Exhibit 4.4

                              SECOND AMENDMENT TO
                         REGISTRATION RIGHTS AGREEMENT
                                  AND CONSENT

     THIS SECOND AMENDMENT TO REGISTRATION RIGHTS AGREEMENT AND CONSENT is made
and entered into as of the 23rd day of December, 1998, by and among Maxygen,
Inc., a Delaware corporation (the "Company"), and the undersigned holders (the
"Investors") of the Company's Series A Preferred Stock, $0.0001 par value per
share (the "Series A Preferred"), Series B Preferred Stock, $0.0001 par value
per share (the "Series B Preferred") and Series C Preferred Stock, $0.0001 par
value per share (the "Series C Preferred").

                                    RECITALS

     The Company and certain of the Investors or their predecessors in interest
(the "Series A and Series B Investors") have entered into that certain
Registration Rights Agreement dated the 14th day of March, 1997, as amended the
31st day of July, 1997, attached hereto as Exhibit A (the "Rights Agreement"),
that provides that such Investors are to receive certain registration rights
with respect to Maxygen Common Stock ("Common Stock") and the Series A Preferred
and the Series B Preferred.

     Pioneer Overseas Corporation, an Iowa corporation (the "Series C Investor")
has agreed to purchase shares of the Series C Preferred Stock pursuant to that
certain Series C Stock Purchase Agreement between the Company and the Series C
Investor dated as of December 23, 1998.

     The undersigned Series A and Series B Investors (the "Consenting
Investors") constitute the holders of a majority of the outstanding Registrable
Securities for the purposes of Sections 13 and 17 of the Rights Agreement.

     The Company and the Consenting Investors wish to amend the Rights Agreement
to add the Series C Preferred as Registrable Securities.

     The Consenting Investors wish to consent to the granting of registration
rights to the Series C Investors.

     The Series C Investor wishes to become a party to the Rights Agreement, as
amended hereby.

     NOW, THEREFORE, in consideration of the promises, mutual covenants and
conditions herein contained, and of other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereby
agree as follows:


     1.  Pursuant to Section 17 of the Rights Agreement, the definition of
"Registrable Securities" contained in Section 1(d) of the Rights Agreement is
hereby amended to read in its entirety as follows:

         (d) "Registrable Securities" shall mean (i) shares of Common Stock
         issued or issuable upon conversion of the Series A Preferred, the
         Series B Preferred or the Series C Preferred, (ii) shares of Common
         Stock held of record by any of the Investors, (iii) shares of Common
         Stock issued or issuable upon conversion of any other series of
         Preferred Stock of the Company as shall be agreed to in writing by a
         majority of the then outstanding Registrable Securities, and (iv)
         shares of Common Stock issued as a dividend or distribution with
         respect to, or in exchange or in replacement of, the foregoing;

     2.  By executing this Agreement on the Series C Investor Signature Page,
the Series C Investor becomes a party to the Rights Agreement, as amended, as
though the Series C Investor had executed the Rights Agreement as of the date
thereof.

     3.  Pursuant to Section 13 of the Rights Agreement, the Consenting
Investors consent to the Company entering into the Rights Agreement with the
Series C Investor.

     This Agreement may be signed in any number of counterparts and by the
parties hereto in separate counterparts, each of which when so executed shall be
deemed to be an original and all of which taken together shall constitute one
and the same agreement.

                                       2


     IN WITNESS WHEREOF, the parties have executed this Second Amendment and
Consent as of the date first above written.

MAXYGEN, INC.
3410 Central Expressway
Santa Clara, California 95051


/s/ Russell Howard
- ------------------
Signature


Russell Howard
- --------------
Name (please print)

President & CEO
- ---------------
Title

                                       3


SERIES A INVESTORS

AFFYMAX TECHNOLOGIES N.V.
Glaxo Wellcome House
Berkeley Avenue
Greenford, Middlesex,
United Kingdom UB6 0NN
Attn:
     -----------------
Fax:
    ------------------


/s/ Adrian Hennah
- -----------------
Signature

Adrian Hennah
- -------------
Name (please print)

Director
- --------
Title


TECHNOGEN ASSOCIATES, L.P.
525 University Avenue, Suite 700
Palo Alto, California  94301

By:   TECHNOGEN MANAGERS, L.L.C.,
      its general partner


/s/ Isaac Stein
- ---------------
Signature

Isaac Stein
- -----------
Name (please print)

Manager
- -------
Title

                                       4


SERIES B INVESTORS

TECHNOGEN ASSOCIATES, L.P.
525 University Avenue, Suite 700
Palo Alto, California  94301

By:   TECHNOGEN MANAGERS, L.L.C.,
      its general partner


/s/ Isaac Stein
- ---------------
Signature

Isaac Stein
- -----------
Name (please print)

Manager
- -------
Title


TECHNOGEN ENTERPRISES, LLC
525 University Avenue, Suite 700
Palo Alto, California  94301


/s/ Isaac Stein
- ---------------
Signature

Isaac Stein
- -----------
Name (please print)

Member
- ------
Title

                                       5


SERIES B INVESTORS continued


- ----------------------
Entity (if applicable)


/s/
- ---
Signature

- -------------------
Name (please print)

- ---------------------
Title (if applicable)

                                       6


                               SERIES C INVESTOR
                               SIGNATURE PAGE TO
                    REGISTRATION RIGHTS AGREEMENT CONTINUED


SERIES C INVESTOR

PIONEER OVERSEAS CORPORATION
800 Capital Square
400 Locust Street
Des Moines, Iowa 50309
Attn:
     ------------------
Fax:
    -------------------


/s/ Bill DeMeulenaere
- ---------------------
Signature

Bill DeMeulenaere
- -----------------
Name (please print)

Assistant Secretary
- -------------------
Title

                                       7