) and
patent applications (including, without limitation, all continuations,
continuations-in-part and divisions thereof) owned or Controlled by Medarex
during the term of the Agreement, in each case, which claims an invention which
is necessary for the use of the Antibodies to make, use or sell Products.

     1.13 "Mice" shall mean immunizable transgenic mice containing unrearranged
           ----
human immunoglobulin genes. "Mouse" shall mean one of the Mice.
                             -----

     1.14 "Mice Materials" shall mean any parts or derivatives of the Mice
           --------------
prepared by Medarex in connection with the Evaluation, including without
limitation, cells, hybridomas Antibodies, genes, DNA sequences or other
biological materials derived directly or indirectly from the Mice.

     1.15 "Net Sales" shall mean [*****].
           ---------

     1.16 "Phase I", "Phase II"and "Phase III" shall mean Phase I (or Phase
           -------    --------      ---------
I/II), Phase II, and Phase III clinical trials, respectively, in each case as
prescribed by the U.S. Food and Drug Administration or a corresponding foreign
entity.

     1.17 "Product" shall mean any product for the diagnosis, prophylaxis or
           -------
treatment of human disease containing one or more Antibodies, or a portion
thereof.

     1.18 "Sublicensee" shall mean a third party to whom Leukosite has granted a
           -----------
license or sublicense to make, have made, import, use, sell, offer for sale or
otherwise exploit Products in the Territory. As used in this Agreement,
"Sublicensee" shall also include a third party to whom Leukosite has granted the
right to distribute a Product.

     1.19 [*****].

     1.20 "Territory" shall mean all countries of the world.
           ---------

2.   EVALUATION

     2.1  Evaluation. Medarex will provide Mice to Leukosite for use during the
          ----------
Evaluation Period to allow Leukosite to immunize the Mice for the purpose of
determining whether Leukosite wishes to obtain a commercial license with respect
to one or more Antigens as provided in Section 3.3.

***** 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.

                                      -3-
<PAGE>

          2.1.1  Provision of Mice. Each year during the Evaluation Period,
                 -----------------
subject to Leukosite's payment of the applicable fees in Section 4. 1, Medarex
shall provide Leukosite with up to [*****] Mice per year. Medarex shall provide
Leukosite with approximately [*****] Mice per quarter. If any Mice delivered by
Medarex die of natural causes before commencement of the relevant immunization
protocol or for any reason during the immunization protocol, they shall be
replaced without cost by Medarex, provided that their death was not due to
Leukosite's misfeasance or negligence, or the mice are rendered unusable by a
failure by Leukosite to commence immunization of the Mice within three (3)
months of delivery. The first [*****] Mice shall be shipped to Leukosite within
[*****] after the Effective Date.

          2.1.2  Evaluation License. Subject to the terms and conditions of this
                 ------------------
Agreement, Medarex hereby grants to Leukosite an non-exclusive, non-transferable
license under the Medarex Technology to immunize the Mice with Antigens and use
the Mice solely for conducting the Evaluation during the Evaluation Period.

          2.1.3  Extension of Evaluation Period. Leukosite will have the option
                 ------------------------------
to extend the term of the Evaluation Period and Evaluation license beyond
[*****] the Effective Date for [*****] by providing Medarex notice at least
[*****] before the end of the then-current term of Evaluation Period and
concurrently paying to Medarex the applicable extension fee due pursuant to
Section 4.1.3. Subject to Section 8.3, Medarex shall be entitled to make a press
release announcing each such extension of the Evaluation Period following
receipt of Leukosite's notice of its wish to extend the Evaluation Period.

     2.2  Ownership.
          ---------

          2.2.1  Mice. Title to the Mice and, subject to the options and
                 ----
licenses granted in Article 3, the Mice Materials, shall at all times remain
with Medarex.

          2.2.2  Intellectual Property. Any invention made by Leukosite or its
                 ---------------------
employees, consultants or agents in the course of activities in connection with
the Evaluation that relate to Mice or Mice Materials shall be owned by Medarex.
Leukosite shall promptly notify Medarex of any such invention, and cooperate
with Medarex, at Medarex's request and expense, in the preparation, filing,
prosecution, enforcement and defense of patent applications and patents relating
thereto. Inventions made by Leukosite or its employees, consultants or agents in
connection with the Evaluation which relate solely to the Antigens used to
immunize Mice shall be owned by Leukosite.

     2.3  Limited Use.  Leukosite agrees that the Mice will be used solely for
          -----------
the purpose of conducting the Evaluation and for no other purpose. All Mice
shall be kept and used only at Leukosite's facilities. Leukosite shall not make
any effort, directly or indirectly, to breed, clone or

***** 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.

                                      -4-
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otherwise reproduce the Mice by any means, sexual or asexual. In no event shall
Leukosite transfer the Mice to any person or entity without the prior written
approval of Medarex.

     2.4  Care in Use of Mice. It is understood and agreed that the Mice are
          -------------------
experimental in nature and may have unknown characteristics and Leukosite
therefore agrees to use prudence and reasonable care in the use, handling,
storage, transportation and disposition and containment of the Mice, and to
maintain the Mice under suitable containment conditions in compliance with all
applicable national, state and local laws, regulations, rules and ordinances.

     2.5  Records. Leukosite will prepare and maintain complete and accurate
          -------
written records of all uses made of, and all persons allowed access to, the Mice
and the Mice Materials, and copies of such records will be furnished to Medarex
on a confidential basis, upon Medarex's request.

     2.6  [*****].

3.   OPTION; LICENSE RIGHTS

     3.1  Option for Commercial Licenses. Subject to the availability of a
          ------------------------------
particular Antigen for exclusive licensing, during the term of the Evaluation
Period, Leukosite shall have an option to obtain exclusive commercial licenses,
in each case, to develop and commercialize Products based on Antibodies against
a specific Antigen for commercial use, as set forth below. Leukosite may
exercise its option with notice to Medarex as provided in Section 3.2 below.

     3.2  Antigen Availability.
          --------------------

          3.2.1  Antigen Identification. At any time during the Evaluation
                 ----------------------
Period, Leukosite may notify Medarex that it wishes to acquire an exclusive
commercial license to use Mice to prepare Antibodies with respect to a
particular Antigen identified by Leukosite. Each Antigen shall be a specific
molecular target or biochemical entity, and the parties shall agree on a written
description of such Antigen.

          3.2.2  Notice of Availability; Notice Date. Within [*****] following
                 -----------------------------------
receipt of notice from Leukosite regarding its desire to obtain an exclusive
commercial license with regard to a particular Antigen, Medarex will notify
Leukosite whether the exclusive rights requested by Leukosite are available for
licensing to Leukosite. It is understood and agreed that an Antigen may not be
available for Leukosite for exclusive commercial use if Medarex is actively
engaged in discussions with a third party, or has previously granted a third
party an option, right of first refusal or other rights to use the Mice with
respect to such Antigen, or if Medarex has initiated an active program of
development or commercialization with respect to such Antigen or has an intent
to initiate an active program with regard to such Antigen.

***** 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.

                                      -5-
<PAGE>

          3.2.3  License Fee. If Medarex notifies Leukosite that a particular
                 -----------
Antigen is available for exclusive licensing by Leukosite, within [*****] of
Medarex's notice of such availability, Leukosite shall pay to Medarex the
initial commercial license fee, if any, due pursuant to Section 4.2 with respect
to such Antigen.

          3.2.4  Unavailability. In the event that Medarex notifies Leukosite
                 --------------
that exclusive rights are not available with regard to a particular Antigen,
Leukosite shall have no further license or other rights with regard to
Antibodies generated against such Antigen unless otherwise agreed in writing by
the parties.

     3.3  Commercial License.
          ------------------

          3.3.1  Grant. If Leukosite elects to exercise its option to acquire a
                 -----
commercial license with respect to a particular Antigen which is available for
commercial licensing to Leukosite, subject to the terms and conditions of this
Agreement, Medarex shall grant to Leukosite the following licenses, on an
Antigen-by-Antigen basis:

          (a)    a worldwide, exclusive, non-transferable, royalty bearing
license under the Medarex Technology to immunize the Mice to make Antibodies
against such Antigen, and

          (b)    a worldwide exclusive, royalty bearing license under the
Medarex Technology, with the right to sublicense, to use such Antibodies against
such Antigen to make, have made, import, have imported, use, offer for sale and
sell Products.

          3.3.2  Sublicenses. Leukosite shall have no right to grant sublicenses
                 -----------
to use the Mice, but may grant sublicenses under the Medarex Technology to the
extent necessary to develop, make, have made, import, use, offer for sale and
sell Products; provided, prior to the execution of any sublicense, Leukosite
shall provide Medarex with at least the following information with respect to
each potential Sublicensee: (i) the identity of the Sublicensee; (ii) a
description of the Product, and the rights being granted to the Sublicensee; and
(iii) the territory in which the Product will be sold. Each sublicense granted
by Leukosite shall be consistent with all the terms and conditions of this
Agreement, and subordinate thereto, and Leukosite shall remain responsible to
Medarex for the compliance of each such Sublicensee with the financial and other
obligations due under this Agreement.

     3.4  Retained Rights; No Further Rights. Only the license granted pursuant
          ----------------------------------
to the express terms of this Agreement shall be of any legal force or effect. No
other license rights shall be granted or created by implication, estoppel or
otherwise. It is understood and agreed that Medarex shall retain rights to make,
have made, import, use, offer for sale, sell and otherwise commercialize the
Mice itself or with third parties for any uses.

***** 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.

                                      -6-
<PAGE>

     3.5  Use of Mice. Any use of the Mice by Leukosite or its Sublicensees
          -----------
pursuant to a license granted pursuant to Section 3.3 shall be subject to the
provisions of Sections 2.3, 2.4 and 2.5.

     3.6  No Patent Filings. Unless and until Leukosite acquires a commercial
          -----------------
license pursuant to Section 3.3 to use the Mice and Mice Materials to prepare
Antibodies against a particular Antigen, Leukosite and its Affiliates shall not
file any patent applications disclosing or claiming the use of the Mice or any
Mice Materials with regard to such Antigen or any related antigen.

4.   CONSIDERATION

     4.1  Evaluation Fees.
          ---------------

          4.1.1  Mouse Fee. Leukosite shall pay to Medarex [*****] per Mouse
                 ---------
delivered to Leukosite for the conduct of the Evaluation. Leukosite shall pay
such Mice fees to Medarex with thirty (30) days following receipt of an invoice
therefore.

          4.1.2  Extension Fee. If Leukosite elects to extend the Evaluation
                 -------------
Period pursuant to Section 2.1.3, it shall pay to Medarex [*****] concurrently
with its notice that Leukosite wishes to extend the Evaluation Period.

     4.2  Commercial License Fee. [*****]. The [*****] commercial license fee
          ----------------------
shall be paid to Medarex in cash within [*****] of Medarex's notice to Leukosite
of the availability of the Antigen for exclusive licensing by Leukosite.

     4.3  Milestone Payments. Within thirty (30) days following the occurrence
          ------------------
of the relevant events specified below with respect to each Product which
contains one of the Antibodies licensed by Leukosite, Leukosite shall pay to
Medarex the following amounts:

[*****]

     4.4  Royalties.
          ----------

          4.4.1  Royalty on Net Sales. In partial consideration for the
                 --------------------
commercial license, Leukosite shall pay to Medarex a royalty on annual Net Sales
of Products, on a Product-by-Product basis, as follows:

                 Annual Net Sales                 Royalty Rate
                 ----------------                 ------------

                      [*****]                       [*****]

***** 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.

                                      -7-
<PAGE>

          4.4.2  [*****].

          4.4.3  Royalty Term. The royalties due pursuant to this Section 4.4
                 ------------
shall be payable on a Product-by-Product and country-by-country basis in each
country, as follows: [*****].

     4.5  [*****].

5.   PAYMENTS

     5.1  Timing of Royalty Payments.  All royalties due to Medarex shall be
          --------------------------
paid [*****].

     5.2  Payment Method. All amounts due Medarex hereunder shall be paid in
          --------------
U.S. dollars [*****].

     5.3  Currency; Foreign Payments. If any currency conversion shall be
          --------------------------
required in connection with the payment of any royalties hereunder, such
conversion shall be made by using the exchange rate for the purchase of U.S.
dollars reported by the Chase Manhattan Bank on the last business day of the
calendar quarter to which such royalty payments relate. If at any time legal
restrictions prevent the prompt remittance of any royalties owed on Net Sales in
any jurisdiction, Leukosite may notify Medarex and make such payments by
depositing the amount thereof in local currency in a bank account or other
depository in such country in the name of Medarex, and Leukosite shall have no
further obligations under this Agreement with respect thereto.

     5.4  Taxes. All royalty amounts required to be paid to Medarex pursuant to
          -----
this Agreement may be paid with deduction for withholding for or on account of
any taxes (other than taxes imposed on or measured by net income) or similar
governmental charge imposed by a jurisdiction other than the United States
("Withholding Taxes"). At Medarex's request, Leukosite shall provide Medarex a
certificate evidencing payment of any Withholding Taxes hereunder and shall
reasonably assist Medarex to obtain the benefit of any applicable tax treaty.

6.   REPORTS AND RECORDS

     6.1  Royalty Reports. Leukosite shall deliver to Medarex [*****] after the
          ---------------
end of each calendar quarter in which Products are sold a report setting forth
in reasonable detail the calculation of the royalties payable to Medarex for
such calendar quarter, including the Products sold in each country by Leukosite
and its Sublicensees, the Net Sales thereof, and all amounts received from
Sublicensees for sales of Products. Such reports shall be Confidential
Information of Leukosite subject to Article 9 herein.

***** 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.

                                      -8-
<PAGE>

     6.2  Inspection of Books and Records. Leukosite and its Affiliates and
          -------------------------------
Sublicensees shall maintain accurate books and records which enable the
calculation of royalties payable hereunder to be verified. Leukosite and its
Affiliates and Sublicensees shall retain the books and records for each
quarterly period for [*****] years after the submission of the corresponding
report under Section 5.1 hereof. Upon [*****] days prior notice to Leukosite,
independent accountants selected by Medarex and reasonably acceptable to
Leukosite, may have access to Leukosite's and its Affiliates' and Sublicensees'
books and records during normal business hours to conduct a review or audit, for
the purpose of verifying the accuracy of Leukosite's payments and compliance
with this Agreement. Any independent accountant from any of the "Big Six"
accounting firms (or a successor thereof) shall be and is hereby agreed to be
acceptable to Leukosite. Any such inspection or audit shall be at Medarex's
expense, however, in the event an inspection reveals underpayment of [*****] or
more in any quarter, Leukosite shall pay the costs of the inspection [*****].

7.   DILIGENCE

     7.1  Reasonable Efforts. Leukosite shall use Commercially Reasonable
          ------------------
Efforts to achieve regulatory approvals for the sale of Products in countries
solely chosen by Leukosite in the Territory. The term "Commercially Reasonable
Efforts" shall with respect to a Product mean [*****].

          7.1.1  Commencing as of the Effective Date, Leukosite shall use
Commercially Reasonable Efforts to develop, clinically test, manufacture and
commercialize Products. All costs of development, clinical testing,
manufacturing and commercialization shall be borne by Leukosite, its Affiliates
or Sublicensees. In the event that Leukosite at any point elects not to initiate
or continue developing a Product in any country in the Territory, Leukosite
shall notify Medarex of such abandonment; effective as of such notice, all
rights granted to Leukosite hereunder applicable to such Product in such country
shall revert to Medarex. Notwithstanding any other provision in this Agreement,
if within [*****] years after receiving a commercial license for a Product,
Leukosite has not [*****], Leukosite shall pay Medarex a fee of [*****] per year
for such Product until such time that Leukosite either [*****] or abandons
development of such Product, whichever occurs earlier. [*****].

          7.1.2  Notwithstanding the above, if within [*****] years after the
Effective Date, Leukosite has not [*****], Medarex may require Leukosite, within
[*****] days of receipt of written notice from Medarex, to establish
Leukosite's, its Affiliates, or Sublicensees use of Commercially Reasonable
Efforts in developing such Product. If within such [*****] days, Leukosite fails
to establish such Commercially Reasonable Efforts to develop such Product, in
Medarex's reasonable opinion, then Leukosite shall have until [*****]. If by
[*****], Leukosite has not [*****], all rights granted to Leukosite hereunder
with respect to such Product (and corresponding Antibodies) shall revert to
Medarex.

***** 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.

                                      -9-
<PAGE>

     7.2  Reports to Medarex. During the term of this Agreement, Leukosite shall
          ------------------
keep Medarex informed of its development and commercialization activities
subject to this Agreement, and on January 31 of each year shall provide Medarex
with a written summary of such events and activities in the preceding year. When
the registration package requesting Approval for commercial sale of any Product
receives Approval in the U.S., Europe and Japan, Leukosite will notify Medarex
in writing within [*****] days thereof.

     7.3  Reports to Medarex. During the term of this Agreement, Leukosite shall
          ------------------
keep Medarex fully informed of its activities subject to this Agreement,
including without limitation, the commercialization of Products, and on January
31 of each year shall provide Medarex with a written report detailing such
events and activities. When the registration package requesting approval for
commercial sale of the Product is first filed in the U.S., the European Union
and Japan, and in each case when approval is received therefor, Leukosite will
notify Medarex in writing within [*****] days.

     7.4  Regulatory Filings. Leukosite shall submit registration packages
          ------------------
requesting approval for commercial sale of the Product as soon as reasonably
practicable. Leukosite (or its designee) shall file and hold title to all
regulatory applications, approvals and supplements thereto.

     7.5  Abandoned Products. Leukosite shall promptly notify Medarex should it
          ------------------
elect to abandon its rights to pursue commercialization of any Product in any
country. Leukosite may voluntarily abandon its right hereunder to market the
Product [*****]. Such notice will effectuate Leukosite's voluntary abandonment
of its right hereunder to market the Product in such country; provided, the
abandonment of the Product in any particular country hereunder shall not be
construed to be a termination of this Agreement.

8.   CONFIDENTIALITY

     8.1  Confidential Information. Except as expressly provided herein, the
          ------------------------
parties agree that, for the term of this Agreement and for five (5) years
thereafter, the receiving party shall keep completely confidential and shall not
publish or otherwise disclose and shall not use for any purpose except for the
purposes contemplated by this Agreement any Confidential Information furnished
to it by the disclosing party hereto pursuant to this Agreement, except that to
the extent that it can be established by the receiving party by competent proof
that such Confidential Information:

                         (i)  was already known to the receiving party, other
than under an obligation of confidentiality, at the time of disclosure;

                         (ii) was generally available to the public or otherwise
part of the public domain at the time of its disclosure to the receiving party;

***** 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.

                                     -10-
<PAGE>

                         (iii) became generally available to the public or
otherwise part of the public domain after its disclosure and other than through
any act or omission of the receiving party in breach of this Agreement;

                         (iv)  was independently developed by the receiving
party as demonstrated by documented evidence prepared contemporaneously with
such independent development; or

                         (v)   was subsequently lawfully disclosed to the
receiving party by a person other than a party hereto.

     8.2  Permitted Use and Disclosures. Each party hereto may use or disclose
          -----------------------------
information disclosed to it by the other party to the extent such use or
disclosure is reasonably necessary in complying with applicable governmental
regulations or otherwise submitting information to tax or other governmental
authorities, conducting clinical trials, or making a permitted sublicense or
otherwise exercising its rights hereunder, provided that if a party is required
to make any such disclosure of another party's confidential information, other
than pursuant to a confidentiality agreement, it will give reasonable advance
notice to the latter party of such disclosure and, save to the extent
inappropriate in the case of patent applications, will use its best efforts to
secure confidential treatment of such information prior to its disclosure
(whether through protective orders or otherwise).

     8.3  Public Disclosure. Except as otherwise required by law, neither party
          -----------------
shall issue a press release or make any other public disclosure of the terms of
this Agreement without the prior approval of such press release or public
disclosure. Each party shall submit any such press release or public disclosure
to the other party, and the receiving party shall have [*****] days to review
and approve any such press release or public disclosure, which approval shall
not be unreasonably withheld. If the receiving party does not respond within
such [*****] day period, the press release or public disclosure shall be deemed
approved. In addition, if a public disclosure is required by law, including
without limitation in a filing with the Securities and Exchange Commission, the
disclosing party shall provide copies of the disclosure reasonably in advance of
such filing or other disclosure for the nondisclosing party's prior review and
comment.

     8.4  Confidential Terms. Except as expressly provided herein, each party
          ------------------
agrees not to disclose any terms of this Agreement to any third party without
the consent of the other party; provided, disclosures may be made as required by
securities or other applicable laws, or to actual or prospective investors or
corporate partners, or to a party's accountants, attorneys and other
professional advisors.

***** 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.

                                      -11
<PAGE>

9.   REPRESENTATIONS AND WARRANTIES

     9.1  Medarex.  Medarex represents and warrants that: (i) it is a
          -------
corporation duly organized validly existing and in good standing under the laws
of the State of New Jersey; (ii) the execution, delivery and performance of this
Agreement have been duly authorized by all necessary corporate action on the
part of Medarex; (iii) it is the sole and exclusive owner of all right, title
and interest in the Mice; (iv) it has the right to grant the rights and licenses
granted herein; [*****].

     9.2  Leukosite. Leukosite represents and warrants that: (i) it is a
          ---------
corporation duly organized validly existing and in good standing under the laws
of the State of Delaware; and (ii) the execution, delivery and performance of
this Agreement have been duly authorized by all necessary corporate action on
the part of Leukosite.

     9.3  Disclaimer of Warranties. THE MICE ARE PROVIDED "AS IS", AND EXCEPT AS
          ------------------------
OTHERWISE EXPRESSLY SET FORTH IN THIS AGREEMENT, MEDAREX AND ITS RESPECTIVE
AFFILIATES MAKE NO REPRESENTATIONS AND EXTEND NO WARRANTIES OR CONDITIONS OF
ANY KIND, EITHER EXPRESS OR IMPLIED, WITH RESPECT TO THE MICE, ANTIBODIES,
PRODUCTS OR MEDAREX TECHNOLOGY, INCLUDING, BUT NOT LIMITED TO, WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, VALIDITY OF THE PATENT RIGHTS
LICENSED HEREUNDER, OR NONINFRINGEMENT OF THE INTELLECTUAL PROPERTY RIGHTS OF
THIRD PARTIES.

     9.4  Disclaimer. Nothing in this Agreement is or shall be construed as:
          ----------

          9.4.1  A warranty or representation by Medarex as to the validity or
scope of any claim or patent within the Patent Rights;

          9.4.2  A warranty or representation that anything made, used, sold, or
otherwise disposed of under any license granted in this Agreement is or will be
free from infringement of any patent rights or other intellectual property right
of any third party;

          9.4.3  An obligation to bring or prosecute actions or suits against
third parties for infringement of any of the Patent Rights; or

          9.4.4  Granting by implication, estoppel, or otherwise any licenses or
rights under patents or other rights of Medarex or third parties, regardless of
whether such patents or other rights are dominant or subordinate to any patent
within the 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.

                                      -12-
<PAGE>

10.  INTELLECTUAL PROPERTY

     10.1 [*****].

          10.1.1 [*****] Technology License. Leukosite hereby grants to
                         ------------------
Medarex an exclusive, worldwide, royalty-free license to Leukosite's entire
right, title and interest in all patent applications, patents, know-how and
tangible materials (including, without limitation, hybridomas and antibodies)
relating to [*****] and/or their manufacture or use, conceived, reduced to
practice or otherwise developed in connection with the conduct of the
Cooperative Research Agreement entered by GenPharm International, Inc. and
Leukosite, Inc. effective January 1, 1995 (collectively, the "[*****]
Technology"). Leukosite retains this non-transferable right to use the [*****]
Technology for its own research purposes. Leukosite hereby represents and
warrants that, as of the Effective Date (i) Leukosite has not previously
licensed or assigned to any third party any right or interest in any portion or
aspect of the [*****] Technology, and will not do so during the term of this
Agreement, and (ii) Leukosite has not filed any patent application claiming any
[*****] and/or methods relating to their manufacture or use of [*****] which is
not included within the [*****] Technology.

          10.1.2 Data. Medarex (or its designee) shall have access to and the
                 ----
right to use and reference, without charge, all data owned or Controlled by
Leukosite relating to the [*****] Technology (collectively, the "Data"), and
Leukosite agrees to cooperate with Medarex to enable Medarex (or its designee)
to practice the foregoing rights.

          10.1.3 Delivery. Within [*****] days following the Effective Date,
                 --------
Leukosite shall deliver to Medarex a list of all patent applications and patents
within the [*****] Technology, and all tangible materials within the [*****]
Technology, and all Data subject to Section 10.1.3.

     10.2 Patent Rights. If Leukosite acquires a commercial license to a
          -------------
particular Antigen, [*****].

     10.3 Failure to Prosecute. [*****].
          --------------------

     10.4 Cooperation. [*****].
          -----------

     10.5 Infringement Claims. If the manufacture, importation, sale or use of
          -------------------
the Product pursuant to this Agreement results in any claim, suit or proceeding
alleging patent infringement against Medarex or Leukosite, such party shall
promptly notify the other party hereto. The defendant shall keep each other
party hereto reasonably informed of all material developments in connection with
any such claim, suit or proceeding.

***** 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.

                                      -13-
<PAGE>

11.  DISPUTE RESOLUTION

     11.1 Mediation. If a dispute arises out of or relates to this Agreement, or
          ---------
the breach thereof, and if said dispute cannot be settled through negotiation,
the parties agree first to try in good faith to settle the dispute by mediation
under the Commercial Mediation Rules of the American Arbitration Association,
before resorting to arbitration, litigation, or some other dispute resolution
procedure.

     11.2 Arbitration. Subject to Sections 11.1 and 14.6, Medarex and Leukosite
          -----------
agree that any dispute or controversy arising out of, in relation to, or in
connection with this Agreement, or the validity, enforceability, construction,
performance or breach thereof, shall be settled by binding arbitration in New
York, New York, United States of America, under the then-current Rules of
Commercial Arbitration of the American Arbitration Association by one (1)
arbitrator appointed in accordance with such Rules. The arbitrators 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
arbitrators shall permit such discovery as they deem necessary to achieve an
equitable resolution of the dispute. The decision and/or award rendered by the
arbitrator shall be written, final and non  appealable and may be entered in any
court of competent jurisdiction. The parties agree that, any provision of
applicable law notwithstanding, they will not request, and the arbitrator shall
have no authority to award, punitive or exemplary damages against any party. The
costs of any arbitration, including administrative fees and fees of the
arbitrator, shall be shared equally by the parties. Each party shall bear the
cost of its own attorneys' fees and expert fees.

12.  INDEMNIFICATION

     12.1 Medarex. Medarex shall indemnify, defend and hold harmless Leukosite
          -------
and its directors, officers and employees (each a "Leukosite Indemnitee") from
and against any and all liabilities, damages, losses, costs or expenses
(including attorneys' and professional fees and other expenses of litigation
and/or arbitration) (a "Liability") resulting from a claim, suit or proceeding
made or brought by a third party against a Leukosite Indemnitee arising from or
occurring as a result of any breach of the representations and warranties set
forth in Section 9.1, except to the extent caused by the negligence or willful
misconduct of Leukosite.

     12.2 Leukosite. Leukosite shall indemnify, defend and hold harmless Medarex
          ---------
and its directors, officers and employees (each a "Medarex Indemnitee") from and
against any and all liabilities, damages, losses, costs or expenses (including
attorneys' and professional fees and other expenses of litigation and/or
arbitration) (a "Liability") resulting from a claim, suit or proceeding made or
brought by a third party against a Medarex Indemnitee, arising from or occurring
as a result of (i) any breach of the representations and warranties set forth in
Section 9.2, (ii) the conduct of the Evaluation or the practice by Leukosite of
any right granted herein, or (iii) any development, testing, manufacture,
importation, use, offer for sale, sale or other distribution of any Product by
Leukosite or its Affiliates Sublicensees (including, without limitation, product
liability claims), except in each case to the extent caused by the negligence or
willful misconduct of Medarex.

                                      -14-
<PAGE>

     12.3 Procedure. In the event that any Indemnitee intends to claim
          ---------
indemnification under this Article 12 it shall promptly notify the other party
(the "Indemnitor") in writing of such alleged Liability. The Indemnitor shall
have the sole right to control the defense and settlement thereof. The
Indemnitees shall cooperate with the Indemnitor and its legal representatives in
the investigation of any action, claim or liability covered by this Article 12.
The Indemnitee shall not, except at its own cost, voluntarily make any payment
or incur any expense with respect to any claim or suit without the prior written
consent of the Indemnitor, which the Indemnitor shall not be required to give.

13.  TERM AND TERMINATION

     13.1 Term. The term of this Agreement shall commence on the Effective Date.
          ----
Unless earlier terminated as provided in this Article 13, this Agreement shall
continue in full force and effect on a country-by-country and Product-by-Product
basis until there are no remaining royalty payment obligations in a country, at
which time the Agreement shall expire in its entirety in such country.

     13.2 Termination for Cause. Either party may terminate this Agreement in
          ---------------------
the event the other party has materially breached or defaulted in the
performance of any of its obligations hereunder, and such default has continued
for sixty (60) days after written notice thereof was provided to the breaching
party by the nonbreaching party. Any termination shall become effective at the
end of such sixty (60) day period unless the breaching party has cured any such
breach or default prior to the expiration of the sixty (60) day period.
Notwithstanding the above, in the case of a failure to timely pay any amounts
due hereunder, the period for cure of any subsequent default following notice
thereof shall be ten (10) days and, unless payment is made within such period
the termination shall become effective at the end of such period.

     13.3 Termination for Insolvency. If voluntary or involuntary proceedings by
          --------------------------
or against a party are instituted in bankruptcy under any insolvency law, or a
receiver or custodian is appointed for such party, or proceedings are instituted
by or against such party for corporate reorganization or the dissolution of such
party, which proceedings, if involuntary, shall not have been dismissed within
sixty (60) days after the date of filing, or if such party makes an assignment
for the benefit of creditors, or substantially all of the assets of such party
are seized or attached and not released within sixty (60) days thereafter, the
other party may immediately terminate this Agreement effective upon notice of
such termination.

     13.4 Permissive Termination.
          ----------------------

          13.4.1 Leukosite. Leukosite may terminate the Evaluation Program and
                 ---------
this Agreement with [*****] days written notice to Medarex.

***** 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.

                                      -15-
<PAGE>

          13.4.2 Medarex. In the event that Leukosite fails to obtain a
                 -------
commercial license pursuant to Section 3.2 on or before the end of the
Evaluation Period, Medarex may terminate the Agreement with written notice to
Leukosite.

     13.5 Effect of Termination.
          ---------------------

          13.5.1 Accrued Rights and Obligations. Termination of this Agreement
                 ------------------------------
for any reason shall not release any party hereto from any liability which, at
the time of such termination, has already accrued to the other party or which is
attributable to a period prior to such termination nor preclude either party
from pursuing any rights and remedies it may have hereunder or at law or in
equity with respect to any breach of this Agreement. It is understood and agreed
that monetary damages may not be a sufficient remedy for any breach of this
Agreement and that the non-breaching party may be entitled to injunctive relief
as a remedy for any such breach.

          13.5.2 Return of Confidential Information. Upon any termination of
                 ----------------------------------
this Agreement, Leukosite and Medarex shall promptly return to the other party
all Confidential Information of the other; provided counsel of each party may
retain one (1) copy of such Confidential Information solely for archival
purposes and for ensuring compliance with Article 8.

          13.5.3 Stock on Hand. In the event this Agreement is terminated for
                 -------------
any reason, Leukosite shall have the right to sell or otherwise dispose of the
stock of any Product subject to this Agreement then on hand, subject to Articles
4, 5 and 6 until the first anniversary of the effective date of such
termination.

          13.5.4 Return of Mice and Mice Materials. Upon any termination of this
                 ---------------------------------
Agreement, Leukosite shall promptly return to Medarex all Mice, or destroy all
materials derived from the Mice, including without limitation, all Antibodies,
hybridomas, all cells capable of producing Antibodies and other biological
materials, and an officer of Leukosite shall provide Medarex with written
certification thereof.

          13.5.5 Licenses. The option and license rights granted in Sections
                 --------
3.1, 3.2 and 3.3 shall terminate upon any termination of this Agreement, and in
such event Leukosite and its Affiliates and Sublicensees shall immediately cease
all development and commercialization of Products.

     13.6 Survival. Sections 2.2, 2.5, 3.4, 3.6, 13.5 and 13.6, and Articles 6,
          --------
8, 9, 10, 11, 12 and 14 of this Agreement shall survive termination of this
Agreement for any reason.

14.  MISCELLANEOUS

     14.1 Governing Law. This Agreement and any dispute, including without
          -------------
limitation any arbitration, arising from the performance or breach hereof shall
be governed by and construed and

                                      -16-
<PAGE>

enforced in accordance with the laws of the state of New Jersey, without
reference to conflicts of laws principles.

     14.2 Independent Contractors. The relationship of the parties hereto is
          -----------------------
that of independent contractors. The parties hereto are not deemed to be agents,
partners or joint venturers of the others for any purpose as a result of this
Agreement or the transactions contemplated thereby.

     14.3 Assignment.  This Agreement shall not be assignable by Leukosite to
          ----------
any third party hereto without the written consent of Medarex which consent
shall not be unreasonably withheld; except Leukosite may assign this Agreement,
without such consent, to an entity that acquires all or substantially all of its
business or assets to which this Agreement pertains, whether by merger,
reorganization, acquisition, sale or otherwise. Medarex may assign this
Agreement. This Agreement shall be binding upon and inure to the benefit of the
parties and their successors and assigns.

     14.4 Notices. All notices, requests and other communications hereunder
          -------
shall be in writing and shall be personally delivered or sent by telecopy or
other electronic facsimile transmission or by registered or certified mail,
return receipt requested, postage prepaid, in each case to the respective
address specified below, or such other address as may be specified in writing to
the other parties hereto:

     If to Medarex:      Medarex, Inc.
                         1545 Route 22 East
                         Annandale, New Jersey 08801
                         Attn: President

     If to Leukosite:    Leukosite, Inc.
                         215 First Street
                         Cambridge, Massachusetts 02142
                         Attn: ______________________

     14.5 Force Majeure. Neither party shall lose any rights hereunder or be
          -------------
liable to the other party for damages or losses (except for payment obligations)
on account of failure of performance by the defaulting party if the failure is
occasioned by war, strike, fire, Act of God, earthquake, flood, lockout,
embargo, governmental acts or orders or restrictions, failure of suppliers, or
any other reason where failure to perform is beyond the reasonable control and
not caused by the negligence, intentional conduct or misconduct of the
nonperforming party has exerted all reasonable efforts to avoid or remedy such
force majeure; provided, however, that in no event shall a party be required to
settle any labor dispute or disturbance.

     14.6 Injunctive Relief. Leukosite acknowledges that limitations and
          -----------------
restrictions on its possession and use of Mice and Mice Materials hereunder are
necessary and reasonable to protect Medarex, and expressly agrees that monetary
damages would be inadequate to compensate Medarex for any violation by Leukosite
or Leukosite of any such limitations or restrictions. The parties agree

                                      -17-
<PAGE>

that any such violation would cause irreparable injury to Medarex and agrees
that without resorting to prior mediation or arbitration, and, in addition to
any other remedies that may be available in law, in equity or otherwise, either
party shall be entitled to obtain temporary and permanent injunctive relief
against any threatened violation of such limitations or restrictions or the
continuation of any such violation in any court of competent jurisdiction,
without the necessity of proving actual damages or the posting of any bond.

     14.7  Advice of Counsel. Medarex and Leukosite have each consulted counsel
           -----------------
of their choice regarding this Agreement, and each acknowledges and agrees that
this Agreement shall not be deemed to have been drafted by one party or another
and will be construed accordingly.

     14.8  Compliance with Laws. Each party shall furnish to the other party any
           --------------------
information requested or required by that party during the term of this
Agreement or any extensions hereof to enable that party to comply with the
requirements of any U.S. or foreign federal, state and/or government agency.

     14.9  Further Assurances. At any time or from time to time on and after the
           ------------------
date of this Agreement, either party shall at the request of the other party
hereto (i) deliver to the requesting party any records, data or other documents
consistent with the provisions of this Agreement, (ii) execute, and deliver or
cause to be delivered, all such consents, documents or further instruments of
transfer or license, and (iii) take or cause to be taken all such actions, as
the requesting party may reasonably deem necessary in order for the requesting
party to obtain the full benefits of this Agreement and the transactions
contemplated hereby.

     14.10 Export Controls. Leukosite agrees that it will take all actions
           ---------------
necessary to insure compliance with all U.S. laws, regulations, orders or other
restrictions on exports and further will not sell, license or reexport,
directly, or indirectly, the Product(s) to any person or entity for sale in any
country or territory, if, to the knowledge of Leukosite based upon reasonable
inquiry, such sale, would cause the parties to be in violation of any such laws
or regulations now or hereafter in effect. Leukosite agrees to secure from any
recipient of Product(s) adequate manually signed written assurances prior to
shipment from the United States as are required by the U.S. Export Regulations.

     14.11 Severability. In the event that any provisions of this Agreement are
           ------------
determined to be invalid or unenforceable by a court of competent jurisdiction,
the remainder of the Agreement shall remain in full force and effect without
said provision. In such event, the parties shall in good faith negotiate a
substitute clause for any provision declared invalid or unenforceable, which
shall most nearly approximate the intent of the parties in entering this
Agreement.

     14.12 Waiver. It is agreed that no waiver by either party hereto of any
           ------
breach or default of any of the covenants or agreements herein set forth shall
be deemed a waiver as to any subsequent and/or similar breach or default.

                                      -18-
<PAGE>

     14.13 Complete Agreement. This Agreement, with its Exhibits, constitutes
           ------------------
the entire agreement, both written and oral, between the parties with respect
to the subject matter hereof, and that all prior agreements respecting the
subject matter hereof, either written or oral, expressed or implied, are merged
and canceled, and are null and void and of no effect. No amendment or change
hereof or addition hereto shall be effective or binding on either of the parties
hereto unless reduced to writing and duly executed on behalf of both parties.

     14.14 Use of Name. Neither party shall use the name or trademarks of the
           -----------
other party without the prior written consent of such other party.

     14.15 Headings. The captions to the several Sections and Articles hereof
           --------
are not a part of this Agreement, but are included merely for convenience of
reference only and shall not affect its meaning or interpretation.

     14.16 Counterparts. This Agreement may be executed in two counterparts,
           ------------
each of which shall be deemed an original and which together shall constitute
one instrument.

     IN WITNESS WHEREOF, Medarex and Leukosite have executed this Agreement by
their respective duly authorized representatives.

MEDAREX, INC.                                LEUKOSITE, INC.

By: /s/ Michael Appelbaum                    By: /s/ A. Lawlor
   --------------------------------             --------------------------------
Print Name: Michael Appelbaum                Print Name: A. Lawlor
           ------------------------                     ------------------------
Title: Executive V.P.                        Title: V.P.
      -----------------------------                -----------------------------

                                      -19-<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

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