SIXTH AMENDMENT TO LEASE

Exhibit 10.10.6

SIXTH AMENDMENT TO LEASE

This Sixth Amendment to Lease (“Amendment”) is entered into, and dated for reference purposes, as of February 6, 2009 (the “Execution Date”) by and between METROPOLITAN LIFE INSURANCE COMPANY, a New York corporation (“Metropolitan”), as Landlord (“Landlord”), and MAXYGEN, INC., a Delaware corporation (“Maxygen”), as Tenant (“Tenant”), with reference to the following facts (“Recitals”):

A. Landlord and Tenant entered into that Lease (the “Original Lease”) dated as of October 21, 1998 for certain premises consisting of the entire building known as 515 Galveston Drive (the “Original Premises” or “515 Galveston Premises”) Redwood City, California, as amended by that First Amendment to Lease dated as of February 26, 1999 (the “First Amendment”) for the lease of Expansion Space A (which may also be referred to as the “220 Penobscot Premises”), that certain written Second Amendment To Lease dated October 24, 2000 (the “Second Amendment”) for the lease of Expansion Space B (which may also be referred to as the “200 Penobscot Premises”), that certain written Third Amendment To Lease dated October 22, 2003 (the “Third Amendment”) for the surrender of all of the 220 Penobscot Premises and part of the 200 Penobscot Premises, that certain written Fourth Amendment To Lease dated December, 2004 (the “Fourth Amendment”) for extension of the Term of the 515 Galveston Premises, and that certain written Fifth Amendment to Lease dated as of August 24, 2008 (“Fifth Amendment”), all as more particularly described in the Original Lease, First Amendment, Second Amendment, Third Amendment, Fourth Amendment and Fifth Amendment (collectively, the “Existing Lease”, and the Premises as of the Execution Date is collectively referred to as the “Premises”).

B. Landlord and Tenant desire to extend the current Term for the Premises in accordance with Tenant’s exercise of its Option To Extend, as set forth in Section 6 of the Fifth Amendment, which is also known as the 515 Galveston Premises and which consists of an agreed 31,166 square feet of Rentable Area, and provide for other amendments of the Existing Lease as more particularly set forth below.

NOW, THEREFORE, in consideration of the foregoing, and of the mutual covenants set forth herein and of other good and valuable consideration, the receipt and adequacy of which are hereby acknowledged, the parties hereto agree as follows:

Section 1 . Scope of Amendment; Defined Terms . Except as expressly provided in this Amendment, the Existing Lease shall remain in full force and effect. Should any inconsistency arise between this Amendment and the Existing Lease as to the specific matters which are the subject of this Amendment, the terms and conditions of this Amendment shall control. The term “Lease” as used herein and in the Existing Lease shall refer to the Existing Lease as modified by this Amendment, except as expressly provided in this Amendment. All capitalized terms used in this Amendment and not defined herein shall have the meanings set forth in the Existing Lease unless the context clearly requires otherwise.

Section 2 . Extension of Term for 515 Galveston Premises . Notwithstanding any provision of the Existing Lease to the contrary, Landlord and Tenant acknowledge and agree as follows:

(a) The current Term pursuant to the Existing Lease will expire on February 28, 2009.

(b) The Term is hereby extended for the period of twelve (12) months (the “Third Extended Term”) commencing on March 1, 2009 (the “Third Extension Commencement Date”) and expiring February 28, 2010 (hereafter, the “Expiration Date” in lieu of the date provided in the Existing Lease), unless sooner terminated pursuant to the terms of the Lease.

(c) Landlord and Tenant acknowledge and agree that this Amendment provides all rights and obligations of the parties with respect to extension of the current Term, whether or not in accordance with any other provisions, if any, of the Existing Lease regarding renewal or extension, and any such provisions, options or rights for renewal or extension provided in the Existing Lease are hereby deleted as of the Execution Date, and without

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limiting the generality of the foregoing, Tenant and Landlord acknowledge and agree that the Option to Extend set forth in Section 6 of the Fifth Amendment is hereby deleted as of the Execution Date.

Section 3 . Monthly Base Rent for Third Extended Term . Notwithstanding any provision of the Existing Lease to the contrary, commencing on the Third Extension Commencement Date and continuing through the Expiration Date of the Third Extended Term, the amount of Monthly Base Rent due and payable by Tenant for the Premises shall be as set forth in the table below: