EXHIBIT 4.3 AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT

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                                                                     EXHIBIT 4.3

                          TRUBION PHARMACEUTICALS, INC.

                               AMENDMENT NO. 1 TO

                 AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT

     This Amendment No. 1 to the Amended and Restated Investor Rights Agreement
(the "RIGHTS AGREEMENT") dated as of July 13, 2004 is entered into as of
________, 2005, by and among Trubion Pharmaceuticals, Inc., a Delaware
corporation (the "COMPANY"), Wyeth, a Delaware corporation ("WYETH"), and the
investors set forth on EXHIBIT A hereto (collectively the "INVESTORS" and each
individually an "INVESTOR").

                                    RECITALS

     A. The Company and the Investors are parties to the Rights Agreement.

     B. The Company and Wyeth have entered into a Common Stock Purchase
Agreement dated as of December ___, 2005 (the "PURCHASE AGREEMENT") pursuant to
which the Company will sell to Purchaser and Purchaser will purchase from the
Company shares of the Company's Common Stock concurrent with and conditioned
upon the closing of the Company's initial public offering (the "CLOSING"). A
condition to the Purchaser's obligations under the Purchase Agreement is that
the Rights Agreement be amended in order to provide Purchaser with certain
rights to register shares of the Company's Common Stock.

     C. Pursuant to Section 6.5 of the Rights Agreement, the written consent of
the Company and the Investors holding a majority of the Registrable Securities
(the "REQUISITE HOLDERS") is required to amend the Rights Agreement.

     D. The Company and the Requisite Holders desire to induce Purchaser to
enter into the Purchase Agreement by agreeing to the terms and conditions set
forth herein.

     NOW, THEREFORE, in consideration of the mutual promises, representations,
warranties, covenants and conditions set forth in this Agreement, the parties
hereto agree as follows:

     1. Definitions. Capitalized terms used herein without definition shall have
the meaning ascribed to them in the Rights Agreement.

     2. Addition of Purchaser as a Party to the Rights Agreement. Effective upon
the Closing pursuant to the Purchase Agreement, the parties hereby agree to add
Purchaser as a party to the Rights Agreement and Purchaser shall be deemed a
"Holder" of Registrable Securities for purposes of Sections 1, 2 and 6 of the
Rights Agreement and subject to all of the rights and obligations of such
Sections. For purposes of clarification, Purchaser shall not be entitled to the
rights or subject to the obligations set forth in Sections 3, 4 and 5 of the
Rights Agreement and Purchaser shall not be deemed an "INVESTOR" for purposes of
the Rights Agreement..

     3. Amendment to Section 1.1. The definition of "Registrable Securities" set
forth in Section 1.1 is hereby amended and restated to read in its entirety as
follows:

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          "REGISTRABLE SECURITIES" means (a) Common Stock of the Company issued
or issuable upon conversion of the Shares, (b) Common Stock of the Company
issued to Frazier Healthcare Fund ("FRAZIER"), ARCH Venture Fund ("Arch") and
Scott Minick ("MINICK") pursuant to those certain Common Stock Purchase
Agreements dated November 19, 2002 by and between the Company and each of
Frazier, Arch and Minick, (c) Common Stock of the Company issued to Wyeth
pursuant to the Purchase Agreement, and (d) any Common Stock of the Company
issued as (or issuable upon the conversion or exercise of any warrant, right or
other security which is issued as) a dividend or other distribution with respect
to, or in exchange for or in replacement of, the securities described in (a),
(b) and (c) above; provided, however, the shares referred to in clause (c) above
shall not qualify as Registrable Securities for the purposes of Sections 2.2
hereof until the 15 month anniversary of the Closing. For the avoidance of
doubt, in the event that the Company effects a registration under the Securities
Act pursuant to Section 2.2 hereof prior to the 15 month anniversary of the
Closing, in connection with such registration the Shares referred to in clause
(c) above shall qualify as Registrable Securities for the purposes of Section
2.3. Notwithstanding the foregoing, Registrable Securities shall not include any
securities sold by a person to the public pursuant to a registration statement
or Rule 144 or sold in a private transaction in which the transferor's rights
under SECTION 2 of this Agreement are not assigned.

     4. Amendment to Section 2.1(a)(ii). Section 2.1(a)(ii) is amended effective
immediately following the expiration of the "Market Stand-Off" period set forth
in Section 2.12 hereof, by deleting the last sentence thereof and substituting
therefor the following:

     "Subject to the other terms of this Agreement (including without limitation
     the restrictions on assignment of registration rights set forth in Section
     2.10 and Sections 2.1(b) and (d)), it is agreed that the restrictions
     contained in this Section 2.1(a)(ii) shall not apply to dispositions of
     Shares or Registrable Securities made pursuant to Rule 144 promulgated
     under the Securities Act."

     5. Amendment to Section 6.5. Section 6.5 is hereby amended by adding, after
the final sentence thereof, the following:

          Notwithstanding the foregoing, neither this Agreement nor any term
hereof may be amended, waived, discharged or terminated in any way that
diminishes or eliminates the rights particular to Wyeth hereunder and in a
manner different than the other holders of Registrable Securities, such action
shall require the prior written consent of Wyeth.

     6. Waiver of Right of Participation. Each Investor on behalf of itself and
all other Investors and holders of Registrable Securities hereby waives any
right of participation set forth in Section IV of the Rights Agreement with
respect to the sale and issuance of the shares of Company Common Stock to Wyeth
pursuant to the Purchase Agreement.

     7. No Other Amendments. Except as expressly amended or waived as set forth
above, the Rights Agreement shall remain in full force and effect in accordance
with its terms.

     8. Counterparts. This Amendment may be executed in any number of
counterparts, eac