October 24, 1999

Ms. Jean J. Lockhart
510 Torrey Point Road
Del Mar, California 92014

Dear Jean:

This letter is a formal offer setting forth the principal terms for you to
join Ambryx Inc., a Delaware corporation (the "Company"), which is located in
San Diego, California.

POSITION:     Executive Director, Finance & Administration. You will report
              to the President and Chief Executive Officer.

COMPENSATION: Monthly compensation will be $11,500 per month.

EQUITY:       You will be entitled to purchase at fair market value at the
              time of acceptance of employment 80,000 shares of the common
              stock of the Company (approximately 1.75% of the total common
              stock outstanding and reserved for founders). Your shares will
              vest according to the following schedules (i) twenty five
              percent (25%) of the total (20,000) will vest upon the first
              anniversary of the commencement of your employment with the
              Company; (ii) one forty-eighth (1/48) of the total
              (approximately 1,667 shares) will vest each month beginning at
              the end of the first year and will continue to vest over a
              three year period until the total 80,000 shares has vested. The
              Company has the right to repurchase any unvested shares in the
              event of termination, except in the event of involuntary
              termination as a result of a change of control.

BENEFITS:     You will be entitled to receive standard medical, life and
              dental insurance benefits similar to those typically afforded
              in similar-sized biotechnology companies. Such benefits will be
              established within six (6) months of starting employment or a
              provision of up to five hundred ($500.00) will be made.

<PAGE>

Ms. Jean J. Lockhart
October 22, 1999
Page 2


VACATION:     You will be entitled to 17 days per year of paid personal
              leave. If Ambryx establishes a company policy to close between
              Christmas and New Year's, such time off will not count against
              the 17 days.

EMPLOYMENT
AT WILL:      Your employment will be at will, which means it may be
              terminated at any time by you or the Company with or without
              cause.

START DATE:   December 1, 1999, or at another mutually agreeable date. This
              offer is subject to the approval of the Board of Directors of
              Ambryx Inc. and if not accepted by October 25, 1999, it will
              lapse.

We are excited about Ambryx's prospects for success, and anticipate that your
contribution will substantially enhance our progress. I look forward to your
joining the Company, and to working with you as we develop our business.

Sincerely,


/s/  Paul A. Grayson
     Paul A. Grayson
     President and Chief Executive Officer

SIGNED AND AGREED TO:

/s/  JEAN J. LOCKHART
------------------------
    (Jean J. Lockhart)

October 24, 1999

cc: Lubert Stryer

Source: OneCLE Business Contracts.