Battery Office Lease

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                                                                   EXHIBIT 10.10

February 1, 2006

Mr. Hans van Houte
TRUBION PHARMACEUTICALS, INC.
2401 Fourth Avenue, Suite 1050
Seattle, Washington 98121

Dear Hans:

Please refer to the Fourth and Battery Office Lease dated April 28, 2003 and all
addenda and amendments thereto (the "Office Lease" or "Lease") for the space
Trubion Pharmaceuticals (hereinafter referred to as "Trubion" or "Lessee")
occupies within the Fourth and Battery building. This letter (this "Amendment")
shall constitute an amendment to the Lease. Capitalized terms used but not
otherwise defined herein shall have the meanings ascribed to such terms in the
Lease.

PREMISES

     Effective as of the Expansion Space Effective Date (defined below), Trubion
     Pharmaceuticals, Inc. agrees to lease the additional space (the "Expansion
     Space") of approximately 15,892 rentable square feet which is the entire
     11th floor of the Fourth and Battery Building, (location attached as
     Exhibit "A") so that, from and after the Expansion Space Effective Date,
     the term "Premises" as defined in Paragraph 1 of your Lease (Description),
     shall be changed from your existing space (10th and 12th floor Premises) of
     31,507 rentable square feet ("Existing Space") to 47,399 rentable square
     feet. The floor plans attached as Exhibit A to the Lease will be
     supplemented by the floor plan of the Expansion Space attached hereto as
     Exhibit A. Lessor shall deliver the Expansion Space to Lessee on the
     Expansion Space Effective Date in the condition described in Section 2 of
     the Lease and the parties agreed that the rights and obligations of Lessee
     and Lessor set forth in Section 2 of the Lease shall apply with respect to
     the Expansion Space.

LEASE TERM

     Lessor shall cause the lease of the current occupant of the Expansion Space
     to terminate as to the Expansion Space, and shall relocate such occupant
     from the Expansion Space, on or prior to May 1, 2006. The term of the Lease
     with

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Mr. Hans van Houte
TRUBION PHARMACEUTICALS, INC.
February 1, 2006
Page 2


     respect to the Expansion Space shall be eighty-four (84) months commencing
     the later of May 1, 2006 and the date Lessor has complied with its
     obligations in the first sentence of this paragraph and delivers to Lessee
     possession of the Expansion Space in the required condition (the "Expansion
     Space Effective Date") and expiring April 30, 2013. The term of the Lease
     for the Existing Space shall be extended so it is coterminous. Lessee and
     Lessee's agents and employees may access the Expansion Space from time to
     time prior to the Expansion Space Effective Date to facilitate Lessee's
     space planning and physical due diligence. If the Expansion Space Effective
     Date is delayed for any reason beyond June 1, 2006, then in addition to
     Lessee's other rights or remedies, Lessee may terminate this Amendment by
     written notice to Lessor, or, at Lessee's election, the date Lessee is
     otherwise obliged to commence payment of rent for the Expansion Space shall
     be delayed by one additional calendar day for each calendar day that the
     Expansion Space Effective Date is delayed beyond such date.

RENT

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