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                                                                    EXHIBIT 10.9

December 8, 2004

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

Dear Hans:

Please refer to your lease dated April 28, 2003 and all subsequent addenda
thereto (the "Lease") for the space you occupy within the Fourth and Battery
Building. This letter (this "Amendment") shall constitute an amendment to that
Lease. Capitalized terms used but not otherwise defined herein shall have the
meanings ascribed to such terms in the Lease.

The name of the Lessee in the above referenced Lease is hereby changed from
Genecraft, Inc. to Trubion Pharmaceuticals, Inc.

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 7,417 rentable square feet on the 10th 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 of 24,090 rentable square feet to 31,507
     rentable square feet, the reference in such Paragraph 1 to the square
     footage of the Premises located on the tenth (10th) floor of the Building
     shall be changed from 8,198 rentable square feet to 15,615 rentable square
     feet and the floor plan 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.

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Mr. Hans van Houte
TRUBION PHARMACEUTICALS, INC.
December 8, 2004
Page 2


LEASE TERM

     The term of the Lease with respect to the Expansion Space shall be
     sixty-nine (69) months commencing January 1, 2005 (the "Expansion Space
     Effective Date") and expiring September 30, 2010. 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.

RENT

     The Base Rent per rentable square foot for the Expansion Space shall be at
     the annual rate of $18.00 ($11,125 per month) for months 1-24 (the
     Effective Date through December 31, 2006) and $19.00 ($11,743.58 per month)
     for months 25-69 (January 1, 2007 through September 30, 2010).

     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 2005 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 January 1, 2006. The provisions of
     Section 19 of the Lease shall apply with respect to the Expansion Space,
     provided, however, that references therein to 2004 shall be deemed
     references to 2005 with respect to the Expansion Space.

TENANT IMPROVEMENT ALLOWANCE

     Lessee shall be provided an allowance of $50,000 toward design and tenant
     improvements for Expansion Space. This amount shall be given to Lessee as a
     rent credit to be offset against rents due for the Expansion Space from and
     after the Expansion Space Effective Date. Lessee may apply this allowance
     to the balance of any previous tenant improvement work that has been
     mutually agreed upon by Lessee and Lessor.

PARKING

     Effective as of the Expansion Space Effective Date, Lessee shall have the
     right to lease five (5) additional parking spaces outside of the parking
     garage at the market rate.

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Mr. Hans van Houte
TRUBION PHARMACEUTICALS, INC.
December 8, 2004
Page 3


REAL ESTATE COMMISSION

     Lessor agrees to pay a real estate commission equivalent to $5.00 per
     rentable square foot to Colliers International, one-half (1/2) upon lease
     execution and the balance payable upon lease commencement. If not paid by
     Lessor within sixty (60) days of being due, then the commission amount may
     be paid directly to Colliers International by Lessee and the amount so paid
     given to Lessee as a rent credit.

PERMITTED ALTERATIONS

     Lessee may construct tenant improvements in the Expansion Space generally
     consistent with the specifications and objectives described on Exhibit B
     attached hereto (the "Permitted Improvements") and in accordance with the
     terms of Section 2 of Exhibit C to the lease. Provided however, the
     location of Lessee's rooftop air conditioning unit shall be set back a
     minimum of five (5) feet from the inside wall of the parapet. In addition,
     the placement of the air conditioning unit, supporting structure and duct
     work, shall be with Lessor's prior approval of Lessee's drawings. 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.

     Paragraph 8 of your Lease, ALTERATIONS, is amended to provide that any
     equipment, supports, ductwork or fencing installed on the rooftop area of
     the 10th floor shall be removed by Lessee upon the expiration of the lease
     term at the request of Lessor. Lessee shall repair any areas of the rooftop
     damaged due to the installation or removal of such equipment and
     repair/restore any penetrations into the roof or into the building.

SIGNAGE

     Lessor shall install, at Lessor's cost and expense, signage with respect to
     the Expansion Space consistent with Section 11 of the Lease.

MISCELLANEOUS

     This Amendment (together with the Lease) constitutes the entire agreement
     between Lessor and Lessee regarding the Expansion Space and the subject

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Mr. Hans van Houte
TRUBION PHARMACEUTICALS, INC.
December 8, 2004
Page 4


     matter contained herein and supersedes any and all prior and/or
     contemporaneous oral or written negotiations, agreements or understandings.
     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 Lease. If you are in agreement with the above, please sign
below where indicated and return all three copies to me for Martin Selig's
signature. Upon full execution, I shall return one copy to you for your 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/ Mike Brixner
-------------------------------------
Mike Brixner

TRUBION1117.04

                              AGREED AND ACCEPTED:

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


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

Dated: January 7, 2005                  Dated: December 23, 2004

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                                   EXHIBIT B

                            (LAYOUT OF OFFICE SPACE)

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               (TRUBION 10TH FLOOR EVALUATION OPTION 1 10/29/04)