December 10, 2007 Waverley Associates, Inc. P.O. Box 2088 Menlo Park, CA 94026 Attn: Isaac Stein Re: Extension of Consulting Agreement Dear Isaac:

Exhibit 10.18.1

December 10, 2007

Waverley Associates, Inc.
X.X. Xxx 0000
Xxxxx Xxxx, XX 00000
Attn: Xxxxx Xxxxx

     Re: Extension of Consulting Agreement

Dear Xxxxx:

     This will confirm the agreement of Maxygen, Inc. (“Maxygen”) and Waverley Associates, Inc. (“Waverley”) with respect to that certain Consulting Agreement entered effective April 1, 2006 (the “Agreement”), to extend the term of the Agreement one year such that it will terminate as of April 1, 2009, if not earlier terminated in accordance with its terms.

Yours sincerely,

/s/ Xxxxx Xxxxx

Xxxxx Xxxxx
Chairman of Maxygen
Compensation Committee

UNDERSTOOD AND AGREED
BY WAVERLEY ASSOCIATES, INC.

Signature: /s/ Xxxxx Xxxxx

Date: 12/19/07