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                                                                   EXHIBIT 10.73


                              [EPIMMUNE LETTERHEAD]


October 6, 1999



Stephen F. Keane
2780 Angell Avenue
San Diego, California 92122

Dear Steve,

This letter defines the benefits to which you will be entitled upon termination
of your employment with Epimmune Inc. (the "Company") under the circumstances
described in this Agreement.

We agree that you are employed by the Company as an "at-will" employee and that
either you or the Company has the right at any time to terminate your employment
with the Company, with or without cause or advance notice, for any reason or for
no reason.

For purposes of this Agreement, the following terms will have the meanings set
forth below:

        "BASE SALARY" means your salary (excluding bonus, any other incentive or
        other payments and stock option exercises) paid by the Company in
        consideration for your service during the 12 months prior to the
        termination of your employment which is includable in your gross income
        for federal income tax purposes or which would have been includable in
        gross income except for an election either under Section 125 or
        402(e)(3) of the Internal Revenue Code of 1986, as amended (the "Code"),
        or under the terms of a nonqualified deferred compensation arrangement
        sponsored by the Company.

        "BENEFIT" means any payment received or to be received by you pursuant
        to this Agreement.

        "CAUSE" means (i) your material breach of any provision of this
        Agreement; or (ii) your engaging or in any manner participating in any
        activity which is directly competitive with or intentionally injurious
        to the Company or which violates any provision of any confidential
        information agreement or nonsolicitation agreement between you and the
        Company; or (iii) your commission any fraud against the Company; or (iv)
        your intentional improper use or appropriation for your personal use or
        benefit of any funds or properties of the Company not authorized by the
        Board of Directors of the Company or an authorized committee of such
        Board of Directors to be so used or appropriated; or (v) your conviction
        of any crime involving dishonesty or moral turpitude.


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Stephen F. Keane
October 6, 1999
Page 2


        "CHANGE IN CONTROL" of the Company will be deemed to have occurred if
        and when (i) any person or entity (other than the Company) or group of
        persons and/or entities acting in concert acquires, directly or
        indirectly, beneficial ownership of more than 50% of the outstanding
        shares of voting stock of the Company; or (ii) the Company is a
        participant in a merger or consolidation in which it does not survive as
        an independent company; or (iii) the business or businesses of the
        Company for which your services are principally performed are disposed
        of by the Company pursuant to a partial or complete liquidation of the
        Company.

        "COMPANY" for the purposes of this Agreement shall include the Company's
        successor if a Change in Control occurs.

        "EXCISE TAX" means the excise tax imposed by Section 4999 of the Code.

        "GOOD REASON" means (i) reduction of your rate of compensation as in
        effect immediately prior to the effective date of this Agreement; or
        (ii) the Company's failure to provide to you a package of welfare
        benefit plans which, taken as a whole, provide substantially similar
        benefits to those in which you are entitled to participate immediately
        prior to the occurrence of the termination of your employment (except
        that employee contributions may be raised to the extent of any cost
        increases imposed by third parties) or any action by the Company which
        would adversely affect your participation or reduce your benefits under
        any of such plans; or (iii) a change in your responsibilities,
        authority, title or office resulting in diminution of your position,
        excluding for this purpose an isolated, insubstantial and inadvertent
        action not taken in bad faith which is remedied by the Company promptly
        after written notice thereof is given by you; or (iv) a request that you
        relocate to a worksite that is more than 35 miles from your prior
        worksite, unless you accept such relocation opportunity; or (v) a
        material reduction in your duties; or (vi) a failure or refusal of a
        successor to the Company to assume the Company's obligations under this
        Agreement; or (vii) a material breach by the Company or any successor to
        the Company of any of the material provisions of this Agreement.

Although the Company has no policy or procedure for providing severance
benefits, in exchange for the promises and covenants set forth herein, and in
consideration thereof, the Company agrees that if, within 365 days following the
occurrence of a Change in Control, your employment is terminated without Cause
by the Company or you terminate your employment for Good Reason, then upon your
furnishing to the Company an executed waiver and release of claims (a form of
which is attached hereto as Exhibit A) ("Release") and subject to your ongoing
obligations under the Company's Proprietary Information and Inventions
Agreement, (1) within 30 days of the Effective Date of the Release you will
receive a lump-sum payment equal to six


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Stephen F. Keane
October 6, 1999
Page 3


months of Base Salary subject to any applicable withholding of federal, state or
local taxes and (2) the vesting of any stock option granted to you for Company
common stock and the time during which such stock option may be exercised
immediately will be accelerated as to 50% of the then unvested stock options,
such that you will have the right to immediately exercise such stock options on
the date of termination.

If any Benefit received or to be received by you pursuant to this Agreement
would constitute a "parachute payment" within the meaning of Section 280G of the
Code and but for this paragraph be subject to the Excise Tax, then, upon written
request to the Commune by you, such Benefit shall be reduced to the extent
necessary so that no portion of the Benefit would be subject to the Excise Tax,
as determined in good faith by the Company; provided, however, that if, in the
absence of any such reduction (or after such reduction), you believe that the
Benefit or any portion thereof (as reduced, if applicable) would be subject to
the Excise Tax, the Benefit shall be reduced (or further reduced) to the extent
determined by you in your discretion so that the Excise Tax would not apply.
Your determination of any required reduction pursuant to this paragraph shall be
conclusive and binding on the Company. If, notwithstanding any such reduction
(or in the absence of such reduction), the Internal Revenue Service determines
that you are liable for the Excise Tax as a result of the Benefit, you shall be
obligated to return to the Company, within 30 days of such determination by the
Internal Revenue Service, a portion of the Benefit sufficient such that none of
the Benefit retained by you constitutes a "parachute payment" within the meaning
of Code Section 280G that is subject to the Excise Tax (the "Repayment Amount").
The Repayment Amount shall be the smallest such amount, if any, as shall be
required to be paid to the Company so that your net proceeds with respect to the
Benefit (after taking into account the payment of the Excise Tax imposed on such
Benefit) shall be maximized. Notwithstanding the foregoing, the Repayment Amount
with respect to the Benefit shall be zero if a Repayment Amount of more than
zero would not eliminate the Excise Tax imposed on such Benefit. If the Excise
Tax is not eliminated pursuant to your compliance with this paragraph, you agree
to pay the Excise Tax.

This Agreement may be changed or terminated only upon the mutual written consent
of the Company and you. Nothing in this Agreement will prevent or limit your
continuing or future participation in any benefit, bonus, incentive or other
plans, programs, policies or practices provided by the Company and for which you
may otherwise qualify, nor will anything herein limit or otherwise affect such
rights as you may have under any stock option or other agreements with the
Company.

If either party hereto brings any action to enforce such party's rights
hereunder, the prevailing party in any such action will be entitled to recover
such party's reasonable attorneys' fees and costs incurred in connection with
such action.


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This Agreement constitutes the entire agreement between you and the Company
regarding the subject matter herein. It is entered into without reliance on any
promise or representation other than those expressly contained herein. All
questions concerning the construction, validity and interpretation of this
Agreement will be governed by the law of the State of California.

Very truly yours,


Deborah A. Schueren
President and CEO

DAS/dw


Agreed and Accepted:

/s/ Stephen F. Keane
-------------------------------
Stephen F. Keane

Dated:  October 8, 1999


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                                    EXHIBIT A
                          RELEASE AND WAIVER OF CLAIMS



        In exchange for consideration provided to me in the Agreement to which
this form is attached, I hereby furnish Epimmune Inc. (the "Company") with the
following release and waiver.

        I hereby release, and forever discharge the Company, its officers,
directors, agents, employees, stockholders, successors, assigns and affiliates,
of and from any and all claims, liabilities, demands, causes of action, costs,
expenses, attorneys fees, damages, indemnities and obligations of every kind and
nature, in law, equity, or otherwise, known and unknown, suspected and
unsuspected, disclosed and undisclosed, arising at any time prior to and
including my employment termination date with respect to any claims relating to
my employment and the termination of my employment, including but not limited
to, claims pursuant to any federal, state or local law relating to employment,
including, but not limited to, discrimination claims, claims under the
California Fair Employment and Housing Act, and the Federal Age Discrimination
in Employment Act of 1967, as amended (ADEA), or claims for wrongful
termination, breach of the covenant of good faith, contract claims, tort claims,
and wage or benefit claims, including but not limited to, claims for salary,
bonuses, commissions, stock, stock options, vacation pay, fringe benefits,
severance pay or any form of compensation (other than the obligations under the
Agreement.)

        I also acknowledge that I have read and understand Section 1542 of the
California Civil Code which reads as follows: "A general release does not extend
to claims which the creditor does not know or suspect to exist in his favor at
the time of executing the release, which if known by him must have materially
affected his settlement with the debtor." I hereby expressly waive and
relinquish all rights and benefits under that section and any law of any
jurisdiction of similar effect with respect to any claims I may have against the
Company.

        I acknowledge that, among other rights, I am waiving and releasing any
rights I may have under ADEA, that this waiver and release is knowing and
voluntary, and that the consideration given for this waiver and release is in
addition to anything of value to which I was already entitled as an employee of
the Company. I further acknowledge that I have been advised, as required by the
Older Workers Benefit Protection Act, that: (a) the waiver and release granted
herein does not relate to claims which may arise after this agreement is
executed; (b) I have the right to consult with an attorney prior to executing
this agreement (although I may choose voluntarily not to do so); (c) I have
twenty-one (21) days from the date I receive this agreement in which to consider
this agreement (although I may choose voluntarily to execute this agreement
earlier); (d) I have seven (7) days following the execution of this agreement to
revoke my consent to the agreement; and (e) this agreement shall not be
effective until the seven (7) day revocation period has expired.


Dated:                                  By:
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