Document:

<PAGE>

                                                                  EXHIBIT 10.284

                               [Ligand Letterhead]

March 11, 2005

Andres Negro-Vilar, M.D., Ph.D.
Executive Vice President, Research and
Development, & Chief Scientific Officer
LIGAND PHARMACEUTICALS INCORPORATED
10275 Science Center Drive
San Diego, CA 92121

Dear Andres:

            The purpose of this letter agreement is to document the terms of the
severance package to which you will be entitled should your employment with
Ligand Pharmaceuticals Incorporated (the "Company") terminate under certain
specified circumstances.

            Part One of this letter agreement sets forth certain definitional
provisions to be in effect for purposes of determining your benefit
entitlements. Part Two specifies the terms and conditions upon which you may
become entitled to receive severance benefits. Severance benefits accrue under
this letter agreement in the event your employment with the Company were to be
terminated involuntarily in connection with certain changes in control of the
Company. Part Three concludes this letter agreement with a series of general
terms and conditions applicable to your severance benefits.

                             PART ONE -- DEFINITIONS

            Definitions. For purposes of this letter agreement, including in
particular the application of the special benefit limitations of Part Three, the
following definitions will be in effect:

   1. Average Compensation means your average W-2 wages from the Company for the
      five (5) calendar years completed immediately prior to the calendar year
      in which the Change in Control is effected. Any W-2 wages for a partial
      year of employment will be annualized, in accordance with the frequency
      with which such wages are paid during such partial year, before inclusion
      within your Average Compensation.

   2. Board means the Company's Board of Directors.

<PAGE>

   3. Change in Control means any of the following events:

            (i) a merger or consolidation in which the Company is not the
      surviving entity, except for a transaction the principal purpose of which
      is to change the state in which the Company is incorporated,

            (ii) the sale, transfer or other disposition of all or substantially
      all of the assets of the Company other than in the ordinary course of
      business,

            (iii) any reverse merger in which the Company ceases to exist as an
      independent corporation and becomes the subsidiary of another corporation,
      except where there is an insubstantial change in the de facto voting
      control of the Company (e.g. the creation of a holding company),

            (iv) any Hostile Take-Over,

            (v) the acquisition by any person (or related group of persons),
      whether by tender or exchange offer made directly to the Company's
      stockholders, private purchases from one or more of the Company's
      stockholders, open market purchases or any other transaction, of
      beneficial ownership of securities possessing more than thirty percent
      (30%) of the total combined voting power of the Company's outstanding
      securities,

            (vi) the acquisition by any person (or related group of persons),
      whether by tender or exchange offer made directly to the Company's
      stockholders, private purchases from one or more of the Company's
      stockholders, open market purchases or any other transaction, of
      additional securities of the Company which increase the total holdings of
      such person (or group) to a level of securities possessing more than fifty
      percent (50%) of the total combined voting power of the Company's
      outstanding securities, or

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 3

             (vii) the acquisition by any person (or related group of persons),
       whether by tender or exchange offer made directly to the Company's
       stockholders, private purchases from one or more of the Company's
       stockholders, open market purchases or any other transaction, of
       securities of the Company possessing sufficient voting power in the
       aggregate to elect an absolute majority of the members of the Board
       (rounded up to the nearest whole number).

   4.  COBRA means the continuation-of-coverage provisions of the Consolidated
       Omnibus Budget Reconciliation Act of 1985, as amended.

   5.  Code means the Internal Revenue Code of 1986, as amended.

   6.  Common Stock means the Company's common stock, par value $0.001 per
       share.

   7.  Equity Incentive Plans means any of the following equity incentive plans
       of the Company: 1992 Stock Option/Stock Issuance Plan, the 2002 Stock
       Incentive Plan, and the Restricted Stock Purchase Plan, together with any
       amendments or successors to such plans.

   8.  Equity Parachute Payment means, with respect to any Option (whether
       Acquisition-Accelerated or Severance-Accelerated) or unvested Stock
       Issuance, the portion deemed to be a parachute payment under Code Section
       280G and the Treasury Regulations issued thereunder. Such Equity
       Parachute Payment shall be calculated in accordance with the valuation
       provisions established under Code Section 280G and the applicable
       Treasury Regulations and will include an appropriate dollar adjustment to
       reflect the lapse of your obligation to remain in the Company's employ as
       a condition to your vesting in the accelerated portion of such Option or
       Stock Issuance.

   9.  ERISA means the Employee Retirement Income Security Act of 1974, as
       amended.

   10. Health Care Coverage means the health care benefits provided by the
       Company to you and your eligible dependents for which you are eligible to
       continue coverage under the provisions of COBRA.

   11. Hostile Take-Over means either of the following events:

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 4

            (i) the acquisition by any person (or related group of persons)
      whether by tender or exchange offer made directly to the Company's
      stockholders, private purchases from one or more of the Company's
      stockholders, open market purchases or any other transaction, of
      beneficial ownership of securities possessing more than thirty percent
      (30%) of the total combined voting power of the Company's outstanding
      securities pursuant to a tender offer made directly to the Company's
      stockholders which the Board does not recommend such stockholders to
      accept, or

            (ii) a change in the composition of the Board over a period of
      thirty-six (36) consecutive months or less such that a majority of the
      Board members (rounded up to the next whole number) ceases, by reason of
      one or more contested elections for Board membership, to be comprised of
      individuals who either (a) have been Board members continuously since the
      beginning of such period or (b) have been elected or nominated for
      election as Board members during such period by at least a majority of the
      Board members described in clause (a) who were still in office at the time
      such election or nomination was approved by the Board.

   12. Involuntary Termination means the termination of your employment with the
       Company:

            (i) upon your involuntary discharge or dismissal, or

            (ii) upon your resignation in connection with any of the following
      changes to the terms and conditions of your employment: (A) a change in
      your position with the Company which materially reduces your level of
      responsibility, (B) a greater than ten percent (10%) reduction in your
      level of compensation (including base salary, fringe benefits and
      participation in non-discretionary bonus programs under which awards are
      payable pursuant to objective financial or performance standards, but
      excluding equity compensation) or (C) a relocation of your principal place
      of employment by more than fifty (50) miles.

            The following guidelines shall determine whether one or more
      reductions in compensation should be taken into account for purposes of
      clause (ii)(B):

            (a) Any reduction in compensation which occurs in connection

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 5

       with an across-the-board reduction in the level of compensation payable
       to the Company's executive officers or senior management shall not
       constitute grounds for a clause (ii)(B) resignation, unless implemented
       within eighteen (18) months after a Change in Control.

            (b) In the event of a Hostile Take-Over, the greater than ten
       percent (10%) standard of clause (ii)(B) shall be reduced to zero percent
       (0%) so that any reduction in the level of your compensation shall
       constitute grounds for a clause (ii)(B) resignation.

            In no event shall an Involuntary Termination be deemed to occur
       should your employment terminate by reason of death or permanent
       disability.

   13. Option means any option granted to you under any of the Equity Incentive
       Plans which is outstanding at the time of your Involuntary Termination or
       any earlier Change in Control. Your outstanding options are to be divided
       into two separate categories as follows:

            (i) Acquisition-Accelerated Options: any outstanding Option (or
       installment thereof) which accelerates upon a Change in Control in
       accordance with the automatic acceleration provisions of the Equity
       Incentive Plans.

            (ii) Severance-Accelerated Options: any outstanding Option (or
       installment thereof) which is not an Acquisition-Accelerated Option but
       which accelerates upon your Involuntary Termination, whether or not in
       connection with a Change in Control, as part of your severance benefits
       under this letter agreement.

   14. Other Parachute Payments mean any payments in the nature of compensation
       to which you may become entitled under this letter agreement (other than
       the Equity Parachute Payment) or any other arrangement with the Company,
       to the extent such payments qualify as parachute payments within the
       meaning of Code Section 280G(b)(2) and the Treasury Regulations issued
       thereunder or would so qualify if the aggregate present value of such
       payments exceeded the amount specified in Code Section 280G(b)(2)(ii).

   15. Stock Issuance means the issuance of unvested shares of Common Stock
       under the Company's Restricted Stock Plan or any other Equity Incentive
       Plan.

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 6

   16. Termination for Cause means an Involuntary Termination or resignation of
       your employment with the Company by reason of your conviction of any
       felony or other criminal act, your commission of any act of fraud or
       embezzlement, your unauthorized use or disclosure of confidential or
       proprietary information or trade secrets of the Company or its
       subsidiaries, or any other intentional misconduct on your part which
       adversely affects the business or affairs of the Company in a material
       manner.

                  PART TWO -- INVOLUNTARY TERMINATION BENEFITS

            You will be entitled to receive the severance benefits specified
below should there occur an Involuntary Termination of your employment during
the term of this letter agreement effected in connection with a Change in
Control, other than a Termination for Cause. However, in the absence of a
Hostile Take-Over, these benefits will continue to be paid you only for so long
as you remain available for any consulting services required of you under Part
Two, Paragraph 4 and abide by the restrictive covenants set forth in Part Two,
Paragraph 5.

   1.  Severance Payments. You will receive severance payments from the Company
       for a period of twelve (12) months following your Involuntary Termination
       in an aggregate amount equal to the sum of (A) one (1) times the annual
       rate of base salary in effect for you at the time of your Involuntary
       Termination or at the time of the relevant Change in Control, whichever
       is higher plus (B) one (1) times the average of the bonuses (excluding
       any signing bonus) paid to you for services rendered in the two (2)
       fiscal years immediately preceding the fiscal year of your Involuntary
       Termination (annualized if paid for a partial fiscal year). If a bonus is
       paid to you for only one of those years, then the bonus amount under
       Clause (B) will be equal to one (1) times such bonus amount. The
       aggregate severance payments shall be paid to you in equal installments
       over the twelve-month period in accordance with the Company's normal
       payroll practices and subject to all applicable withholding taxes. The
       severance payments will immediately terminate if and only if (i) you
       should cease to remain available for the consulting services required of
       you under Section 4, or (ii) you fail to abide by the restrictive
       covenants set forth in Section 5 . However, in the event your Involuntary
       Termination occurs in connection with a Hostile Take-Over, your severance
       payments will be paid to you in the form of a single lump sum amount
       within thirty (30) days after such Involuntary Termination, and the
       provisions of

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 7

      Sections 4 and 5 of this Part Two will not apply.

   2. Health Care Coverage. The Company will, at its expense, make any COBRA
      payments for you and your eligible dependents in order to continue your
      Health Care Coverage until the earlier of (i) twelve (12) months after the
      effective date of your Involuntary Termination (other than a Termination
      for Cause) or (ii) the first date that you are covered under another
      employer's (or, in the event of rehire, the Company's) health benefit
      program which provides substantially the same level of benefits without
      exclusion for pre-existing medical conditions. Such payments will be in
      lieu of any other continued health care coverage to which you or your
      dependents would otherwise be entitled pursuant to the requirements of
      Code Section 4980B by reason of your termination of employment.

   3. Option Acceleration and Lapse of Restrictions. Each of your outstanding
      Options under the Equity Incentive Plans will (to the extent not then
      otherwise exercisable) automatically accelerate so that each such Option
      will become immediately exercisable for the total number of shares of
      Common Stock at the time subject to that Option. Each such accelerated
      Option, together with all of your other vested Options, will remain
      exercisable for a period of twelve (12) months following your Involuntary
      Termination until the end of the specified ten (10)-year option term. Such
      Option(s) may be exercised for any or all of the option shares in
      accordance with the exercise provisions of the option agreement evidencing
      the grant. In addition, all restrictions applicable to the Stock Issuances
      you hold (to the extent those restrictions have not previously lapsed in
      accordance with the terms of the issuance agreements) will automatically
      lapse upon your Involuntary Termination (except a Termination for Cause).

   4. Consulting Services. Unless your Involuntary Termination occurs in
      connection with a Hostile Take-Over, you will make yourself available to
      perform consulting services reasonably requested of you during the twelve
      (12)-month period following your Involuntary Termination. You will be
      compensated at an hourly rate to be agreed upon by you and the Company at
      the time such consulting services are to be rendered, and you will be
      reimbursed for all reasonable out-of-pocket expenses incurred in rendering
      such services upon your submission of appropriate documentation for those
      expenses.

   5. Restrictive Covenants. For the one hundred twenty (120)-day period
      following your Involuntary Termination:

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 8

            (i) You will not directly or indirectly, whether for your own
      account or as an employee, director, consultant or advisor, provide
      services to any business enterprise which is at the time in competition
      with any of the Company's then existing or formally planned product lines
      and which is located geographically in an area where the Company maintains
      substantial business activities, unless you obtain the prior written
      consent of the Board of Directors.

            (ii) You will not directly or indirectly encourage or solicit any
      individual to leave the Company's employ for any reason or interfere in
      any other manner with the employment relationships at the time existing
      between the Company and its current or prospective employees.

            (iii) You will not induce or attempt to induce any customer,
      supplier, distributor, licensee or other business relation of the Company
      to cease doing business with the Company or in any way interfere with the
      existing business relationship between any such customer, supplier,
      distributor, licensee or other business relation and the Company.

      You acknowledge that monetary damages may not be sufficient to compensate
      the Company for any economic loss which may be incurred by reason of your
      breach of the foregoing restrictive covenants. Accordingly, in the event
      of any such breach, the Company shall, in addition to the cessation of the
      severance benefits provided you under this letter agreement and any
      remedies available to the Company at law, be entitled to obtain equitable
      relief in the form of an injunction precluding you from continuing to
      engage in such breach.

      None of the foregoing restrictive covenants in this section 5 shall be
      applicable in the event your Involuntary Termination occurs in connection
      with a Hostile Take-Over.

   6. Benefit Reduction.

            (i) Benefit Reduction. If the Change in Control does not constitute
      a Hostile Take-Over, first the dollar amount of your severance payment
      under Paragraph 1 will be reduced to the extent necessary to assure that
      the present value of those benefits will not, when added to the present
      value of your Equity Parachute Payment and your Other Parachute Payments,
      exceed 2.99 times your Average Compensation. In the event of a Hostile
      Take-Over, no reduction will be made to your severance payment (or any
      other benefit to which you become entitled hereunder), unless necessary to
      provide you with the

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 9

      maximum after-tax benefit available, after taking into account any
      parachute excise tax which might otherwise be payable by you under Code
      Section 4999 and any analogous State income tax provision.

            (ii) Resolution of Disputes. In the event there is any disagreement
      between you and the Company as to whether one or more benefits to which
      you become entitled (whether under this letter agreement or otherwise) in
      connection with a Change in Control constitute Equity Parachute Payments
      or Other Parachute Payments, such dispute is to be resolved as follows:

                  A. The matter shall be submitted for resolution to independent
      counsel mutually acceptable to you and the Company ("Independent
      Counsel"). The resolution reached by Independent Counsel shall be final
      and controlling. However, should the Independent Counsel determine that
      the status of the benefits in dispute can be resolved by obtaining a
      private letter ruling from the Internal Revenue Service, a formal and
      proper request for such ruling shall be prepared and submitted by
      Independent Counsel, and the determination made by the Internal Revenue
      Service in the issued ruling shall be controlling. All expenses incurred
      in connection with the retention of Independent Counsel and (if
      applicable) the preparation and submission of the ruling request shall be
      paid by the Company.

                  B. The present value of each Equity Parachute Payment and each
      of the Other Parachute Payments (including your severance payment and
      Health Care Coverage) shall be determined in accordance with the
      provisions of Code Section 280G(d)(4) and the Treasury Regulations issued
      thereunder.

      The full amount of your severance benefit under Paragraph 1 shall not be
      paid to you until any amounts in dispute under this Paragraph 6(ii) have
      been resolved in accordance herewith. However, any portion of such
      severance payment which would not otherwise exceed the benefit limitation
      of Paragraph 6(i) even if all amounts in dispute under this Paragraph
      6(ii) were to be resolved against you will be paid to you in accordance
      with the applicable provisions of this letter agreement.

            (iii) Overriding Limitation. You will in all events be entitled to
      receive the full amount of your severance payment under Paragraph 1, to
      the extent those benefits, when added to the present value of your Equity
      Parachute Payment and your Other Parachute Payments (excluding such
      severance payment), will nevertheless qualify as reasonable compensation
      within the

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 10

      standards established under Code Section 280G(b)(4).

            (iv) Interpretation. The provisions of this Section 6 shall in all
      events be interpreted in such manner as will avoid the imposition of
      excise taxes under Code Section 4999, and the disallowance of deductions
      under Code Section 280G(a), with respect to your severance benefits under
      this letter agreement.

                     PART THREE -- MISCELLANEOUS PROVISIONS

   1. Termination for Cause. Should your termination constitute a Termination
      for Cause, then the Company shall only be required to pay you (i) any
      unpaid compensation earned for services previously rendered through the
      date of such termination and (ii) any accrued but unpaid vacation benefits
      or sick days, (iii) any reimbursements then owed to you by the Company and
      no benefits will be payable to you under this letter agreement.

   2. Term of Agreement. The provisions of this letter agreement will continue
      in effect for a period of five (5) years from the date hereof.

   3. General Creditor Status. The benefits to which you may become entitled
      under this letter agreement (except those attributable to your Options or
      Stock Issuances) will be paid, when due, from the general assets of the
      Company. Your right (or the right of the executors or administrators of
      your estate) to receive any such payments will at all times be that of a
      general creditor of the Company and will have no priority over the claims
      of other general creditors of the Company.

   4. Death. Should you die before receipt of all benefits to which you become
      entitled under this letter agreement, then the payment of such benefits
      will be made, on the due date or dates hereunder had you survived, to the
      executors or administrators of your estate. Should you die before you
      exercise your Severance-Accelerated Options (if any) or any other of your
      outstanding vested Options, then each such Option may be exercised, during
      the applicable exercise period in effect hereunder for those options at
      the time of your death, by the executors or administrators of your estate
      or by person to whom the Option is transferred pursuant to your will or in
      accordance with the laws of inheritance.

   5. Miscellaneous. The provisions of this letter agreement will be construed
      and interpreted under ERISA. To the extent ERISA is inapplicable, then the
      laws of the State of California shall control, without regard to that
      state's choice of law

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 11

      provisions. This letter agreement incorporates the entire agreement
      between you and the Company relating to the subject of severance benefits
      and supersedes all prior agreements and understandings with respect to
      such subject matter. This letter agreement may only be amended by written
      instrument signed by you and another duly-authorized officer of the
      Company. If any provision of this letter agreement as applied to any party
      or to any circumstance should be adjudged by an arbitrator or court of
      competent jurisdiction to be void or unenforceable for any reason, the
      invalidity of that provision shall in no way affect (to the maximum extent
      permissible by law) the application of such provision under circumstances
      different from those so adjudicated, the application of any other
      provision of this letter agreement, or the enforceability or invalidity of
      this letter agreement as a whole. Should any provision of this letter
      agreement become or be determined to be invalid, illegal or unenforceable
      in any jurisdiction by reason of the scope, extent or duration of its
      coverage, then such provision shall be deemed amended to the extent
      necessary to conform to applicable law so as to be valid and enforceable
      or, if such provision cannot be so amended without materially altering the
      intention of the parties, then such provision shall be stricken and the
      remainder of this letter agreement shall continue in full force and
      effect.

   6. Remedies. All rights and remedies provided pursuant to this letter
      agreement or by law will be cumulative, and no such right or remedy will
      be exclusive of any other. A party may pursue any one or more rights or
      remedies hereunder or may seek damages or specific performance in the
      event of another party's breach hereunder or may pursue any other remedy
      by law or equity, whether or not stated in this letter agreement.

   7. Arbitration. Any controversy which may arise between you and the Company
      with respect to the construction, interpretation or application of any of
      the terms, provisions or conditions of this letter agreement or any
      monetary claim arising from or relating to this letter agreement will be
      submitted to and exclusively decided by final and binding arbitration in
      San Diego, California in accordance with the rules of the American
      Arbitration Association then in effect.

   8. No Employment or Service Contract. Nothing in this letter agreement shall
      confer upon you any right to continue in the employment of the Company for
      any period of specific duration or interfere with or otherwise restrict in
      any way the rights of the Company or you, which rights are hereby
      expressly reserved by each, to terminate your employment at any time for
      any reason whatsoever, with or without cause.

<PAGE>

Andres Negro-Vilar, M.D., Ph.D.
March 11, 2005
Page 12

   9. Proprietary Information. You hereby acknowledge that the Company may, from
      time to time during your employment with the Company, disclose to you
      confidential information pertaining to the Company's business and affairs.
      All information and data, whether or not in writing, of a private or
      confidential nature concerning the business or financial affairs of the
      Company is and will remain subject to a separate Proprietary Information
      and Inventions Agreement (or the like) between you and the Company.

            Please indicate your acceptance of the foregoing provisions of this
severance agreement by signing the enclosed copy of this letter agreement and
returning it to the Company.

Very truly yours,

LIGAND PHARMACEUTICALS INCORPORATED

/s/ David E. Robinson

David E. Robinson
Chairman, President and CEO

ACCEPTED BY AND AGREED TO

Signature: /s/ Andres Negro-Vilar

Dated: 3/11/05<PAGE>

                                                                  EXHIBIT 10.285

                 CONFIDENTIAL INTERFERENCE SETTLEMENT AGREEMENT

      This Confidential Interference Settlement Agreement (the "Agreement"),
effective March 11, 2005, is entered into by and between: Ligand Pharmaceuticals
Incorporated, a Delaware corporation having its principal place of business at
10275 Science Center Drive, San Diego, California 92121 (hereinafter "LIGAND");
and SRI International, a California non-profit and public benefit corporation,
having its principal place of business at 333 Ravenswood Avenue, Menlo Park,
California 94025 (hereinafter "SRI") and The Burnham Institute (hereinafter
"BURNHAM"), a California non-profit and public benefit corporation, having its
principal place of business at 10901 North Torrey Pines Road, San Diego,
California 92037.

      WHEREAS, each of SRI and BURNHAM (hereinafter collectively "SRI/BURNHAM")
jointly owns all right, title and interest in and to Dawson et al., United
States Patent No. 5,466,861 (hereinafter "Dawson '861"), which issued on
November 14, 1999, from United States Patent Application No. 07/982,305; and
Dawson et al., United States Patent No. 5,837,725 (hereinafter "Dawson '725"),
which issued on November 17, 1998, from United States Patent Application No.
08/448,991 (hereinafter collectively the "Dawson Patents");

      WHEREAS, LIGAND owns all right, title and interest in and to Boehm et al.,
United States Patent Application No. 08/141,496 filed on October 22, 1993
(hereinafter the "Boehm Application");

      WHEREAS, the Dawson Patents and the Boehm Application are currently
involved in Patent Interference No. 105,256 declared on December 22, 2004,
containing two counts and styled "Dawson v. Boehm";

      WHEREAS, Count 1 of said Interference is directed to the compound
4-[1-(3,5,5,8,8,-pentamethyl-5,6,7,8-tetrahydro-2-naphthalenyl)-ethenyl]-benzoic
acid, also known as 4-[1-(3,5,5,8,8,-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)
-ethenyl]-benzoic acid and the parties agree that Claims 1, 3, 4, 6 and 13 of
Dawson '861 and Claims 70 and 74 of the Boehm Application should be designated
as corresponding to said count;

                                       1.
<PAGE>

      WHEREAS, Count 2 of the Interference is directed to the compound
4-[1-(3,8,8,-trimethyl-5,6,7,8-tetrahydro-2-naphthenyl)-cyclopropyl]-benzoic
acid, and the parties agree that Claims 1, 3, 4 and 7 of Dawson '861, Claims
1-3, 5, 7, and 13-15 of Dawson '725 and Claims 75 and 76 of the Boehm
Application should be designated as corresponding to said count;

      WHEREAS, each of SRI/BURNHAM and LIGAND have carefully reviewed the Counts
of the Interference and the claims that have been designated as corresponding to
the Counts, and have determined that the claims that correspond in fact to the
Counts are as set forth above, but that in addition, there is further
interfering subject matter between the parties that would properly correspond to
two further counts as follows:

1. Further Count 3 would be directed to pharmaceutical compositions comprising
the compound of Count 1. The claims that correspond to Count 3 would be:

      Dawson '861: Claims 2, 15 and 23.
      Dawson '725: None.
      Boehm Application: Claims 78 and 80.

2. Further Count 4 would be directed to pharmaceutical compositions comprising
and methods of treatment using the compound of Count 2. The claims that
correspond to Count 4 would be:

      Dawson '861: Claims 2, 15 and 23.
      Dawson '725: Claims 26-29.
      Boehm Application: Claim 81.

3. The claims that do not or would not correspond to Counts 1-4 are:

      Dawson '861: Claims 5, 8-12, 14 and 16-22.
      Dawson '725: Claims 4, 6, 8-12, 16-25 and 30-32.
      Boehm Application: Claims 71-73, 77 and 79.

      WHEREAS, the parties' expressed intentions with respect to patent rights
in Bexarotene and payments to SRI and BURNHAM are that LIGAND be awarded
priority on the claims listed above as corresponding in fact to Count 1 and
Count 2, that the parties agree that LIGAND

                                       2.
<PAGE>

would be entitled to priority were Counts 3 and 4 included in the Interference,
that each of SRI/BURNHAM shall assign the Assigned Patents (as defined herein)
to LIGAND, and that LIGAND shall make payments to SRI/BURNHAM as set forth in
this Agreement;

      WHEREAS, each of SRI/BURNHAM and LIGAND desire to settle this Interference
with minimum further expense;

      WHEREAS, each of SRI/BURNHAM and LIGAND have previously entered into that
certain Settlement Agreement, License and Mutual General Release effective
August 23, 1995 (the "Settlement Agreement"), pursuant to which, among other
things, each of SRI/BURNHAM has granted LIGAND a license to the Dawson Patents
in consideration of LIGAND making certain payments to each of SRI/BURNHAM; and

      WHEREAS, the parties desire to enter into this Agreement and, except as
expressly noted otherwise herein, concurrently to terminate and to supersede the
terms of the Settlement Agreement by entering into this Agreement;

      NOW, THEREFORE, intended to be bound legally hereby, and in consideration
of the promises and covenants herein contained, the parties agree as follows:

I.    DEFINITIONS

      A. The term "Affiliate" means (i) any corporation, firm, partnership,
individual or other form of business organization which is now or hereafter
owned or controlled by or under common control with a party to this Agreement,
(ii) any corporation in which a party to this Agreement owns at least fifty
percent (50%) of the stock entitled to vote for directors, and (iii) any
corporation, firm, partnership, individual or other form of business
organization in which a party to this Agreement has the maximum ownership
interest it is permitted to have in the country where such business organization
exists.

                                       3.
<PAGE>

      B. The term "Assigned Patents" means the patents and applications listed
on Schedule A attached hereto.

      C. The term "Bexarotene" means the compound of Count 1 of the
Interference, i.e., 4-[1-(3,5,5,8,8,-pentamethyl-5,6,7,
8-tetrahydro-2-naphthalenyl)-ethenyl]-benzoic acid, also known as 4-[1-(3,5,5,8
8,-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)-ethenyl]-benzoic acid.

      D. The term "Board" means the Board of Patent Appeals & Interferences of
the United States Patent & Trademark Office.

      E. The term "Boehm Patents" means any patent that issues on the Boehm
Application and any other United States patent that currently or in the future
claims and is entitled to the benefit of priority to Application No. 07/872,707.

      F. The term "GAAP" means U.S. generally accepted accounting principles
consistently applied.

      G. The term "Interference" means pending Interference No. 105,256 or any
re-declaration thereof or any subsequent interference arising therefrom or any
appeal from any of the preceding.

      H. The term "Licensed Product" means any drug or other product, in any
form, the importation, making, using or selling of which is (i) within the scope
of any valid claim in any Assigned Patent, as issued or reissued, or (ii)
Bexarotene within the scope of any valid claim of a Boehm Patent (whether
formulated alone or in combination with any other compound).

      I. The term "Ligand Patented Technology" means and is limited to United
States Patents Nos. 4,981,784, 5,071,773, 5,091,518 and 5,171,671 and any
foreign counterparts thereof. Upon expiration, disclaimer or a holding from
which no appeal is or can be taken of invalidity of any patent included in the
Ligand Patented Technology or any claim or claims thereof, then Ligand Patented
Technology shall mean and be limited to the unexpired patents

                                       4.
<PAGE>

included in the Ligand Patented Technology and the patents or claims thereof not
affected by such disclaimer or holding of invalidity.

      J. The term "Net Sales" means the aggregate gross sales of Licensed
Product by LIGAND or its Affiliates, licensees or sublicensees (each a "Selling
Party") in the United States, less amounts actually deducted by such Selling
Party and recognized in its financial statements in calculating net sales of
such Licensed Product in accordance with GAAP and with the Selling Party's
standard accounting principles (consistent with GAAP), including, but not
limited to the following: (a) reserves and allowances (including those for
rebates and chargebacks (e.g. those associated with Medicare, Medicaid or
managed care), wholesaler fees, rejections, returns, and discounts granted to
customers of products (including volume or other incentives, and any prompt
payment discount), in cash); (b) to the extent separately stated on purchase
orders, invoices or other documents of sale, freight, transport packing and
insurance charges associated with transportation paid by or on behalf of the
Selling Party; and (c) taxes, tariff or import/export duties based on sales when
included in gross sales, but not value-added taxes or taxes assessed on income
derived from such sales in the United States. For clarity, the date on which Net
Sales are deemed to occur for the purposes of this Agreement shall be the date
on which the relevant product sale is made and booked by the Selling Party
according to GAAP. In the event that the Selling Party sells a product that is a
combination of Licensed Product and one or more other active ingredients, the
Net Sales for said product will be determined by multiplying the Net Sales of
said combination product determined in accordance with this paragraph by a
fraction, the numerator of which will be the Selling Party's sales price of
Licensed Product and the denominator of which will be the sum of the Selling
Party's sales price of each active ingredient in the combination product,
including Licensed Product.

II.   REPRESENTATIONS AND WARRANTIES

      A. Each of SRI and BURNHAM represent and warrant that they solely or
jointly with each other own all right, title and interest in and to the Assigned
Patents.

                                       5.
<PAGE>

      B. BURNHAM represents and warrants that SelectRA Pharmaceuticals, Inc.,
formerly a California corporation has been previously wound up, has no
successors and its consent to this Agreement is not required.

      C. LIGAND represents and warrants that it owns all right, title and
interest in and to the Boehm Application and the Boehm Patents. LIGAND
represents and warrants that there are no pending applications owned by LIGAND
that were filed on or before June 7, 1995 and that claim or could be amended to
claim the composition of matter of Bexarotene other than United States Patent
Application No. 08/141,496 and United States Patent Application No. 08/141,246.

      D. LIGAND represents and warrants that Allergan Ligand, formerly a
California partnership has been previously dissolved, has no successors and its
consent to this Agreement is not required.

      E. Each of SRI, BURNHAM and LIGAND hereby respectively represents and
warrants as of the date of this Agreement as follows:

                  (a) Such party is a corporation or, in the case of
SRI/BURNHAM, non-profit, public benefit corporation, duly organized, validly
existing and in good standing under the laws of the state in which it is
incorporated.

                  (b) Such party (i) has the corporate power and authority and
the legal right to enter into the Agreement and perform its obligations
hereunder, and (ii) has taken all necessary corporate action on its part
required to authorize the execution and delivery of the Agreement and the
performance of its obligations hereunder. The Agreement has been duly executed
and delivered on behalf of such party, and constitutes a legal, valid, binding
obligation of such party and is enforceable against it in accordance with its
terms subject to the effects of bankruptcy, insolvency or other laws of general
application affecting the enforcement of creditor rights and judicial principles
affecting the availability of specific performance and general principles of
equity whether enforceability is considered a proceeding at law or equity.

                                       6.
<PAGE>

                  (c) The execution and delivery of the Agreement and the
performance of such party's obligations hereunder (i) do not conflict with or
violate any requirement of applicable law or regulation or any provision of
articles of incorporation or bylaws of such party in any material way, and (ii)
do not conflict with, violate or breach or constitute a default or require any
consent under, any contractual obligation or court or administrative order by
which such party is bound.

                  (d) Such party will use its reasonable efforts to cause its
employees and former employees to cooperate and perform any acts, including
execution of documents, that are necessary or expedient in carrying out the
objectives of this Agreement, including without limitation taking those actions
or refraining from taking actions as set forth in Article III below.

III.  DETERMINATION OF PRIORITY; ASSIGNMENT OF PATENTS AND POSSIBLE FUTURE
      INFRINGEMENT ACTIONS

      A. LIGAND is senior party in the Interference, having been accorded
benefit of earlier applications, whose earliest filing date is April 22, 1992.
SRI/BURNHAM are jointly the junior party in the Interference, with an effective
filing date for the Dawson Patents being November 25, 1992. Each of SRI/BURNHAM
and LIGAND have determined that LIGAND is entitled to priority of invention in
the Interference.

      B. Each of SRI/BURNHAM shall file in the Interference: (i) a statutory
disclaimer of the claims that would correspond to Counts 3 and 4; and (ii) a
document seeking to correct the listing of claims corresponding to Counts 1 and
2, which the parties agree to file jointly. Within five (5) days after the Board
issues an order correcting the designation of claims that correspond to Counts 1
and 2, SRI/BURNHAM will file with the Board a document conceding that LIGAND has
priority as to both counts of the Interference and seeking entry of adverse
judgment under Board Rule 127(b), as to Counts 1 and 2 of the Interference or
taking such other action as LIGAND reasonably deems necessary or advisable to
resolve or dismiss the Interference, and each of SRI/BURNHAM will provide
assistance to LIGAND as LIGAND may reasonably deem necessary to accomplish the
foregoing (including using reasonable efforts to

                                       7.
<PAGE>

have their employees and former employees help to accomplish the same). If the
foregoing does not result in a final judgment from the Board that LIGAND is
entitled to its claims properly corresponding to Counts 1 and 2 within thirty
(30) days from the effective date of this Agreement, then each of SRI/BURNHAM
shall, at LIGAND's request, file a statutory disclaimer of the claims that
should correspond to Counts 1 and 2 (i.e., as listed herein). If requested to do
so by LIGAND, each of SRI/BURNHAM will cooperate in the filing of the assignment
of the Dawson Patents and this Agreement in the Interference.

      C. In consideration of the terms and conditions of this Agreement,
including paragraph IV, each of SRI/BURNHAM does hereby agree to sell, assign,
convey and transfer to LIGAND all rights, title, and interests in and to the
Assigned Patents other than the Dawson Patents as of the effective date of this
Agreement and to the Dawson Patents upon both: (1) either of (a) receipt of a
final judgment from the Board that LIGAND is entitled to its claims properly
corresponding to Counts 1 and 2, and that SRI/BURNHAM are not entitled to their
claims properly corresponding to Counts 1 and 2, or (b) the filing of a
statutory disclaimer by SRI/BURNHAM of the claims that should correspond to
Counts 1 and 2 (whether subsection (a) or (b) applies to be determined in
accordance with Section III.B.), and (2) the filing of a statutory disclaimer by
SRI/BURNHAM of the claims that would correspond to Counts 3 and 4. Each of
SRI/BURNHAM will, upon request of LIGAND, execute such additional assignments,
documents, certificates and other writings and, at the expense of LIGAND, do
such additional acts as LIGAND may reasonably deem necessary or desirable to
perfect LIGAND's rights in the Assigned Patents or to prosecute or maintain the
Boehm Application, a Boehm Patent or an Assigned Patent (including using their
reasonable efforts to cause its employees or former employees to execute any
such documents or perform such acts). Each of SRI/BURNHAM shall deliver to
LIGAND, or a party that LIGAND designates in writing, any and all sealed
notebooks and invention records relating to the Assigned Patents, with LIGAND to
pay for the reasonable out-of-pocket costs of SRI/BURNHAM for photocopying and
shipping. In the situation where SRI or BURNHAM has received funding from the
United States Government in support of research activities which have resulted
in Assigned Patents, LIGAND acknowledges that LIGAND's rights in the Assigned
Patents are subject to the rights of the United States Government which arise or
result from the receipt of research support from the United States

                                       8.
<PAGE>

Government by SRI or BURNHAM, including, without limitation, (i) the grant to
the United States of a nonexclusive, irrevocable, royalty-free license to
Assigned Patents for governmental purposes, (ii) the right of the United States
to exercise "march-in" rights to force certain non-exclusive licensing if LIGAND
is not diligently commercializing certain Licensed Products, and (iii) the
obligation of LIGAND to manufacture substantially in the United States those
Licensed Products which are sold in the United States, unless a waiver is
obtained from the appropriate agency of the United States.

      D. The parties shall cooperate in obtaining extensions of time in this
Interference, as needed to facilitate resolution of this Interference in
accordance with this Agreement. Neither of SRI/BURNHAM nor LIGAND shall seek to
file or file any other substantive motions in the Interference (i.e., including
any appeal therefrom in any forum or action), except as permitted by paragraph
III.B. or III.C. Neither of SRI/BURNHAM shall challenge the validity or
enforceability of any Boehm Application, Boehm Patent or Assigned Patent or
provide information to a third party (except as may be required by law, legal
process or governmental regulation) in order to have the third party bring such
a challenge.

      E. Each of SRI/BURNHAM hereby covenant and agree, for themselves and their
successors and assigns, not to make, have made, import, use, sell, market or
distribute in the United States any products or carry out any processes which
would infringe claims in the Ligand Patented Technology, the Assigned Patents or
the Boehm Patents or to induce or contribute to the infringement of the Ligand
Patented Technology, the Assigned Patents or the Boehm Patents by others.

      F. Each of SRI/BURNHAM will not after the effective date of this Agreement
seek in any proceeding, or assist any other party except under judicial
compulsion, to have any patent or any claim thereof in the Ligand Patented
Technology, the Assigned Patents or the Boehm Patents declared, determined or
adjudicated invalid or unenforceable. As used in this Agreement, "proceeding"
includes a reexamination proceeding in the United States Patent and Trademark
Office.

                                       9.
<PAGE>

      G. Each of SRI/BURNHAM agree that they will not assert any rights in
compounds synthesized in the laboratory of Marcia Dawson, Ph.D. through May 17,
1993 inconsistent with or in derogation of LIGAND's rights set forth in the
Compound Evaluation Agreement of May 17, 1990.

      H. In the event LIGAND, its Affiliates or its sublicensees shall prosecute
any legal action to enforce any of the Assigned Patents or Boehm Patents against
infringement by an unlicensed third party or parties, it shall be at their own
expense. Each of SRI/BURNHAM shall cooperate with LIGAND and render reasonable
assistance in enforcing the Assigned Patents. LIGAND shall pay the reasonable
out-of-pocket costs of SRI/BURNHAM arising out of such cooperation. Any recovery
realized as a result of such legal action, shall belong to LIGAND, provided that
to the extent any such recovery realized by LIGAND represents lost profits, it
shall be treated as Net Sales for purposes of this Agreement after deducting
therefrom a pro-rata portion of the out-of-pocket expenses of LIGAND in
connection with such action including the cost of capital for such expenses. The
pro-rata portion of such expenses to be deducted from any lost profits recovery
shall be based on the ratio of any lost profits recovery to the total recovery.
The "cost of capital" shall mean interest on such expenses from the time
disbursed at the currently prevailing FAR rate.

      I. As used in Sections III.H. above, "cooperate" means with respect to
each of SRI/BURNHAM that each will, at LIGAND's request and expense, take
actions, including but not limited to, executing documents, providing
declarations and affidavits, supplying documents and giving depositions and
in-court testimony, and otherwise affirmatively assisting in the matters and
proceedings which are the subject of those sections without subpoena.

      J. Each party to this Agreement shall bear its own costs and expenses,
including attorneys' fees, in connection with the Interference and this
Agreement, except as otherwise provided herein.

      K. Each of SRI/BURNHAM shall promptly upon execution of this Agreement
provide any valid, documented information it may have concerning the joint
inventorship of any

                                      10.
<PAGE>

Boehm Patents or the Boehm Application and any other patents or applications
owned or licensed exclusively to LIGAND. LIGAND shall make a good faith
determination, in consultation with its outside patent counsel, as to whether
there is proper joint inventorship for any such patents or applications. If any
such error in the current inventorship is identified by LIGAND, then LIGAND
shall seek to correct such error(s). If any employee or former employee of
SRI/BURNHAM is added as an inventor to any such patent or application, then each
of SRI/BURNHAM shall assign any rights they have in such patent or application
to LIGAND.

IV.   PAYMENTS

      A. In consideration of the agreement to assign the Assigned Patents by
each of SRI/BURNHAM to LIGAND and related matters under Section III, LIGAND
shall pay to each of SRI and BURNHAM beginning on January 1, 2005 a payment of
1.5% of Net Sales of Licensed Product in the United States (for a total payment
of 3.0% of Net Sales) until October 5, 2016, but only so long as a Boehm Patent
or Assigned Patent contains a valid claim covering such Licensed Product or its
use. After October 5, 2016, LIGAND shall pay each of SRI and BURNHAM a payment
of 1.0% of Net Sales (for a total payment of 2.0% of Net Sales) for (a) a
Licensed Product (other than Bexarotene) covered by an Assigned Patent so long
as there is a valid claim in such Assigned Patent covering such Licensed Product
or its use, and (b) a Bexarotene product so long as there is a valid claim in
any issued patent resulting from United States Patent Application No. 08/141,496
or United States Patent Application No. 08/141,246 covering the composition of
matter of the Bexarotene product. Only one payment will be owed on each Licensed
Product, e.g., in the circumstance where a Licensed Product is covered by
multiple claims in one or more Boehm Patents and/or Assigned Patents. No other
payments to each of SRI/BURNHAM arising from sales of Licensed Product will be
required, including without limitation payments on sales of Licensed Product
outside the United States or Licensed Product manufactured in the United States
but sold outside the United States. The term "valid claim" as used in this
Agreement shall mean an enforceable claim that has not been invalidated by a
final unappealable or unappealed decision of a court or governmental agency of
competent jurisdiction.

                                      11.
<PAGE>

      B. LIGAND shall provide quarterly reports to SRI within sixty (60) days of
each March 31, June 30, and September 30 and within seventy-five (75) days of
December 31 during the period when payments are DUE. Such reports will state the
quantity and description of products subject to payment sold during the
preceding payment period, the Net Sales thereof and the calculation of the
payment due. All payments due hereunder will be paid simultaneously with the
submission of such reports. If any taxes are imposed on the payments due each of
SRI/BURNHAM and are required to be withheld therefrom, such taxes will be for
the account of each of SRI/BURNHAM and will reduce the payments required to be
made hereunder. All payments will be made in U.S. Dollars.

      C. LIGAND shall keep true and accurate records and/or books of account
containing information reasonably required for the computation and verification
of payments to be paid to each of SRI/BURNHAM hereunder, which records and/or
books will at all reasonable and mutually convenient times during ordinary
business hours be open for periodic inspection, not more than once each calendar
year and for inspection of no more than the three (3) prior years' records
and/or books, by an independent certified public accountant selected by each of
SRI/BURNHAM and who is reasonably acceptable to LIGAND, for the sole purpose of
and only to the extent reasonably necessary for verification of the amount of
payments due and payable under this Agreement. The expense of the audit will be
borne by each of SRI/BURNHAM and the results thereof will be binding on both
parties; provided, however, if the audit discloses an aggregate underpayment of
more than ten percent (10%) for any audited period, the expenses for said audit
will be reimbursed by LIGAND. Any underage in a payment revealed by the audit
will be due and payable with thirty (30) days of the audit results being
disclosed by notice to LIGAND, unless contested during the thirty (30) day
period by LIGAND which may at its expense retain a second independent certified
public accountant to audit the payment obligation. Any overage in a payment
revealed by the audit will be due and payable with thirty (30) days of the audit
results being disclosed by notice to LIGAND, unless contested during the thirty
(30) day period by each of SRI/BURNHAM which may at its expense retain a second
independent certified public accountant to audit the payment obligation. The
payment obligation calculated by the second accountant will be disclosed to each
of SRI/BURNHAM and averaged with the

                                      12.
<PAGE>

results disclosed by the first auditor and any underage resulting from such
averaging will be payable within thirty (30) days of the second audit. The
provisions of this Section IV.C. shall also apply to any payments made by LIGAND
to SRI/BURNHAM under the Settlement Agreement for any period prior to January 1,
2005.

      D. The terms of this Agreement supersede in their entirety the terms of
the Settlement Agreement with respect to the Assigned Patents (which are the
same as the Licensed Patents under the Settlement Agreement as of January 1,
2005), and the parties hereby agree that the Settlement Agreement is terminated
as it applies to the Assigned Patents, except that such termination shall not
terminate any right or obligation of a party with respect to the Assigned
Patents that arose under the Settlement Agreement prior to such termination,
including, without limitation, the right to receive any payment due under the
Settlement Agreement prior to January 1, 2005. For clarification, the following
provisions of the Settlement Agreement shall remain in effect: Section 2.09
(Authorized Comments); and Article 3 (Release). For further clarification, all
Assigned Patents shall cease to be Licensed Patents for purposes of the
Settlement Agreement effective as of the assignment thereof to LIGAND hereunder.

      E. In addition, the parties agree that the terms of the Settlement
Agreement shall continue in effect from and after January 1, 2005 only as
between LIGAND and BURNHAM and only with respect to Option Patents that are
Option Patents as of the date of this Agreement and Option Patents arising after
the date of this Agreement that cover Option Technology invented, discovered or
developed by employees, consultants or agents of BURNHAM, which was enabled by,
reduced to practice or otherwise derived from or facilitated by an act or acts
within the scope of the claims of any patent within the Ligand Patented
Technology or, after the assignment of the Assigned Patents pursuant to this
Agreement, the Assigned Patents. For clarification, the following provisions of
the Settlement Agreement shall continue in effect as between LIGAND and BURNHAM
(and not as to SRI) but only as they apply to the Option Patents and Option
Technology described in the first sentence of this Section IV.E.: Sections 1.01
(Party), except that such term shall only apply to LIGAND and BURNHAM, including
their respective Affiliates, 1.02 (Affiliate), 1.03 (Ligand Patented
Technology), 1.04 (Option Technology), as modified by this Section IV.E., 1.05
(Option Patents), as modified by

                                      13.
<PAGE>

this Section IV.E., 1.06 (Licensed Patent), 1.07 (Licensed Product), 1.08
(Licensed Process), 1.09 (Net Sales), replaced and superseded by Section I.J. of
this Agreement, 1.10 (Field), provided that references therein to Ligand
Patented Technology shall also include Assigned Patents, as issued or reissued,
but shall not include (i) Bexarotene, its salts, or esters, or (ii) in vivo (in
animal) administration of metabolites of Bexarotene known as of the effective
date of this Agreement or in vivo (in animal) administration of likely
metabolites (i.e., mono- or di-hydroxy, mono or di-oxo, or mono-hydroxy,
mono-oxo metabolites or metabolites with amino acid conjugation at the
carboxylic acid) of Bexarotene and 1.11 (Territory); Section 2.10 (Restrictions
on Seeking Governmental License); Article 5 (Basic Research Sublicense),
provided that references therein to Ligand Patented Technology shall also
include Assigned Patents, as issued or reissued, but shall not include (i)
Bexarotene or the salts, or esters thereof, or (ii) in vivo (in animal)
administration of the metabolites of Bexarotene known as of the effective date
of this Agreement or in vivo (in animal) administration of likely metabolites
(i.e., mono- or di-hydroxy, mono or di-oxo, or mono-hydroxy, mono-oxo
metabolites or metabolites with amino acid conjugation at the carboxylic acid)
of Bexarotene; Article 6 (Option for Exclusive License); Article 7
(Confidentiality); Article 8 (Arbitration); Article 9 (Term and Termination);
and Article 10 (Miscellaneous). For further clarification, no breach of any
provision of the Settlement Agreement, nor any termination or expiration of the
terms of the Settlement Agreement, arising after the date of this Agreement
shall have any effect on the other terms of this Agreement, and no payment made
under the Settlement Agreement terms after the date of this Agreement shall have
any effect on, or serve to reduce, any payment made or to be made under the
terms of this Agreement.

V.    FILING OF THIS AGREEMENT

      LIGAND shall, in accordance with 35 U.S.C. Section 135(c) file with the
Board a true copy of this Agreement, in a sealed envelope along with a request,
in accordance with Board Rule 205(c), that said copy be kept separate from the
file of the Interference and be made available only to authorized
representatives of United States government agencies or to others only on
petition under Board Rule 205(d).

                                      14.
<PAGE>

VI.   ENTIRE AGREEMENT

      This Agreement constitutes the entire agreement between the parties hereto
relating to the specific subject matter hereof, and supersedes all other
previous understandings, agreements, and negotiations with respect to the
specific subject matter hereof except as set forth in Sections IV.D. and IV.E.
above. No variation or modification of this Agreement or waiver of any terms or
provisions hereof will be deemed valid unless made in writing and signed by all
parties hereto.

VII.  NOTICES

      Any notice required or provided for by the terms of this Agreement must be
in writing and, if sent by express mail or registered mail, prepaid and properly
addressed to the party to be served therewith at the addresses set forth below
(or any other address provided by notice in accordance with this provision) and
if sent by facsimile, properly addressed to the party to be served therewith at
the facsimile number set forth below (or any other facsimile number provided by
notice in accordance with this provision) with receipt confirmed, shall be
deemed to have been given or made on the date upon which said notice was
received. Notices to SRI will be addressed to: SRI INTERNATIONAL 333 Ravenswood
Ave. Menlo Park, CA 94025 Attn: Vice President, Legal and Business Affairs and
General Counsel Facsimile: (650) 859-3834

Notices to SRI will be addressed to:

       SRI INTERNATIONAL
       333 Ravenswood Ave.
       Menlo Park, CA  94025
       Attn:    Vice President, Legal and Business Affairs and General Counsel
       Facsimile:        (650) 859-3834

Notices to BURNHAM will be addressed to:

       The Burnham Institute
       10901 North Torrey Pines
       La Jolla, CA  92037
       Attn:    Chief Operating Officer
       Facsimile:        (858) 646-3105

                                       15.

<PAGE>

Notices to LIGAND will be addressed to:

            Ligand Pharmaceuticals Incorporated
            10275 Science Center Drive
            San Diego, California 92121
            Attn:  General Counsel
            Facsimile:   (858) 550-1825

VIII. SUCCESSION

      This Agreement will be binding upon and will inure to the benefit of the
respective successors of the parties hereto. However, this Agreement and the
rights and obligations of SRI or BURNHAM hereunder will not be assignable by SRI
or BURNHAM without prior written consent of LIGAND except to a non-profit entity
which acquires substantially all of the assets of SRI or BURNHAM. Further, this
Agreement and the rights and obligations of LIGAND hereunder will not be
assignable by LIGAND without prior written consent of SRI and BURNHAM except to
a successor in interest of substantially all of the assets to which this
Agreement pertains or to an Affiliate of LIGAND or to a successor entity in the
case of a merger, acquisition or other combination in which LIGAND is not a
surviving entity, in which case written notice (but not approval) is required.

IX.   CONFIDENTIALITY

      No party to this Agreement shall disclose any materials or information
relating to this Interference to any third party. The use of the name of SRI or
BURNHAM in connection with the advertising or sale of any Licensed Product is
expressly prohibited. No party shall make any written public release or written
public statement regarding this Agreement and the underlying Interference
(including, without limitation, press releases and disclosures in Securities and
Exchange Commission filings) unless such written public release or written
public statement has previously been reviewed and approved by all parties;
provided, however, that a party shall not be required to secure the approval of
any other party to make a disclosure which, upon advice of independent counsel,
it is required to make by law, regulation or legal process, and further provided
that where approval is required that no party shall unreasonably withhold its
approval. Once approved, such written public release or written public statement
may be freely reissued or

                                       16.

<PAGE>

repeated in writing or orally without additional approval. Nothing in this
Section IX shall affect the parties' respective rights to communicate privately,
orally or in writing with employees, vendors, researchers, distributors,
licensees, prospective licensees and other commercially affected persons and
entities as may be necessary to effectuate this Agreement. Each party shall have
the obligation to use reasonable efforts to prevent its agents and employees
from violating the foregoing. The provisions of this Section IX shall also apply
to any confidential or proprietary information supplied by one party to another
party under the Settlement Agreement prior to the date of this Agreement.

X.    INDEMNITY

      LIGAND hereby agrees to indemnify, defend and hold harmless each of
SRI/BURNHAM and their respective officers, trustees, employees and agents
(hereinafter "Indemnitees") from and against any liability or expense arising
from any product liability claim asserted by any party as the result of the use
of any Licensed Product or any product liability claim arising from the use of
any Assigned Patent by LIGAND, its Affiliates, assignees or licensees. Such
indemnity and defense obligation shall apply to any product liability claim,
including, without limitation, personal injury, death or property damage.
Notwithstanding the above, LIGAND shall have no obligation to indemnify, defend
or hold harmless the Indemnitees for claims arising from the negligence or
willful misconduct of an Indemnitee and is conditioned on the prompt notice of
and reasonable assistance in the defense of such claims. The provisions of this
Section X shall also apply to any liability or expense arising from any product
liability claim asserted by any party as the result of the use of any Licensed
Product or Process or any product liability claim arising from the use of any
Licensed Patent by LIGAND, its Affiliates, assignees or licensees under the
Settlement Agreement prior to the date of this Agreement.

XI.   TERM

      A. Unless sooner terminated in accordance with the provisions of this
Agreement, or as agreed in writing by the parties, this Agreement shall
terminate upon the last to expire of the Boehm Patents and Assigned Patents.

                                       17.

<PAGE>

      B. In the event of a breach of any of the obligations here under by SRI,
BURNHAM or LIGAND, as the case may be, the other party may advise the breaching
party of the breach in writing. The parties shall meet within thirty (30) days
after receipt by the breaching party of such notice. If the breach is not
corrected within thirty (30) days of such meeting, the other party shall have
the right to seek such relief with respect to such breach as may be provided by
law or equity. The failure of a party to advise the other of a breach shall not
constitute a waiver of any of the party's rights hereunder.

      C. Expiration of this Agreement shall not affect any right or obligation
of a party that arose under this Agreement prior to such expiration, including,
without limitation, the right to receive any payment due under paragraph IV
prior to expiration, and the following provisions of this Agreement shall
survive expiration of this Agreement: paragraph I, paragraph II.A. and B.;
paragraph IV.C., D. and E.; paragraph VI, paragraph VIII; paragraph VIII;
paragraph IX; paragraph X; paragraph XI.B. and C.; paragraph XIII; paragraph
XIV; paragraph XVI; paragraph XVII; and paragraph XVIII.

XII.  FORCE MAJEURE

      Neither of the parties shall be responsible to the other for delay or
failure in performance of any of the obligations imposed by this Agreement,
provided such failure shall be occasioned by an Act of God beyond the reasonable
control and without fault or negligence of such party.

XIII. AMENDMENT AND WAIVER

      No provision of this Agreement shall be deemed waived, amended, or
modified by any party unless such waiver, amendment, or modification shall be in
writing and signed by the party against the waiver or modification is sought to
be enforced.

                                       18.

<PAGE>

XIV.  SEPARABILITY

      In the event that any provision of this Agreement is found to be void or
unenforceable, such provision shall be construed to be independent of and
separate from the other provisions of this Agreement, which other provisions
shall retain full force and effect.

XV.   HEADINGS

      The headings of the several sections are inserted for convenience of
reference only and are not intended to be part of or to affect the meaning or
interpretation of this Agreement.

XVI.  DISCLAIMER

      EXCEPT AS EXPRESSLY PROVIDED HEREIN, NO PARTY MAKES ANY REPRESENTATION OR
WARRANTY OF MERCHANTIBILITY, FITNESS FOR A PARTICULAR PURPOSE OR NONINFRINGEMENT
OR REGARDING THE VALIDITY OF ANY PATENT. The provisions of this Section XVI
shall also apply as a disclaimer of representations and warranties with respect
to Assigned Patents under the Settlement Agreement.

XVII. SEPARATE LIABILITY

      The parties agree that the obligations and duties of each party arising
under this Agreement, regardless whether shared, identical or otherwise similar,
are separate and distinct from the obligations and duties of any other party.
Actions or failures to act by one party shall not confer joint and several
liability to the other parties.

XVIII. APPLICABLE LAW

      This Agreement shall be construed in accordance with the laws of the State
of California, without reference to its conflict of laws provisions.

                                       19.

<PAGE>

      IN WITNESS WHEREOF, the parties have caused this Agreement to be executed
in multiple counterparts (each of which shall be deemed an original) by their
duly authorized representatives, each of whom warrant that he/she has authority
to do so, this Agreement to be effective as of the date and year first written
above.

                             LIGAND PHARMACEUTICALS INCORPORATED

                             By: /s/ Warner R. Broaddus

      Date:________________  Title: General Counsel, Vice President & Secretary

                             SRI INTERNATIONAL

                             By:    /s/ Richard Abramson

      Date: 3/11/05          Title: VP Legal & Business Affairs

                             THE BURNHAM INSTITUTE

                             By:    /s/ Karin Eastham

      Date: 3-11-05          Title: Executive Vice-President & COO

                        ALLERGAN LIGAND RETINOID THERAPEUTICS, INC.

                             By:    /s/ Warner R. Broaddus

      Date:________________  Title  Secretary

                                       20.
<PAGE>

                                   SCHEDULE A

                              THE ASSIGNED PATENTS

LIGAND CASE NUMBER: 015-0003
TITLE: METHOD OF INHIBITING TRANSCRIPTION UTILIZING NUCLEAR RECEPTORS

<TABLE>
<CAPTION>
COUNTRY                      STATUS      APPLICATION NUMBER   PATENT NUMBER
--------------------------   ---------   ------------------   -------------
<S>                          <C>         <C>                  <C>
Canada                       Abandoned   2,093,811
European Patent Convention   Abandoned   91920385.1
Japan                        Abandoned   3-518584
Patent Cooperation Treaty    Closed      US91/07572
United States of America     Issued      08/182,735           5,643,720
United States of America     Issued      08/757,349           6,004,748
United States of America     Issued      09/139,525           6,683,047
</TABLE>

LIGAND CASE NUMBER: 015-0004
TITLE: RXR Homodimer Formation

<TABLE>
<CAPTION>
COUNTRY                      STATUS      APPLICATION NUMBER   PATENT NUMBER
--------------------------   ---------   ------------------   -------------
<S>                          <C>         <C>                  <C>
United States of America     Abandoned   901,719
United States of America     Issued      08/297,706           5,712,093
United States of America     Issued      07/982,174           5,552,271
United States of America     Issued      08/589,528           5,824,484
United States of America     Pending     08/918,154
</TABLE>

LIGAND CASE NUMBER: 015-0005
TITLE: Novel Retinoid X Receptor Heterodimers and Methods of Use

<TABLE>
<CAPTION>
COUNTRY                      STATUS      APPLICATION NUMBER   PATENT NUMBER
--------------------------   ---------   ------------------   -------------
<S>                          <C>         <C>                  <C>
Patent Cooperation Treaty    Closed      US92/11221
United States of America     Pending     07/814,871
United States of America     Pending     09/232,411
</TABLE>

                                      21.
<PAGE>

                                   SCHEDULE A
                              THE ASSIGNED PATENTS

LIGAND CASE NUMBER: 024-0001
TITLE: BRIDGED BICYCLIC AROMATIC COMPOUNDS AND THEIR USE IN MODULATING GENE
EXPRESSION OF RETINOID RECEPTORS

<TABLE>
<CAPTION>
COUNTRY                      STATUS      APPLICATION NUMBER   PATENT NUMBER
--------------------------   ---------   ------------------   -------------
<S>                          <C>         <C>                  <C>
Australia                    Granted     58693/94             700706
Brazil                       Abandoned   9307528
Brazil                       Pending     1101118-1
Canada                       Pending     2,149,882
European Patent Convention   Abandoned   94904805.2
Japan                        Pending     6-513405
Japan                        Pending     2004-2452
Korea, Republic of           Granted     95-702106            353654
Patent Cooperation Treaty    Closed      US93/11492           WO94/12880
United States of America     Issued      07/982,305           5,466,861
United States of America     Issued      08/448,991           5,837,725
</TABLE>

LIGAND CASE NUMBER: 024-0002
TITLE: NOVEL COMPOUNDS USEFUL IN MODULATING GENE EXPRESSION OF RETINOID
RESPONSIVE GENES AND/OR HAVING ANTI-AP-1 ACTIVITY

<TABLE>
<CAPTION>
COUNTRY                      STATUS      APPLICATION NUMBER   PATENT NUMBER
--------------------------   ---------   ------------------   -------------
<S>                          <C>         <C>                  <C>
European Patent Convention   Abandoned   95923801.5
Patent Cooperation Treaty    Closed      US95/07390
United States of America     Abandoned   08/255,345
United States of America     Abandoned   08/468,035
</TABLE>

LIGAND CASE NUMBER: 024-0003
TITLE: RETINOIDS WITH SELECTIVE ANTI-AP-1 ACTIVITY EXHIBIT ANTI-PROLIFERATIVE
ACTIVITY CYCLIC AROMATIC COMPOUNDS AND THEIR USE IN MODULATING GENE EXPRESSION
OF RETINOID RECEPTORS

<TABLE>
<CAPTION>
COUNTRY                      STATUS      APPLICATION NUMBER   PATENT NUMBER
--------------------------   ---------   ------------------   -------------
<S>                          <C>         <C>                  <C>
United States of America     Abandoned   08/326,775
</TABLE>

                                      22.

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00094-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00094-of-00352.parquet"}]]