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                                                                   EXHIBIT 10.12

                          TRUBION PHARMACEUTICALS, INC.

                         COMMON STOCK PURCHASE AGREEMENT

      This Common Stock Purchase Agreement (this "Agreement") is made as of
December 19, 2005 by and between Trubion Pharmaceuticals, Inc., a Delaware
corporation (the "Company"), and Wyeth, a Delaware corporation (the
"Purchaser").

                                    RECITALS

      A. The Purchaser and the Company are entering into a collaboration
agreement of even date herewith (the "Collaboration Agreement");

      B. In connection with the Collaboration Agreement, Purchaser desires to
purchase from the Company shares of its Common Stock (the "Common Stock"),
concurrently with and conditioned upon the closing of the Company's initial
public offering, upon the terms and conditions set forth herein;

      C. The Company and the Purchaser wish to set forth the terms and
conditions upon which the Company will sell the Common Stock to the Purchaser;
and

      D. Concurrent with the execution of this Agreement, the Company and
Purchaser are entering into an amendment (the "Rights Agreement Amendment") to
the Company's Amended and Restated Investor Rights Agreement (the "Rights
Agreement") to provide Purchaser with certain rights and obligations thereunder
upon the issuance of the Common Stock hereunder.

      NOW, THEREFORE, in consideration of the premises and mutual covenants and
conditions contained herein, the Company and the Purchaser hereby agree as
follows:

                                    ARTICLE I

                           PURCHASE AND SALE OF SHARES

      1.1 Purchase Price and Closing. Subject solely to the conditions set forth
in Sections 1.2 - 1.5 and Article IV hereof, the Company will issue and sell to
the Purchaser and, subject to the terms and conditions set forth in this
Agreement, the Purchaser will purchase from the Company (the "Sale"), that
number of shares of Common Stock (the "Shares") equal to the quotient obtained
by dividing Twenty-Five Million Dollars ($25,000,000) (the "Investment Amount")
by the per-share price to the public (the "IPO Price") of shares of Common Stock
in the Company's first underwritten, firm commitment public offering (the "IPO")
pursuant to an effective registration statement (the "Registration Statement")
under the Securities Act of 1933, as amended (the "Securities Act"). The per
share price to Purchaser shall be the IPO Price. The purchase and sale will take
place at a closing (the "Closing") to be held on the date, at the location and
simultaneously with the closing of the IPO, subject to the satisfaction of all
of the conditions to the Closing specified in Article IV herein. At the Closing
the Company will issue and deliver a certificate evidencing the Shares to the
Purchaser against payment of the full purchase price therefor by wire transfer
of immediately available funds to an account designated by the Company.

      1.2 Maximum Share Number. Notwithstanding Section 1.1 above, in the event
the number of Shares would otherwise constitute more than (i) nineteen and
nine-tenths percent (19.9%) of the Actual Voting Power (as defined in Section
5.1(i)) or (ii) twenty percent (20%) of the number of shares issued in the IPO
(including any shares covered by a related registration statement filed pursuant
to Rule 462(b) of

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the Securities Act but excluding any shares issued or to be issued in an
overallotment option), then in either case (i) or (ii) above the Investment
Amount (and correspondingly the number of shares purchased by the Purchaser)
shall be reduced by the minimum dollar amount and share amount necessary to
avoid either such event.

      1.3 Restrictions on Transfer. Pursuant to the Rights Agreement Amendment,
Purchaser agrees and acknowledges that the restrictions set forth in Sections
2.1 and 2.12 of the Rights Agreement shall apply to Purchaser and the Shares.

      1.4 HSR Act. Prior to the execution of the Collaboration Agreement and
this Agreement, the parties made certain filings under the Hart-Scott Rodino
Antitrust Improvements Act of 1976, as amended (the "HSR Act"). If either party
concludes in good faith that additional filings or proceedings are necessary or
desirable as a result of the transactions contemplated hereby either as a result
of the signing of this Agreement or in connection with the Closing or otherwise,
the parties agree to promptly file such additional notices, applications and
documents that may be required under the HSR Act, or any other required foreign
or domestic competition law (collectively, the "Competition Laws") and all
applicable additional filings fees associated therewith shall be paid by the
party required to so pay such additional filing fees under the applicable
Competition Law(s). In connection therewith, the Company and Purchaser each
shall use their commercially reasonable efforts to take such actions as may be
required to cause the expiration or early termination of the notice periods
under the Competition Laws as promptly as possible and to resolve such
objections, if any, as may be asserted with respect to the transactions
contemplated by this Agreement under the Competition Laws; provided, however,
that notwithstanding the foregoing, neither party shall agree to any change or
amendment to this Agreement unless such change or amendment is agreed by the
other party in advance. Nothing in this Agreement shall require either party or
any subsidiary or affiliate of either party to sell, hold separate, license or
otherwise dispose of any assets or conduct its business in a specified manner,
or agree or proffer to sell, hold separate, license or otherwise dispose of any
assets or conduct its business in a specified manner, or permit or agree to the
sale, holding separate, licensing or other disposition of any assets of either
party or any subsidiary or affiliate of either party, whether as a condition to
obtaining any approval from, or to avoid potential litigation or administrative
action by, a governmental entity or any other person or for any other reason.

      1.5 Termination of Purchase Right and Obligation. Notwithstanding any
provision of this Agreement to the contrary, Purchaser's right and obligation to
purchase, and the Company's right and obligation to sell, the Shares shall
terminate if the closing of the IPO has not occurred prior to the earliest to
occur of the following:

            (a) The termination of the Collaboration Agreement; or

            (b) The Company (1) undergoes a Change of Control (as defined in
Section 5.1(iv));; provided, however, the following shall be deemed to not be a
Change of Control for purposes of this Section 1.5(b): (i) a transaction
effected exclusively for the purpose of changing the domicile of the Company, or
(ii) an equity financing in which the Company is the surviving corporation, or
(2) engages in a merger, consolidation, reorganization or similar transaction in
which the surviving entity has a class of equity securities registered under
Section 12 of the Exchange Act (as defined below).

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                                   ARTICLE II

                  REPRESENTATIONS AND WARRANTIES OF THE COMPANY

      The Company hereby represents and warrants to the Purchaser as follows:

      2.1 Corporate Action. The Company has all necessary corporate power and
has taken all corporate action required to enter into and perform this Agreement
and the Rights Agreement Amendment (collectively, the "Financing Documents").
The Financing Documents have been duly executed and delivered, and constitute
valid, legal, binding and enforceable obligations of the Company, enforceable in
accordance with their terms, except (i) as limited by applicable bankruptcy,
insolvency, reorganization, moratorium and other laws of general application
affecting enforcement of creditors' rights generally and (ii) as limited by laws
relating to the availability of specific performance, injunctive relief or other
equitable remedies. The issuance, sale and delivery of the Shares in accordance
with this Agreement have been duly authorized by all necessary corporate action
on the part of the Company. The issuance of the Shares is not subject to
preemptive rights or other preferential rights in any present stockholders of
the Company that have not been waived and will not conflict with any provision
of any agreement or instrument to which the Company is a party or by which it or
its property is bound and to which the Company has not obtained appropriate
waivers.

      2.2 No Conflict. The execution and delivery of this Agreement by the
Company does not, and the consummation of the transactions contemplated hereby
will not, conflict with, or result in any violation of, or default under (with
or without notice or lapse of time, or both), or give rise to a right of
termination, cancellation, modification or acceleration of any obligation under
(i) any provision of the Certificate of Incorporation of the Company or Bylaws
of the Company, (ii) any material mortgage, indenture, lease, contract or other
agreement or instrument, permit, concession, or license to which the Company or
any of its properties or assets is subject or (iii) any judgment, order, decree,
applicable to the Company or its properties or assets. To the Company's
knowledge as of the date hereof, no provision of any applicable law, rule or
regulation and no judgment, order, decree or injunction applicable to the
Company or its properties or assets shall prohibit the consummation of the
Closing nor shall the Closing result in any violation of any such law, rule,
regulation, judgment, order, decree or injunction.

      2.3 Status of Shares. The Shares, when issued and delivered in accordance
with the terms hereof and after payment of the purchase price therefor, will be
duly authorized, validly issued, fully-paid and non-assessable, issued in
compliance with applicable state and federal securities laws (subject, in part,
to the representations and warranties of Purchase in Article III hereof) and
free of restrictions on transfer other than restrictions on transfer under the
Financing Documents and applicable state and federal securities laws.

      2.4 Organization, Good Standing and Qualification. The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of Delaware and has all requisite corporate power and authority to
carry on its business.

      2.5. Collaboration Agreement. The Collaboration Agreement has been duly
authorized, executed, and delivered by the Company and constitutes a valid and
binding obligation of the Company, enforceable in accordance with its terms,
except (i) as limited by applicable bankruptcy, insolvency, reorganization,
moratorium and other laws of general application affecting enforcement of
creditors' rights generally and (ii) as limited by laws relating to the
availability of specific performance, injunctive relief or other equitable
remedies.

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      2.6 Final Prospectus and Registration Statement. The Company,
acknowledging that the Purchaser will be relying on the accuracy and
completeness of the Company's disclosure in connection with the IPO, warrants to
the Purchaser that the Prospectus (as defined below) used in connection with the
Company's IPO will comply, at the time of filing or use, with the requirements
of the Securities Act, and the Prospectus filed or used in connection with the
IPO will not, at such time, contain an untrue statement of a material fact or
omit to state a material fact required to be stated therein or necessary to make
the statements therein, in light of the circumstances under which they were
made, not misleading; the Registration Statement, when it becomes effective,
will comply, in all material respects, with the requirements of the Securities
Act; and the Registration Statement will not, when it becomes effective, contain
an untrue statement of a material fact or omit to state a material fact required
to be stated therein or necessary to make the statements therein; provided,
however, that the Company makes no warranty with respect to any statement
contained in the Registration Statement or a prospectus in reliance upon and in
conformity with information concerning the Purchaser that is furnished by the
Purchaser expressly for use therein. "Prospectus" means the final prospectus (as
such term is defined in Section 2(a)(10) of the Securities Act) as first filed
with the SEC pursuant to paragraph (1) or (4) of Rule 424(b) of the Securities
Act.


                                   ARTICLE III

            REPRESENTATIONS AND WARRANTIES AND COVENANTS BY PURCHASER

      The Purchaser represents and warrants and covenants to the Company that:

      3.1 Purchaser is an "accredited investor" as defined in Rule 501(a) under
the Securities Act of 1933, as amended.

      3.2 Purchaser will acquire the Shares for its own account, for the purpose
of investment and not with a view to distribution or resale thereof.

      3.3 Purchaser has all necessary corporate power and has taken all
corporate action required to enter into and perform the Financing Documents. The
Financing Documents have been duly executed and delivered, and constitute valid,
legal, binding and enforceable obligations of Purchaser, enforceable in
accordance with their terms, except (i) as limited by applicable bankruptcy,
insolvency, reorganization, moratorium and other laws of general application
affecting enforcement of creditors' rights generally and (ii) as limited by laws
relating to the availability of specific performance, injunctive relief or other
equitable remedies.

      3.4 Purchaser has taken no action which would give rise to any claim
against the Company by any other person for any brokerage commissions, finders'
fees or the like relating to this Agreement or the transactions contemplated
hereby.

      3.5 Purchaser has had the opportunity to ask questions of and receive
answers from representatives of the Company concerning the terms of the offering
of the Shares and to obtain additional information concerning the Company and
its business.

      3.6 The acquisition by the Purchaser of the Shares shall constitute a
confirmation of these representations and warranties made by the Purchaser as of
the Closing. Purchaser understands that the Shares are "restricted securities"
under the Securities Act and have not been registered under the Securities Act
in reliance upon an exemption for non-public offerings. The Purchaser further
represents that it understands and agrees that, until registered under the
Securities Act or transferred pursuant to the provisions of Rule 144 as
promulgated by the Commission, all certificates evidencing any of the Shares,

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whether upon initial issuance or upon any transfer thereof, shall be subject to
the transfer restrictions and bear the legends set forth in Section 2.1 of the
Rights Agreement.

      3.7 To the Purchaser's knowledge as of the date hereof, no provision of
any applicable law, rule or regulation and no judgment, order, decree or
injunction applicable to the Purchaser or its properties or assets shall
prohibit the consummation of the Closing nor shall the Closing result in any
violation of any such law, rule, regulation, judgment, order, decree or
injunction.

                                   ARTICLE IV

                              CONDITIONS TO CLOSING

      4.1 Conditions of the Purchaser's Obligation. The obligation of the
Purchaser to purchase and pay for the Shares at the Closing is subject to the
satisfaction of the following conditions:

            (a) Documentation at Closing. The Purchaser shall have received
prior to or at the Closing all of the following documents or instruments, or
evidence of completion thereof, each in form and substance satisfactory to the
Purchaser:

                  (i) A copy of the Certificate of Incorporation of the Company,
certified by the Secretary of State of the State of Delaware, a copy of the
resolutions of the Board of Directors of the Company evidencing the approval of
this Agreement, the issuance of the Shares and the other matters contemplated
hereby, and a copy of the Bylaws of the Company, all of which shall have been
certified by the Secretary of the Company to be true, complete and correct in
every particular, and certified copies of all documents evidencing other
necessary corporate or other action and governmental approvals, if any, with
respect to this Agreement and the Shares.

                  (ii) A customary opinion of counsel to the Company covering
the matters set forth in Exhibit A hereto.

                  (iii) A certificate of the Secretary of the Company which
shall certify the names of the officers of the Company authorized to sign this
Agreement, the certificate for the Shares and the other documents, instruments
or certificates to be delivered pursuant to this Agreement by the Company or any
of its officers, together with the true signatures of such officers.

                  (iv) A certificate of the President of the Company stating (A)
that the representations and warranties made by the Company in this Agreement
are true and correct in all material respects at the date hereof and as of the
Closing with the same force and effect as though all such representations and
warranties had been made as of the Closing, and (B) that all covenants and
conditions required to be performed prior to or at the Closing have been
performed as of the Closing.

                  (v) A Certificate of Good Standing for the Company from the
Secretary of State of the State of Delaware, dated as of a recent date.

            (b) Performance. The Company shall have performed and complied with
in all material respects all agreements, obligations and conditions contained in
this Agreement that are required to be performed or complied with by it on or
before the Closing.

            (c) Consents, Waivers, Etc. The Company shall have obtained all
consents or waivers, if any, necessary to execute and deliver this Agreement,
issue the Shares and to carry out the transactions contemplated hereby and
thereby and the waiting period applicable to this Agreement and the



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Collaboration Agreement under the HSR Act (or any other applicable Competition
Laws) shall have expired or terminated early. All corporate and other action and
governmental filings necessary to effect the terms of this Agreement, the
issuance of the Shares and other agreements and instruments executed and
delivered by the Company in connection herewith shall have been made or taken,
except for any post-sale filing that may be required under federal or state
securities laws.

            (d) Rights Agreement Amendment. The Rights Agreement Amendment shall
have been executed by the Company and by the holders of the requisite majority
of Registrable Securities (as such term is defined in the Rights Agreement);
provided, however, the parties acknowledge that subsequent to the date hereof
the Rights Agreement may be further amended in accordance with its terms;
provided, further, however, Purchaser shall be required to consent to such
amendment or be provided substantially equivalent rights in such amendment or
another written agreement with the Company.

            (e) Collaboration Agreement. The Collaboration Agreement shall have
been duly authorized, executed, and delivered by the Company and constitute a
valid and binding obligation of the Company, enforceable in accordance with its
terms, except (i) as limited by applicable bankruptcy, insolvency,
reorganization, moratorium and other laws of general application affecting
enforcement of creditors' rights generally and (ii) as limited by laws relating
to the availability of specific performance, injunctive relief or other
equitable remedies. The Purchaser shall not have the right to terminate the
Collaboration Agreement for cause pursuant to Section 9.5 thereof (provided,
however, if Purchaser's right to so terminate the Collaboration Agreement for
cause is solely dependent on the lapsing on any applicable "cure" period
pursuant to Section 9.5 thereof, solely for purposes of this Section 4.1(e),
Purchaser shall be deemed to have the right to terminate the Collaboration
Agreement for cause notwithstanding the failure of any such cure period to have
lapsed); and the Company shall not have given notice to the Purchaser of its
intent to terminate the Collaboration Agreement.

            (f) Representations and Warranties. The representations and
warranties made by the Company in this Agreement shall have been true and
correct in all material respects at the date hereof and as of the Closing with
the same force and effect as though all such representations and warranties had
been made as of the Closing.

            (g) No Injunctions. No provision of any applicable law, rule or
regulation and no judgment, order, decree or injunction shall prohibit the
consummation of the Closing.

            (h) Listing. The shares of Common Stock sold in the IPO shall be
listed on the New York Stock Exchange ("NYSE") or traded on the Nasdaq National
Market.

            (i) Closing of IPO. The Closing hereunder shall be concurrent with
the closing of the IPO.

      4.2 Conditions of the Company's Obligation. The obligation of the Company
to sell the Shares at the Closing is subject to the satisfaction of the
following conditions:

            (a) Performance. The Purchaser shall have performed and complied
with in all material respects all agreements, obligations and conditions
contained in this Agreement that are required to be performed or complied with
by it on or before the Closing.

            (b) Consents, Waivers, Etc. Any waiting period applicable to this
Agreement and the Collaboration Agreement under the HSR Act (or any other
applicable Competition Laws) shall have expired or terminated early.

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            (c) Rights Agreement Amendment. The Rights Agreement Amendment shall
have been executed by the Purchaser.

            (d) Collaboration Agreement. The Collaboration Agreement shall have
been duly authorized, executed and delivered by the Purchaser and constitute a
valid and binding obligation of the Purchaser, enforceable in accordance with
its terms, subject to laws of general application relating to bankruptcy,
insolvency and the relief of debtors and rules of law governing specific
performance, injunctive relief or other equitable remedies. The Company shall
not have the right to terminate the Collaboration Agreement for cause pursuant
to Section 9.5 thereof (provided, however, if the Company's right to so
terminate the Collaboration Agreement for cause is solely dependent on the
lapsing on any applicable "cure" period pursuant to Section 9.5 thereof, solely
for purposes of this Section 4.2(d), the Company shall be deemed to have the
right to terminate the Collaboration Agreement for cause notwithstanding the
failure of any such cure period to have lapsed; and the Purchaser shall not have
given notice to the Company of its intent to terminate the Collaboration
Agreement.

            (e) Representations and Warranties. The representations and
warranties made by the Purchaser in this Agreement shall have been true and
correct in all material respects at the date hereof and as of the Closing with
the same force and effect as though all such representations and warranties had
been made as of the Closing.

            (f) No Injunctions; Applicable Law. No provision of any applicable
law, rule or regulation and no judgment, order, decree or injunction shall
prohibit the consummation of the Closing nor shall the Closing result in any
violation of any such law, rule, regulation, judgment, order, decree or
injunction.

            (g) Listing. The shares of Common Stock sold in the IPO shall be
listed on the NYSE or traded on the Nasdaq National Market.

            (h) Closing of IPO. The Closing hereunder shall be concurrent with
the closing of the IPO.

            (i) Securities Regulations. The sale of the Shares to Purchaser
shall not be prohibited under state and federal securities laws and regulations.

                                    ARTICLE V

                              STANDSTILL AGREEMENT

      5.1. Definitions. For the purposes of this Agreement, the following words
and phrases shall have the following meanings:

            (i) "Actual Voting Power" means, as of the date of determination,
the total number of votes attaching to the outstanding securities entitled to
vote for the election of directors of the Company.

            (ii) "Affiliate" shall have the meaning given it in Rule 12b-2 under
the Securities Exchange Act of 1934, as amended (the "Exchange Act").

            (iii) "Beneficial Ownership" "Beneficial Owner" and "Beneficially
Own" shall have the meanings described to those terms in Rule 13d-1 under the
Exchange Act.

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            (iv) "Change of Control" means (1) the acquisition by a Third Party
of more than 50% of the Company's then outstanding Voting Securities or (2) the
consummation of a merger, acquisition, consolidation or reorganization or series
of such related transactions involving the Company, unless immediately after
such transaction or transactions, the Beneficial Owners of the Company
immediately prior to the first such transaction shall Beneficially Own at least
50% of the outstanding Voting Securities of the Company (or, if the Company
would not be the surviving company in such merger, consolidation or
reorganization, the Voting Securities of the surviving corporation issued in
such transaction or transactions in respect of Voting Securities of the Company
shall represent at least 50% of the Voting Securities of such surviving
company).

            (v) "Investor Group" means Purchaser and any member of a 13D Group
to which the Purchaser belongs.

            (vi) "Person" means an individual, corporation, partnership,
association, trust, unincorporated organization or other entity

            (vii) "13D Group" means any group of persons formed for the purpose
of acquiring, holding, voting or disposing of Voting Securities which would be
required under the Exchange Act and the rules and regulations promulgated
thereunder, to file a statement on Schedule 13D with the Securities and Exchange
Commission as a "person" within the meaning of Section 13(d)(3) of the Exchange
Act if such group beneficially owned sufficient securities to require such a
filing under the Exchange Act.

            (viii) "Standstill Period" shall mean the period beginning on the
Closing of the IPO and ending on the date that is one year following the Closing
of the IPO.

            (ix) "Threshold Percentage" means the percentage of Actual Voting
Power owned by the Purchaser immediately following the closing of the IPO and
the sale of Shares hereunder, which in no case shall exceed nineteen and
nine-tenths percent (19.9%) of Actual Voting Power.

            (x) "Third Party" means any Person or two or more Persons acting in
concert, other than the Purchaser and its Affiliates or the Company and its
Affiliates.

            (xii) "Voting Security" means, as of the date of determination, the
Common Stock of the Company, any other security generally entitled to vote for
the election of directors and any outstanding convertible securities, options,
warrants or other rights which are convertible into or exchangeable or
exercisable for securities entitled to vote for the election of directors.

      5.2. Standstill Obligations.

            (a) Limitation. At any time during the Standstill Period, except
with the prior written consent of the Company's Board of Directors, no member of
the Investor Group shall, directly or indirectly:

            (i) acquire any Voting Securities (except by way of stock splits,
stock dividends or other distributions) if the effect of such acquisition or
exercise would be to increase the percentage interest of the Investor Group in
the Actual Voting Power to more than the Threshold Percentage; or

            (ii) publicly propose (on behalf of itself or to or with a Third
Party) any merger, business combination, restructuring, recapitalization or
similar transaction involving the Company or its subsidiaries or the purchase,
sale or other disposition outside the ordinary course of business of any
material portion of the assets of the Company or any of its subsidiaries.

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            (b) Repurchases. Notwithstanding Section 5.2(a), no member of the
Investor Group shall be obligated to dispose of any Voting Securities if the
aggregate percentage ownership of the Investor Group is increased as a result of
a repurchase of Voting Securities by the Company.

            (c) Participation. Except with the prior written consent of the
Company's Board of Directors, during the Standstill Period the Investor Group
will not:

                  (i) solicit proxies (or powers of attorney or similar rights
to vote) in respect of any Voting Securities;

                  (ii) become a "participant" or "participant in a
solicitation", as those terms are defined in Regulation 14A of the General Rules
and Regulations promulgated pursuant to the Exchange Act, in opposition to a
solicitation by the Company; provided, however, that the Investor Group shall
not be deemed to be a "participant" or to have become engaged in a solicitation
hereunder solely by reason of the Company's solicitation of proxies in
connection with any meeting of the stockholders of the Company;

                  (iii) seek to advise or intentionally influence any person or
entity with respect to the voting of Voting Securities in connection with any
such solicitation, in opposition to the recommendation of a majority of the
Board of Directors with respect to any matter relating to a Change of Control;

                  (iv) initiate, propose or otherwise solicit stockholders for
the approval of any stockholder proposal (as described in Rule 14a-8 under the
Exchange Act or otherwise) with respect to the Company that is opposed by the
Board of Directors;

                  (v) form or join any 13D Group for the purpose of voting,
purchasing or disposing of Voting Securities or the acquisition of all or
substantially all of assets of the Company;

                  (vi) deposit any Voting Securities in a voting trust or
subject them to a voting agreement or other arrangement of similar effect,
except in order to comply with Competition Laws or other legal requirements;

                  (vii) otherwise act, alone or in concert with others, in a
manner designed or having the deliberate effect of circumventing the
restrictions otherwise imposed hereunder, publicly announce any intention, plan
or arrangement inconsistent with the foregoing or finance or agree to finance
any other person in connection with any of the activities prohibited by this
Agreement; or

                  (viii) publicly request, propose or otherwise seek any
amendment or waiver of the provisions of this Article 5.

      5.3 Exceptions. The limitations provided in Section 5.2 shall immediately
terminate upon the occurrence of any of the following events:

            (a) the commencement by any Person (other than a member of the
Investor Group or an Affiliate thereof) of a bona fide tender or exchange offer
seeking to acquire Beneficial Ownership of fifty percent (50%) or more of the
outstanding shares of Voting Securities of the Company;

            (b) the execution of an agreement by the Company and any Person
which, if consummated, would result in either (i) a Change of Control of the
Company or (ii) the sale of all or substantially all of the Company's assets; or

                                       9
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            (c) the adoption by the Company of a plan of liquidation or
dissolution with respect to the Company.

      5.4 Exclusion. No action or actions taken by the Purchaser pursuant to the
terms of the Collaboration Agreement or in connection with exercising or
enforcing its rights thereunder shall be deemed to violate the restrictions in
Section 5.2.

                                   ARTICLE VI

                                  MISCELLANEOUS

      6.1 No Waiver. No failure or delay on the part of any party to this
Agreement in exercising any right, power or remedy hereunder shall operate as a
waiver thereof; nor shall any single or partial exercise of any such right,
power or remedy preclude any other or further exercise thereof or the exercise
of any other right, power or remedy hereunder. None of the terms, covenants and
conditions of this Agreement can be waived except by the written consent of the
party waiving compliance.

      6.2 Publicity. The parties may, subject to compliance with the Securities
Act, issue a joint press release announcing this Agreement and the transactions
contemplated hereby following execution of this Agreement. Any proposed
announcement, press release or other public disclosure concerning this Agreement
and/or any of the transactions or relationships contemplated hereby shall be
mutually approved by both parties (which approval shall not be unreasonably
withheld); provided, however, that the restrictions contained in this Section
6.2 do not apply to disclosures required by law, the rules of the NYSE, the NASD
or under U.S. generally accepted accounting principles. The Purchaser agrees and
acknowledges that this Agreement and the transactions contemplated hereby shall
be disclosed in, and filed as an exhibit to, the Registration Statement.

      6.3 Amendments, Waivers and Consents. Any provision in this Agreement to
the contrary notwithstanding, and except as hereinafter provided, changes in or
additions to this Agreement may be made, and compliance with any covenant or
provision set forth herein may be omitted or waived, if the party requesting
such change, addition, omission or waiver shall obtain consent thereto in
writing from the other party. Any waiver or consent may be given subject to
satisfaction of conditions stated therein and any waiver or consent shall be
effective only in the specific instance and for the specific purpose for which
given. Any such amendment or waiver or consent effected in accordance with this
Section 6.3 shall be binding upon the parties and their respective successors
and assigns.

      6.4 Addresses for Notices. Any notice required or permitted by this
Agreement shall be in writing and shall be deemed sufficient upon receipt, when
delivered personally or by courier, overnight delivery service or confirmed
facsimile, or seventy-two (72) hours after being deposited in the regular mail
as certified or registered mail (airmail if sent internationally) with postage
prepaid, if such notice is addressed to the party to be notified at such party's
address or facsimile number as set forth below, or as subsequently modified by
written notice.

      If to the Company:

             Trubion Pharmaceuticals, Inc.
             2401 Fourth Avenue, Suite 1050
             Seattle, WA  98121
             Attn:  Chief Executive Officer and General Counsel
             Facsimile Number:  (206) 838-0503

                                       10
<PAGE>


      If to the Purchaser:

             Wyeth Pharmaceuticals
             500 Arcola Road
             Collegeville, Pennsylvania  19426
             Attn: Senior Vice President, Corporate Business Development
             Fax:  (484) 865-6476

             with a copy to:

             Wyeth
             5 Giralda Farms
             Madison, NJ  07940
             Attn:  Executive Vice President and General Counsel
             Facsimile:  (973) 660-7156

      6.5 Binding Effect; Assignment. This Agreement may not be assigned by
either party without the prior written consent of the other; provided, however,
that the Purchaser may assign its rights and delegate its duties hereunder to an
Affiliate without the prior written consent of the Company; provided, however,
Purchaser shall remain subject to Section 5 hereof regardless of any such
assignment; and provided further that if the Company undergoes a Change of
Control in which (a) the Company is not the surviving entity and (b) this
Agreement does not terminate pursuant to Section 1.5(b) in connection with such
Change of Control, the surviving entity and the Purchaser shall enter into a
replacement agreement with substantially the same terms as this Agreement.
Subject to the foregoing, the terms and conditions of this Agreement shall inure
to the benefit of and be binding upon the respective successors and assigns of
the parties. Nothing in this Agreement, express or implied, is intended to
confer upon any party other than the parties hereto or their respective
successors and assigns any rights, remedies, obligations, or liabilities under
or by reason of this Agreement, except as expressly provided in this Agreement.

      6.6 Entire Agreement. This Agreement and the documents referred to herein
constitute the entire agreement between the parties and supersede any prior
understandings or agreements concerning the subject matter hereof.

      6.7 Specific Performance. The parties acknowledge and agree that
irreparable damage would occur in the event any of the provisions of Article V
of this Agreement were not performed in accordance with their specific terms or
were otherwise breached. Accordingly, it is agreed that the parties shall be
entitled to an injunction or injunctions to prevent or cure breaches of the
provisions of Article V of this Agreement and to enforce specifically the terms
and provisions of such Article in any court of the United States or any state
thereof having jurisdiction, in addition to any other remedy to which they may
be entitled in law or in equity.

      6.8 Severability. The provisions of this Agreement are severable and, in
the event that any court of competent jurisdiction shall determine that any one
or more of the provisions or part of a provision contained in this Agreement
shall, for any reason, be held to be invalid, illegal or unenforceable in any
respect, such invalidity, illegality or unenforceability shall not affect any
other provision or part of a provision of this Agreement; but this Agreement,
shall be reformed and construed as if such invalid or illegal or unenforceable
provision, or part of a provision, had never been contained herein, and such

                                       11
<PAGE>

provisions or part reformed so that it would be valid, legal and enforceable to
the maximum extent possible.

      6.9 Governing Law. This Agreement shall be governed by, and construed and
enforced in accordance with, the laws of the State of Delaware without reference
to Delaware conflicts of law provisions.

      6.10 Headings. Article, Section and subsection headings in this Agreement
are included herein for convenience of reference only and shall not constitute a
part of this Agreement for any other purpose.

      6.11 Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be enforceable against the party actually
executing the counterpart, and all of which together shall constitute one
instrument.

                            [Signature page follows.]


                                       12
<PAGE>




      IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
executed as of the date first above written.

                                   TRUBION PHARMACEUTICALS, INC.

                                   By:      /s/ Peter A. Thompson
                                   Name:    Peter A. Thompson
                                   Title:   President & Chief Executive Officer

                                   WYETH

                                   By:      /s/ William M. Haskel
                                   Name:    William M. Haskel
                                   Title:   Vice President



                 SIGNATURE PAGE TO TRUBION PHARMACEUTICALS, INC.
                         COMMON STOCK PURCHASE AGREEMENT
<PAGE>
                                                                       EXHIBIT A

                    MATTERS TO BE COVERED BY COMPANY COUNSEL

      1. The Company is a corporation validly existing under Delaware law and in
good standing with the Secretary of the State of Delaware and has the corporate
power to execute and deliver the Agreement and to perform its obligations
thereunder.

      2. The Company has duly authorized, executed and delivered the Agreement,
and the Agreement constitutes the Company's valid and binding agreement
enforceable against the Company in accordance with its terms, except as
enforceability may be limited by bankruptcy, insolvency, reorganization,
moratorium or similar laws relating to or affecting creditors' rights generally
or by general equitable principles.

      3. No consent, approval, authorization or order of, or any filing or
declaration with, any court or governmental agency or body is required in
connection with the execution, delivery and performance of the Agreement by the
Company or in connection with the taking by the Company of any action
contemplated thereby, other than as indicated in the Agreement or such as have
been obtained and made and such as may be required under federal and state
securities laws.

      4. The execution, delivery and performance of the Agreement by the
Company, and the consummation by the Company of the transactions contemplated
therein do not and will not (a) violate the Certificate of Incorporation or
By-Laws of the Company, (b) materially violate any judgment, ruling, decree or
order known to such counsel, (c) materially violate any statute or regulation
applicable to the business or properties of the Company, or (d) result in a
material breach or violation of any of the terms or provisions of, or constitute
a default or result in the acceleration of any obligation under any material
contract to which the Company is a party or bound.

      5. The Shares delivered on the date hereof have been duly authorized and
validly issued and are fully paid and non-assessable shares of the Company.


                SIGNATURE PAGE TO TRUBION PHARMACEUTICALS, INC.
                        COMMON STOCK PURCHASE AGREEMENT