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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

     The Base Rent per rentable square foot for the Expansion Space shall be at
     the annual rate of $25.00 ($33,108.33 per month) for months 1 through 84.
     The rent for the Existing Space shall remain as is directed in the Office
     Lease and earlier amendment for the full eighty-four (84) months.

     The "base year" for calculating Lessee's proportionate share of Comparison
     Year Costs (if any) with respect to the Expansion Space, shall be the
     calendar year 2006 and Lessee shall have no obligation to pay its
     proportionate share of Operating Services and Real Estate Taxes with
     respect to the Expansion Space until 13 months after the Expansion Space
     Effective Date. The provision of Section 10 of the Lease shall apply with
     respect to the Expansion Space provided however that references therein to
     2004 shall be deemed references to 2006 with respect to the Expansion
     Space. The base year for the Existing Space shall remain as is directed in
     the Office Lease and earlier amendment.

AS-IS, WHERE-IS

     Lessee acknowledges Lessee has fully inspected the Expansion Space and,
     subject to the delivery requirement set forth above, accepts the Expansion
     Space

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Mr. Hans van Houte
TRUBION PHARMACEUTICALS, INC.
February 1, 2006
Page 3


     in an as-is, where-is condition. Lessee shall not call upon Lessor to
     provide any modifications or improvements to the Expansion Space.

PARKING

     Effective as of the Expansion Space Effective Date, Lessee shall have the
     right to lease eleven (11) additional parking spaces inside the building
     garage at the market rate.

PERMITTED ALTERATIONS

     Lessee may construct tenant improvements in the Expansion Space generally
     consistent with the specifications and objectives described in Exhibit "B"
     attached hereto (the "Permitted Improvements") and consistent with the
     terms of Exhibit C to the Office Lease. Lessor acknowledges and agrees that
     Lessee intends to use the North Half of the tenth floor Expansion Space for
     scientific laboratory space and that Lessee shall have the right to install
     certain of the Permitted Improvements on the adjacent roof of the 9th floor
     of the building. The last sentence of Section 47 of the Lease shall apply
     to the Permitted Improvements, and the references therein to "Tenant
     Improvements" shall also include the Permitted Improvements. Lessor shall
     be responsible for the costs described in Section 4 of Exhibit C to the
     Lease to the extent such costs arise in connection with the construction of
     the Permitted Improvements.

RIGHT OF FIRST OFFER

     Subordinate to existing rights of first offer and first refusal, Lessee
     shall have a right of first offer on all space that may become available in
     the Building on the terms set forth in Section 46 of the Lease, and
     references therein to "Expansion Space" hereinafter shall refer to any
     space in the Building that becomes available.

MISCELLANEOUS

     This Amendment (together with the Lease) constitutes the entire agreement
     between Lessor and Lessee regarding the Expansion Space and the subject
     matter contained herein and supersedes any and all prior and/or
     contemporaneous oral or written negotiations, agreements or understandings.

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Mr. Hans van Houte
TRUBION PHARMACEUTICALS, INC.
February 1, 2006
Page 4


     This Amendment shall be binding upon and inure to the benefit of Lessor and
     Lessee and their respective heirs, legal representatives, successors and
     assigns. No subsequent change or addition to this Amendment shall be
     binding unless in writing and duly executed by both Lessor and Lessee.
     Except as specifically amended hereby, all of the terms and conditions of
     the Lease are and shall remain in full force and effect and are hereby
     ratified and confirmed.

Please consider this document, when fully executed, as our agreement for the
amendment of your Office Lease. If you are in agreement with the above, please
sign below where indicated and return all four copies to me for Martin Selig's
signature. Upon Full execution, I will return two copies for your own files.

Thank you Hans, for this lease of additional space. We appreciate your tenancy
with us and look forward to continuing to satisfy your office and lab space
needs.

Very truly yours,


/s/ Theresa Howard
-------------------------------------
Theresa Howard

TRUBION131.06

                              AGREED AND ACCEPTED:

SELIG REAL ESTATE HOLDINGS EIGHT,       TRUBION PHARMACEUTICALS, INC.
LLC


/s/ Martin Selig                        /s/ Hans van Houte
-------------------------------------   ----------------------------------------
By: Martin Selig                        By: Hans van Houte
Its: Managing Member                    Its: VP, Finance

Dated: February 10, 2006                Dated: February 6, 2006

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                                   EXHIBIT B

                            (LAYOUT OF OFFICE SPACE)

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                            (FLOOR PLAN 10th FLOOR)