October 5, 2006


Trubion Pharmaceuticals, Inc.
2401 4th Avenue
Suite 1050
Seattle, Washington 98121

         RE:      REGISTRATION STATEMENT ON FORM S-1

Ladies and Gentlemen:

         We are acting as counsel to Trubion Pharmaceuticals, Inc., a Delaware
corporation (the "Company"), in connection with the registration of 4,600,000
shares of the Company's Common Stock, par value $0.001 per share (the "Shares"),
pursuant to a Registration Statement on Form S-1 (Registration No. 333-134709),
as amended (the "Registration Statement"), filed with the Securities and
Exchange Commission pursuant to the Securities Act of 1933, as amended.

         As counsel for the Company, we have examined originals or copies,
certified or otherwise identified to our satisfaction, of such documents,
corporate records, certificates of public officials and other instruments as we
have deemed necessary for the purposes of rendering this opinion. In our
examination, we have assumed the genuineness of all signatures, the authenticity
of all documents submitted to us as originals and the conformity with the
originals of all documents submitted to us as copies.

         Based upon the foregoing, we are of the opinion that the Shares have
been duly authorized by the Company, and when issued, delivered and paid for in
accordance with the terms of the underwriting agreement referred to in the
Registration Statement and in accordance with the resolutions adopted by the
Board of Directors of the Company, will be, validly issued, fully paid and
nonassessable.

         We consent to the use of this opinion as an exhibit to the Registration
Statement, and we consent to the reference of our name under the caption "Legal
Matters" in the Prospectus forming a part of the Registration Statement.

                                          Very truly yours,

                                          /s/ WILSON SONSINI GOODRICH & ROSATI
                                          Professional Corporation