<PAGE>

                                                                     Exhibit 3.1

                                    BYLAWS OF

                          TRUBION PHARMACEUTICALS, INC.

                      (as amended through March 22, 2007)
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                                TABLE OF CONTENTS

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                                                                            PAGE
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<S>                                                                         <C>
ARTICLE I - CORPORATE OFFICES............................................     1

   1.1      REGISTERED OFFICE............................................     1
   1.2      OTHER OFFICES................................................     1

ARTICLE II - MEETINGS OF STOCKHOLDERS....................................     1

   2.1      PLACE OF MEETINGS............................................     1
   2.2      ANNUAL MEETING...............................................     1
   2.3      SPECIAL MEETING..............................................     1
   2.4      ADVANCE NOTICE PROCEDURES; NOTICE OF STOCKHOLDERS' MEETINGS..     2
   2.5      MANNER OF GIVING NOTICE; AFFIDAVIT OF NOTICE.................     3
   2.6      QUORUM.......................................................     3
   2.7      ADJOURNED MEETING; NOTICE....................................     4
   2.8      CONDUCT OF BUSINESS..........................................     4
   2.9      VOTING.......................................................     4
   2.10     STOCKHOLDER ACTION BY WRITTEN CONSENT WITHOUT A MEETING......     4
   2.11     RECORD DATE FOR STOCKHOLDER NOTICE; VOTING; GIVING CONSENTS..     4
   2.12     PROXIES......................................................     5
   2.13     LIST OF STOCKHOLDERS ENTITLED TO VOTE........................     5
   2.14     INSPECTORS OF ELECTION.......................................     5

ARTICLE III - DIRECTORS..................................................     6

   3.1      POWERS.......................................................     6
   3.2      NUMBER OF DIRECTORS..........................................     6
   3.3      ELECTION, QUALIFICATION AND TERM OF OFFICE OF DIRECTORS......     7
   3.4      RESIGNATION AND VACANCIES....................................     7
   3.5      PLACE OF MEETINGS; MEETINGS BY TELEPHONE.....................     7
   3.6      REGULAR MEETINGS.............................................     8
   3.7      SPECIAL MEETINGS; NOTICE.....................................     8
   3.8      QUORUM.......................................................     8
   3.9      BOARD ACTION BY WRITTEN CONSENT WITHOUT A MEETING............     9
   3.10     FEES AND COMPENSATION OF DIRECTORS...........................     9
   3.11     REMOVAL OF DIRECTORS.........................................     9

ARTICLE IV - COMMITTEES..................................................     9

   4.1      COMMITTEES OF DIRECTORS......................................     9
   4.2      COMMITTEE MINUTES............................................    10
   4.3      MEETINGS AND ACTION OF COMMITTEES............................    10

ARTICLE V - OFFICERS.....................................................    10

   5.1      OFFICERS.....................................................    10
   5.2      APPOINTMENT OF OFFICERS......................................    11
</TABLE>


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                                TABLE OF CONTENTS
                                   (CONTINUED)

<TABLE>
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                                                                            PAGE
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<S>                                                                         <C>
   5.3      SUBORDINATE OFFICERS.........................................    11
   5.4      REMOVAL AND RESIGNATION OF OFFICERS..........................    11
   5.5      VACANCIES IN OFFICES.........................................    11
   5.6      REPRESENTATION OF SHARES OF OTHER CORPORATIONS...............    11
   5.7      AUTHORITY AND DUTIES OF OFFICERS.............................    11

ARTICLE VI - RECORDS AND REPORTS.........................................    12

   6.1      MAINTENANCE AND INSPECTION OF RECORDS........................    12
   6.2      INSPECTION BY DIRECTORS......................................    12

ARTICLE VII - GENERAL MATTERS............................................    12

   7.1      EXECUTION OF CORPORATE CONTRACTS AND INSTRUMENTS.............    12
   7.2      STOCK CERTIFICATES; PARTLY PAID SHARES.......................    12
   7.3      SPECIAL DESIGNATION ON CERTIFICATES..........................    13
   7.4      LOST CERTIFICATES............................................    13
   7.5      CONSTRUCTION; DEFINITIONS....................................    13
   7.6      DIVIDENDS....................................................    14
   7.7      FISCAL YEAR..................................................    14
   7.8      SEAL.........................................................    14
   7.9      TRANSFER OF STOCK............................................    14
   7.10     STOCK TRANSFER AGREEMENTS....................................    14
   7.11     REGISTERED STOCKHOLDERS......................................    14
   7.12     WAIVER OF NOTICE.............................................    15

ARTICLE VIII - NOTICE BY ELECTRONIC TRANSMISSION.........................    15

   8.1      NOTICE BY ELECTRONIC TRANSMISSION............................    15
   8.2      DEFINITION OF ELECTRONIC TRANSMISSION........................    16
   8.3      INAPPLICABILITY..............................................    16

ARTICLE IX - INDEMNIFICATION.............................................    16

   9.1      INDEMNIFICATION OF DIRECTORS AND OFFICERS....................    16
   9.2      INDEMNIFICATION OF OTHERS....................................    16
   9.3      PREPAYMENT OF EXPENSES.......................................    17
   9.4      DETERMINATION; CLAIM.........................................    17
   9.5      NON-EXCLUSIVITY OF RIGHTS....................................    17
   9.6      INSURANCE....................................................    17
   9.7      OTHER INDEMNIFICATION........................................    17
   9.8      AMENDMENT OR REPEAL..........................................    17

ARTICLE X - AMENDMENTS...................................................    18
</TABLE>


                                      -ii-

<PAGE>

                     BYLAWS OF TRUBION PHARMACEUTICALS, INC.

                          ARTICLE I - CORPORATE OFFICES

     1.1  REGISTERED OFFICE.

     The registered office of Trubion Parmaceuticals, Inc. shall be fixed in the
corporation's certificate of incorporation, as the same may be amended from time
to time.

     1.2  OTHER OFFICES.

     The corporation's board of directors (the "Board") may at any time
establish other offices at any place or places where the corporation is
qualified to do business.

                      ARTICLE II - MEETINGS OF STOCKHOLDERS

     2.1  PLACE OF MEETINGS.

     Meetings of stockholders shall be held at any place, within or outside the
State of Delaware, designated by the Board. The Board may, in its sole
discretion, determine that a meeting of stockholders shall not be held at any
place, but may instead be held solely by means of remote communication as
authorized by Section 211(a)(2) of the Delaware General Corporation Law (the
"DGCL"). In the absence of any such designation or determination, stockholders'
meetings shall be held at the corporation's principal executive office.

     2.2  ANNUAL MEETING.

     The annual meeting of stockholders shall be held each year. The Board shall
designate the date and time of the annual meeting. In the absence of such
designation the annual meeting of stockholders shall be held on the second
Tuesday of May of each year at 10:00 a.m. However, if such day falls on a legal
holiday, then the meeting shall be held at the same time and place on the next
succeeding business day. At the annual meeting, directors shall be elected and
any other proper business may be transacted.

     2.3  SPECIAL MEETING.

     A special meeting of the stockholders may be called at any time by the
Board, chairperson of the Board, chief executive officer or president (in the
absence of a chief executive officer), but such special meetings may not be
called by any other person or persons.

     No business may be transacted at such special meeting other than the
business specified in such notice to stockholders. Nothing contained in this
paragraph of this Section 2.3 shall be construed as limiting, fixing, or
affecting the time when a meeting of stockholders called by action of the Board
may be held.

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     2.4  ADVANCE NOTICE PROCEDURES; NOTICE OF STOCKHOLDERS' MEETINGS.

          (i) At an annual meeting of the stockholders, only such business shall
be conducted as shall have been properly brought before the meeting. To be
properly brought before an annual meeting, business must be: (A) specified in
the notice of meeting (or any supplement thereto) given by or at the direction
of the board of directors; (B) otherwise properly brought before the meeting by
or at the direction of the board of directors; or (C) otherwise properly brought
before the meeting by a stockholder. For business to be properly brought before
an annual meeting by a stockholder, the stockholder must have given timely
notice thereof in writing to the secretary of the corporation. To be timely, a
stockholder's notice must be delivered to or mailed and received at the
principal executive offices of the corporation not less than one hundred twenty
(120) calendar days before the one year anniversary of the date on which the
corporation first mailed its proxy statement to stockholders in connection with
the previous year's annual meeting of stockholders; provided, however, that in
the event that no annual meeting was held in the previous year or the date of
the annual meeting has been changed by more than thirty (30) days from the date
of the prior year's meeting, notice by the stockholder to be timely must be so
received not later than the close of business on the later of one hundred twenty
(120) calendar days in advance of such annual meeting and ten (10) calendar days
following the date on which public announcement of the date of the meeting is
first made. A stockholder's notice to the secretary shall set forth as to each
matter the stockholder proposes to bring before the annual meeting: (a) a brief
description of the business desired to be brought before the annual meeting and
the reasons for conducting such business at the annual meeting; (b) the name and
address, as they appear on the corporation's books, of the stockholder proposing
such business; (c) the class and number of shares of the corporation that are
beneficially owned by the stockholder; (d) any material interest of the
stockholder in such business; and (e) any other information that is required to
be provided by the stockholder pursuant to Regulation 14A under the Securities
Exchange Act of 1934, as amended (the "1934 Act"), in his capacity as a
proponent to a stockholder proposal. Notwithstanding the foregoing, in order to
include information with respect to a stockholder proposal in the proxy
statement and form of proxy for a stockholder's meeting, stockholders must
provide notice as required by the regulations promulgated under the 1934 Act.
Notwithstanding anything in these bylaws to the contrary, no business shall be
conducted at any annual meeting except in accordance with the procedures set
forth in this paragraph (i). The chairman of the annual meeting shall, if the
facts warrant, determine and declare at the meeting that business was not
properly brought before the meeting and in accordance with the provisions of
this paragraph (i), and, if he should so determine, he shall so declare at the
meeting that any such business not properly brought before the meeting shall not
be transacted.

          (ii) Only persons who are nominated in accordance with the procedures
set forth in this paragraph (ii) shall be eligible for election as directors.
Nominations of persons for election to the board of directors of the corporation
may be made at a meeting of stockholders by or at the direction of the board of
directors or by any stockholder of the corporation entitled to vote in the
election of directors at the meeting who complies with the notice procedures set
forth in this paragraph (ii). Such nominations, other than those made by or at
the direction of the board of directors, shall be made pursuant to timely notice
in writing to the secretary of the corporation in accordance with the provisions
of paragraph (i) of this Section 2.4. Such stockholder's notice shall set forth:
(a) as to each person, if any, whom the stockholder proposes to nominate for
election or re-election as a director: (A) the name, age, business address and
residence address of such person; (B) the principal occupation or employment of
such person; (C) the class and number of shares of the corporation that are
beneficially owned by such person; (D) a description of all arrangements or
understandings between the stockholder and each nominee and any other person or
persons (naming such person or persons) pursuant to which the nominations are to
be made by the stockholder; and (E) any other information relating to such
person that is required to be disclosed in solicitations of proxies for
elections of directors, or is otherwise required, in each case pursuant to
Regulation 14A


                                       -2-

<PAGE>

under the 1934 Act (including without limitation such person's written consent
to being named in the proxy statement, if any, as a nominee and to serving as a
director if elected); and (b) as to such stockholder giving notice, the
information required to be provided pursuant to paragraph (i) of this Section
2.4. At the request of the board of directors, any person nominated by a
stockholder for election as a director shall furnish to the secretary of the
corporation that information required to be set forth in the stockholder's
notice of nomination which pertains to the nominee. No person shall be eligible
for election as a director of the corporation unless nominated in accordance
with the procedures set forth in this paragraph (ii). The chairman of the
meeting shall, if the facts warrant, determine and declare at the meeting that a
nomination was not made in accordance with the procedures prescribed by these
bylaws, and if he should so determine, he shall so declare at the meeting, and
the defective nomination shall be disregarded.

     These provisions shall not prevent the consideration and approval or
disapproval at an annual meeting of reports of officers, directors and
committees of the board of directors, but in connection therewith no new
business shall be acted upon at any such meeting unless stated, filed and
received as herein provided. Notwithstanding anything in these bylaws to the
contrary, no business brought before a meeting by a stockholder shall be
conducted at an annual meeting except in accordance with procedures set forth in
this Section 2.4.

     All notices of meetings of stockholders shall be sent or otherwise given in
accordance with either Section 2.5 or Section 8.1 of these bylaws not less than
10 nor more than 60 days before the date of the meeting to each stockholder
entitled to vote at such meeting. The notice shall specify the place, if any,
date and hour of the meeting, the means of remote communication, if any, by
which stockholders and proxy holders may be deemed to be present in person and
vote at such meeting, and, in the case of a special meeting, the purpose or
purposes for which the meeting is called.

     2.5  MANNER OF GIVING NOTICE; AFFIDAVIT OF NOTICE.

     Notice of any meeting of stockholders shall be given:

          (i) if mailed, when deposited in the United States mail, postage
prepaid, directed to the stockholder at his or her address as it appears on the
corporation's records; or

          (ii) if electronically transmitted as provided in Section 8.1 of these
bylaws.

     An affidavit of the secretary or an assistant secretary of the corporation
or of the transfer agent or any other agent of the corporation that the notice
has been given by mail or by a form of electronic transmission, as applicable,
shall, in the absence of fraud, be prima facie evidence of the facts stated
therein.

     2.6  QUORUM.

     The holders of a majority of the stock issued and outstanding and entitled
to vote, present in person or represented by proxy, shall constitute a quorum
for the transaction of business at all meetings of the stockholders. If,
however, such quorum is not present or represented at any meeting of the
stockholders, then either (i) the chairperson of the meeting, or (ii) the
stockholders entitled to vote at the meeting, present in person or represented
by proxy, shall have power to adjourn the meeting from time to time, without
notice other than announcement at the meeting, until a quorum is present or
represented. At such adjourned meeting at which a quorum is present or
represented, any business may be transacted that might have been transacted at
the meeting as originally noticed.


                                       -3-

<PAGE>

     2.7  ADJOURNED MEETING; NOTICE.

     When a meeting is adjourned to another time or place, unless these bylaws
otherwise require, notice need not be given of the adjourned meeting if the
time, place if any thereof, and the means of remote communications if any by
which stockholders and proxy holders may be deemed to be present in person and
vote at such adjourned meeting are announced at the meeting at which the
adjournment is taken. At the adjourned meeting, the corporation may transact any
business which might have been transacted at the original meeting. If the
adjournment is for more than 30 days, or if after the adjournment a new record
date is fixed for the adjourned meeting, a notice of the adjourned meeting shall
be given to each stockholder of record entitled to vote at the meeting.

     2.8  CONDUCT OF BUSINESS.

     The chairperson of any meeting of stockholders shall determine the order of
business and the procedure at the meeting, including such regulation of the
manner of voting and the conduct of business.

     2.9  VOTING.

     The stockholders entitled to vote at any meeting of stockholders shall be
determined in accordance with the provisions of Section 2.11 of these bylaws,
subject to Section 217 (relating to voting rights of fiduciaries, pledgors and
joint owners of stock) and Section 218 (relating to voting trusts and other
voting agreements) of the DGCL.

     Except as may be otherwise provided in the certificate of incorporation or
these bylaws, each stockholder shall be entitled to one vote for each share of
capital stock held by such stockholder.

     2.10 STOCKHOLDER ACTION BY WRITTEN CONSENT WITHOUT A MEETING.

     Subject to the rights of the holders of the shares of any series of
Preferred Stock or any other class of stock or series thereof having a
preference over the Common Stock as dividend or upon liquidation, any action
required or permitted to be taken by the stockholders of the corporation must be
effected at a duly called annual or special meeting of stockholders of the
corporation and may not be effected by any consent in writing by such
stockholders.

     2.11 RECORD DATE FOR STOCKHOLDER NOTICE; VOTING; GIVING CONSENTS.

     In order that the corporation may determine the stockholders entitled to
notice of or to vote at any meeting of stockholders or any adjournment thereof,
or entitled to receive payment of any dividend or other distribution or
allotment of any rights, or entitled to exercise any rights in respect of any
change, conversion or exchange of stock or for the purpose of any other lawful
action, the Board may fix, in advance, a record date, which record date shall
not precede the date on which the resolution fixing the record date is adopted
and which shall not be more than 60 nor less than 10 days before the date of
such meeting, nor more than 60 days prior to any other such action.


                                       -4-

<PAGE>

     If the Board does not so fix a record date:

          (i) The record date for determining stockholders entitled to notice of
or to vote at a meeting of stockholders shall be at the close of business on the
day next preceding the day on which notice is given, or, if notice is waived, at
the close of business on the day next preceding the day on which the meeting is
held.

          (ii) The record date for determining stockholders for any other
purpose shall be at the close of business on the day on which the Board adopts
the resolution relating thereto.

     A determination of stockholders of record entitled to notice of or to vote
at a meeting of stockholders shall apply to any adjournment of the meeting;
provided, however, that the Board may fix a new record date for the adjourned
meeting.

     2.12 PROXIES.

     Each stockholder entitled to vote at a meeting of stockholders may
authorize another person or persons to act for such stockholder by proxy
authorized by an instrument in writing or by a transmission permitted by law
filed in accordance with the procedure established for the meeting, but no such
proxy shall be voted or acted upon after three years from its date, unless the
proxy provides for a longer period. The revocability of a proxy that states on
its face that it is irrevocable shall be governed by the provisions of Section
212 of the DGCL.

     2.13 LIST OF STOCKHOLDERS ENTITLED TO VOTE.

     The officer who has charge of the stock ledger of the corporation shall
prepare and make, at least 10 days before every meeting of stockholders, a
complete list of the stockholders entitled to vote at the meeting, arranged in
alphabetical order, and showing the address of each stockholder and the number
of shares registered in the name of each stockholder. The corporation shall not
be required to include electronic mail addresses or other electronic contact
information on such list. Such list shall be open to the examination of any
stockholder, for any purpose germane to the meeting for a period of at least 10
days prior to the meeting: (i) on a reasonably accessible electronic network,
provided that the information required to gain access to such list is provided
with the notice of the meeting, or (ii) during ordinary business hours, at the
corporation's principal executive office. In the event that the corporation
determines to make the list available on an electronic network, the corporation
may take reasonable steps to ensure that such information is available only to
stockholders of the corporation. If the meeting is to be held at a place, then
the list shall be produced and kept at the time and place of the meeting during
the whole time thereof, and may be inspected by any stockholder who is present.
If the meeting is to be held solely by means of remote communication, then the
list shall also be open to the examination of any stockholder during the whole
time of the meeting on a reasonably accessible electronic network, and the
information required to access such list shall be provided with the notice of
the meeting. Such list shall presumptively determine the identity of the
stockholders entitled to vote at the meeting and the number of shares held by
each of them.

     2.14 INSPECTORS OF ELECTION

     A written proxy may be in the form of a telegram, cablegram, or other means
of electronic transmission which sets forth or is submitted with information
from which it can be determined that the telegram, cablegram, or other means of
electronic transmission was authorized by the person.


                                       -5-

<PAGE>

     Before any meeting of stockholders, the board of directors shall appoint an
inspector or inspectors of election to act at the meeting or its adjournment.
The number of inspectors shall be either one (1) or three (3). If any person
appointed as inspector fails to appear or fails or refuses to act, then the
chairperson of the meeting may, and upon the request of any stockholder or a
stockholder's proxy shall, appoint a person to fill that vacancy.

     Such inspectors shall:

          (i) determine the number of shares outstanding and the voting power of
each, the number of shares represented at the meeting, the existence of a
quorum, and the authenticity, validity, and effect of proxies;

          (ii) receive votes, ballots or consents;

          (iii) hear and determine all challenges and questions in any way
arising in connection with the right to vote;

          (iv) count and tabulate all votes or consents;

          (v) determine when the polls shall close;

          (vi) determine the result; and

          (vii) do any other acts that may be proper to conduct the election or
vote with fairness to all stockholders.

     The inspectors of election shall perform their duties impartially, in good
faith, to the best of their ability and as expeditiously as is practical. If
there are three (3) inspectors of election, the decision, act or certificate of
a majority is effective in all respects as the decision, act or certificate of
all. Any report or certificate made by the inspectors of election is prima facie
evidence of the facts stated therein.

                             ARTICLE III - DIRECTORS

     3.1  POWERS.

     Subject to the provisions of the DGCL and any limitations in the
certificate of incorporation or these bylaws relating to action required to be
approved by the stockholders or by the outstanding shares, the business and
affairs of the corporation shall be managed and all corporate powers shall be
exercised by or under the direction of the Board.

     3.2  NUMBER OF DIRECTORS.

     The authorized number of directors shall be determined from time to time by
resolution of the Board, provided the Board shall consist of at least one
member. No reduction of the authorized number of directors shall have the effect
of removing any director before that director's term of office expires.


                                       -6-

<PAGE>

     3.3  ELECTION, QUALIFICATION AND TERM OF OFFICE OF DIRECTORS.

     Except as provided in Section 3.4 of these bylaws, each director, including
a director elected to fill a vacancy, shall hold office until the expiration of
the term for which elected and until such director's successor is elected and
qualified or until such director's earlier death, resignation or removal.
Directors need not be stockholders unless so required by the certificate of
incorporation or these bylaws. The certificate of incorporation or these bylaws
may prescribe other qualifications for directors.

     If so provided in the certificate of incorporation, the directors of the
corporation shall be divided into three classes.

     3.4  RESIGNATION AND VACANCIES.

     Any director may resign at any time upon notice given in writing or by
electronic transmission to the corporation. When one or more directors so
resigns and the resignation is effective at a future date, a majority of the
directors then in office, including those who have so resigned, shall have power
to fill such vacancy or vacancies, the vote thereon to take effect when such
resignation or resignations shall become effective, and each director so chosen
shall hold office as provided in this section in the filling of other vacancies.

     Unless otherwise provided in the certificate of incorporation or these
bylaws, vacancies and newly created directorships resulting from any increase in
the authorized number of directors elected by all of the stockholders having the
right to vote as a single class may be filled by a majority of the directors
then in office, although less than a quorum, or by a sole remaining director. If
the directors are divided into classes, a person so elected by the directors
then in office to fill a vacancy or newly created directorship shall hold office
until the next election of the class for which such director shall have been
chosen and until his or her successor shall have been duly elected and
qualified.

     If at any time, by reason of death or resignation or other cause, the
corporation should have no directors in office, then any officer or any
stockholder or an executor, administrator, trustee or guardian of a stockholder,
or other fiduciary entrusted with like responsibility for the person or estate
of a stockholder, may call a special meeting of stockholders in accordance with
the provisions of the certificate of incorporation or these bylaws, or may apply
to the Court of Chancery for a decree summarily ordering an election as provided
in Section 211 of the DGCL.

     If, at the time of filling any vacancy or any newly created directorship,
the directors then in office constitute less than a majority of the whole Board
(as constituted immediately prior to any such increase), then the Court of
Chancery may, upon application of any stockholder or stockholders holding at
least 10% of the total number of the shares at the time outstanding having the
right to vote for such directors, summarily order an election to be held to fill
any such vacancies or newly created directorships, or to replace the directors
chosen by the directors then in office as aforesaid, which election shall be
governed by the provisions of Section 211 of the DGCL as far as applicable.

     3.5  PLACE OF MEETINGS; MEETINGS BY TELEPHONE.

     The Board may hold meetings, both regular and special, either within or
outside the State of Delaware.


                                       -7-

<PAGE>

     Unless otherwise restricted by the certificate of incorporation or these
bylaws, members of the Board, or any committee designated by the Board, may
participate in a meeting of the Board, or any committee, by means of conference
telephone or other communications equipment by means of which all persons
participating in the meeting can hear each other, and such participation in a
meeting shall constitute presence in person at the meeting.

     3.6  REGULAR MEETINGS.

     Regular meetings of the Board may be held without notice at such time and
at such place as shall from time to time be determined by the Board.

     3.7  SPECIAL MEETINGS; NOTICE.

     Special meetings of the Board for any purpose or purposes may be called at
any time by the chairperson of the Board, the chief executive officer, the
president, the secretary or a majority of the authorized number of directors.

     Notice of the time and place of special meetings shall be:

          (i)  delivered personally by hand, by courier or by telephone;

          (ii) sent by United States first-class mail, postage prepaid;

          (iii) sent by facsimile; or

          (iv) sent by electronic mail,

directed to each director at that director's address, telephone number,
facsimile number or electronic mail address, as the case may be, as shown on the
corporation's records.

     If the notice is (i) delivered personally by hand, by courier or by
telephone, (ii) sent by facsimile or (iii) sent by electronic mail, it shall be
delivered or sent at least 24 hours before the time of the holding of the
meeting. If the notice is sent by United States mail, it shall be deposited in
the United States mail at least four days before the time of the holding of the
meeting. Any oral notice may be communicated to the director. The notice need
not specify the place of the meeting (if the meeting is to be held at the
corporation's principal executive office) nor the purpose of the meeting.

     3.8  QUORUM.

     At all meetings of the Board, a majority of the authorized number of
directors shall constitute a quorum for the transaction of business. The vote of
a majority of the directors present at any meeting at which a quorum is present
shall be the act of the Board, except as may be otherwise specifically provided
by statute, the certificate of incorporation or these bylaws. If a quorum is not
present at any meeting of the Board, then the directors present thereat may
adjourn the meeting from time to time, without notice other than announcement at
the meeting, until a quorum is present.


                                       -8-
<PAGE>

     A meeting at which a quorum is initially present may continue to transact
business notwithstanding the withdrawal of directors, if any action taken is
approved by at least a majority of the required quorum for that meeting.

     3.9 BOARD ACTION BY WRITTEN CONSENT WITHOUT A MEETING.

     Unless otherwise restricted by the certificate of incorporation or these
bylaws, any action required or permitted to be taken at any meeting of the
Board, or of any committee thereof, may be taken without a meeting if all
members of the Board or committee, as the case may be, consent thereto in
writing or by electronic transmission and the writing or writings or electronic
transmission or transmissions are filed with the minutes of proceedings of the
Board or committee. Such filing shall be in paper form if the minutes are
maintained in paper form and shall be in electronic form if the minutes are
maintained in electronic form.

     3.10 FEES AND COMPENSATION OF DIRECTORS.

     Unless otherwise restricted by the certificate of incorporation or these
bylaws, the Board shall have the authority to fix the compensation of directors.

     3.11 REMOVAL OF DIRECTORS.

     Any director may be removed from office by the stockholders of the
corporation only for cause.

     No reduction of the authorized number of directors shall have the effect of
removing any director prior to the expiration of such director's term of office.

                            ARTICLE IV - COMMITTEES

     4.1 COMMITTEES OF DIRECTORS.

     The Board may, by resolution passed by a majority of the authorized number
of directors, designate one or more committees, each committee to consist of one
or more of the directors of the corporation. The Board may designate one or more
directors as alternate members of any committee, who may replace any absent or
disqualified member at any meeting of the committee. In the absence or
disqualification of a member of a committee, the member or members thereof
present at any meeting and not disqualified from voting, whether or not such
member or members constitute a quorum, may unanimously appoint another member of
the Board to act at the meeting in the place of any such absent or disqualified
member. Any such committee, to the extent provided in the resolution of the
Board or in these bylaws, shall have and may exercise all the powers and
authority of the Board in the management of the business and affairs of the
corporation, and may authorize the seal of the corporation to be affixed to all
papers that may require it; but no such committee shall have the power or
authority to (i) approve or adopt, or recommend to the stockholders, any action
or matter (other than the election or removal of directors) expressly required
by the DGCL to be submitted to stockholders for approval, or (ii) adopt, amend
or repeal any bylaw of the corporation.


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     4.2 COMMITTEE MINUTES.

     Each committee shall keep regular minutes of its meetings and report the
same to the Board when required.

     4.3 MEETINGS AND ACTION OF COMMITTEES.

     Meetings and actions of committees shall be governed by, and held and taken
in accordance with, the provisions of:

          (i) Section 3.5 (place of meetings and meetings by telephone);

          (ii) Section 3.6 (regular meetings);

          (iii) Section 3.7 (special meetings and notice);

          (iv) Section 3.8 (quorum);

          (v) Section 7.12 (waiver of notice); and

          (vi) Section 3.9 (action without a meeting)

with such changes in the context of those bylaws as are necessary to substitute
the committee and its members for the Board and its members. However:

          (i) the time of regular meetings of committees may be determined
either by resolution of the Board or by resolution of the committee;

          (ii) special meetings of committees may also be called by resolution
of the Board; and

          (iii) notice of special meetings of committees shall also be given to
all alternate members, who shall have the right to attend all meetings of the
committee. The Board may adopt rules for the government of any committee not
inconsistent with the provisions of these bylaws.

                              ARTICLE V - OFFICERS

     5.1 OFFICERS.

     The officers of the corporation shall be a president and a secretary. The
corporation may also have, at the discretion of the Board, a chairperson of the
Board, a vice chairperson of the Board, a chief executive officer, a chief
financial officer or treasurer, one or more vice presidents, one or more
assistant vice presidents, one or more assistant treasurers, one or more
assistant secretaries, and any such other officers as may be appointed in
accordance with the provisions of these bylaws. Any number of offices may be
held by the same person.


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     5.2 APPOINTMENT OF OFFICERS.

     The Board shall appoint the officers of the corporation, except such
officers as may be appointed in accordance with the provisions of Sections 5.3
and 5.5 of these bylaws, subject to the rights, if any, of an officer under any
contract of employment.

     5.3 SUBORDINATE OFFICERS.

     The Board may appoint, or empower the chief executive officer or, in the
absence of a chief executive officer, the president, to appoint, such other
officers and agents as the business of the corporation may require. Each of such
officers and agents shall hold office for such period, have such authority, and
perform such duties as are provided in these bylaws or as the Board may from
time to time determine.

     5.4 REMOVAL AND RESIGNATION OF OFFICERS.

     Subject to the rights, if any, of an officer under any contract of
employment, any officer may be removed, either with or without cause, by an
affirmative vote of the majority of the Board at any regular or special meeting
of the Board or, except in the case of an officer chosen by the Board, by any
officer upon whom such power of removal may be conferred by the Board.

     Any officer may resign at any time by giving written notice to the
corporation. Any resignation shall take effect at the date of the receipt of
that notice or at any later time specified in that notice. Unless otherwise
specified in the notice of resignation, the acceptance of the resignation shall
not be necessary to make it effective. Any resignation is without prejudice to
the rights, if any, of the corporation under any contract to which the officer
is a party.

     5.5 VACANCIES IN OFFICES.

     Any vacancy occurring in any office of the corporation shall be filled by
the Board or as provided in Section 5.2.

     5.6 REPRESENTATION OF SHARES OF OTHER CORPORATIONS.

     The chairperson of the Board, the president, any vice president, the
treasurer, the secretary or assistant secretary of this corporation, or any
other person authorized by the Board or the president or a vice president, is
authorized to vote, represent, and exercise on behalf of this corporation all
rights incident to any and all shares of any other corporation or corporations
standing in the name of this corporation. The authority granted herein may be
exercised either by such person directly or by any other person authorized to do
so by proxy or power of attorney duly executed by such person having the
authority.

     5.7 AUTHORITY AND DUTIES OF OFFICERS.

     All officers of the corporation shall respectively have such authority and
perform such duties in the management of the business of the corporation as may
be designated from time to time by the Board or the stockholders and, to the
extent not so provided, as generally pertain to their respective offices,
subject to the control of the Board.


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                        ARTICLE VI - RECORDS AND REPORTS

     6.1 MAINTENANCE AND INSPECTION OF RECORDS.

     The corporation shall, either at its principal executive office or at such
place or places as designated by the Board, keep a record of its stockholders
listing their names and addresses and the number and class of shares held by
each stockholder, a copy of these bylaws as amended to date, accounting books,
and other records.

     Any stockholder of record, or a person who is the beneficial owner of
shares of such stock held either in a voting trust or by a nominee on behalf of
such person, in person or by attorney or other agent, shall, upon written demand
under oath stating the purpose thereof, have the right during the usual hours
for business to inspect for any proper purpose the corporation's stock ledger, a
list of its stockholders, and its other books and records and to make copies or
extracts therefrom. In every instance where the stockholder is other than a
record holder of stock, the demand under oath shall state the person's status as
a stockholder, be accompanied by documentary evidence of beneficial ownership of
the stock, and state that such documentary evidence is a true and correct copy
of what it purports to be. A proper purpose shall mean a purpose reasonably
related to such person's interest as a stockholder. In every instance where an
attorney or other agent is the person who seeks the right to inspection, the
demand under oath shall be accompanied by a power of attorney or such other
writing that authorizes the attorney or other agent so to act on behalf of the
stockholder. The demand under oath shall be directed to the corporation at its
registered office in Delaware or at its principal executive office.

     6.2 INSPECTION BY DIRECTORS.

     Any director shall have the right to examine the corporation's stock
ledger, a list of its stockholders, and its other books and records for a
purpose reasonably related to his or her position as a director. The Court of
Chancery is hereby vested with the exclusive jurisdiction to determine whether a
director is entitled to the inspection sought. The Court may summarily order the
corporation to permit the director to inspect any and all books and records, the
stock ledger, and the stock list and to make copies or extracts therefrom. The
Court may, in its discretion, prescribe any limitations or conditions with
reference to the inspection, or award such other and further relief as the Court
may deem just and proper.

                         ARTICLE VII - GENERAL MATTERS

     7.1 EXECUTION OF CORPORATE CONTRACTS AND INSTRUMENTS.

     The Board, except as otherwise provided in these bylaws, may authorize any
officer or officers, or agent or agents, to enter into any contract or execute
any instrument in the name of and on behalf of the corporation; such authority
may be general or confined to specific instances. Unless so authorized or
ratified by the Board or within the agency power of an officer, no officer,
agent or employee shall have any power or authority to bind the corporation by
any contract or engagement or to pledge its credit or to render it liable for
any purpose or for any amount.

     7.2 STOCK CERTIFICATES; PARTLY PAID SHARES.

     The shares of the corporation shall be represented by certificates,
provided that the Board may provide by resolution or resolutions that some or
all of any or all classes or series of its stock shall be uncertificated shares.


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<PAGE>

Any such resolution shall not apply to shares represented by a certificate until
such certificate is surrendered to the corporation. Notwithstanding the adoption
of such a resolution by the Board, every holder of stock represented by
certificates and upon request every holder of uncertificated shares shall be
entitled to have a certificate signed by, or in the name of the corporation by
the chairperson or vice-chairperson of the Board, or the president or
vice-president, and by the treasurer or an assistant treasurer, or the secretary
or an assistant secretary of such corporation representing the number of shares
registered in certificate form. Any or all of the signatures on the certificate
may be a facsimile. In case any officer, transfer agent or registrar who has
signed or whose facsimile signature has been placed upon a certificate has
ceased to be such officer, transfer agent or registrar before such certificate
is issued, it may be issued by the corporation with the same effect as if he
were such officer, transfer agent or registrar at the date of issue.

     The corporation may issue the whole or any part of its shares as partly
paid and subject to call for the remainder of the consideration to be paid
therefor. Upon the face or back of each stock certificate issued to represent
any such partly paid shares, or upon the books and records of the corporation in
the case of uncertificated partly paid shares, the total amount of the
consideration to be paid therefor and the amount paid thereon shall be stated.
Upon the declaration of any dividend on fully paid shares, the corporation shall
declare a dividend upon partly paid shares of the same class, but only upon the
basis of the percentage of the consideration actually paid thereon.

     7.3 SPECIAL DESIGNATION ON CERTIFICATES.

     If the corporation is authorized to issue more than one class of stock or
more than one series of any class, then the powers, the designations, the
preferences, and the relative, participating, optional or other special rights
of each class of stock or series thereof and the qualifications, limitations or
restrictions of such preferences and/or rights shall be set forth in full or
summarized on the face or back of the certificate that the corporation shall
issue to represent such class or series of stock; provided, however, that,
except as otherwise provided in Section 202 of the DGCL, in lieu of the
foregoing requirements there may be set forth on the face or back of the
certificate that the corporation shall issue to represent such class or series
of stock a statement that the corporation will furnish without charge to each
stockholder who so requests the powers, the designations, the preferences, and
the relative, participating, optional or other special rights of each class of
stock or series thereof and the qualifications, limitations or restrictions of
such preferences and/or rights.

     7.4 LOST CERTIFICATES.

     Except as provided in this Section 7.4, no new certificates for shares
shall be issued to replace a previously issued certificate unless the latter is
surrendered to the corporation and cancelled at the same time. The corporation
may issue a new certificate of stock or uncertificated shares in the place of
any certificate theretofore issued by it, alleged to have been lost, stolen or
destroyed, and the corporation may require the owner of the lost, stolen or
destroyed certificate, or such owner's legal representative, to give the
corporation a bond sufficient to indemnify it against any claim that may be made
against it on account of the alleged loss, theft or destruction of any such
certificate or the issuance of such new certificate or uncertificated shares.

     7.5 CONSTRUCTION; DEFINITIONS.

     Unless the context requires otherwise, the general provisions, rules of
construction, and definitions in the DGCL shall govern the construction of these
bylaws. Without limiting the generality of this provision, the


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<PAGE>

singular number includes the plural, the plural number includes the singular,
and the term "person" includes both a corporation and a natural person.

     7.6 DIVIDENDS.

     The Board, subject to any restrictions contained in either (i) the DGCL, or
(ii) the certificate of incorporation, may declare and pay dividends upon the
shares of its capital stock. Dividends may be paid in cash, in property, or in
shares of the corporation's capital stock.

     The Board may set apart out of any of the funds of the corporation
available for dividends a reserve or reserves for any proper purpose and may
abolish any such reserve. Such purposes shall include but not be limited to
equalizing dividends, repairing or maintaining any property of the corporation,
and meeting contingencies.

     7.7 FISCAL YEAR.

     The fiscal year of the corporation shall be fixed by resolution of the
Board and may be changed by the Board.

     7.8 SEAL.

     The corporation may adopt a corporate seal, which shall be adopted and
which may be altered by the Board. The corporation may use the corporate seal by
causing it or a facsimile thereof to be impressed or affixed or in any other
manner reproduced.

     7.9 TRANSFER OF STOCK.

     Stock of the corporation shall be transferable in the manner prescribed by
law and in these bylaws. Transfers of stock shall be made on the books of the
corporation only by the record holder of such stock or by his or her attorney
lawfully constituted in writing and, if such stock is certificated, upon the
surrender of the certificate therefor, which shall be canceled before a new
certificate shall be issued.

     7.10 STOCK TRANSFER AGREEMENTS.

     The corporation shall have power to enter into and perform any agreement
with any number of stockholders of any one or more classes of stock of the
corporation to restrict the transfer of shares of stock of the corporation of
any one or more classes owned by such stockholders in any manner not prohibited
by the DGCL.

     7.11 REGISTERED STOCKHOLDERS.

     The corporation:

          (i) shall be entitled to recognize the exclusive right of a person
registered on its books as the owner of shares to receive dividends and to vote
as such owner;

          (ii) shall be entitled to hold liable for calls and assessments the
person registered on its books as the owner of shares; and


                                      -14-

<PAGE>

          (iii) shall not be bound to recognize any equitable or other claim to
or interest in such share or shares on the part of another person, whether or
not it shall have express or other notice thereof, except as otherwise provided
by the laws of Delaware.

     7.12 WAIVER OF NOTICE.

     Whenever notice is required to be given under any provision of the DGCL,
the certificate of incorporation or these bylaws, a written waiver, signed by
the person entitled to notice, or a waiver by electronic transmission by the
person entitled to notice, whether before or after the time of the event for
which notice is to be given, shall be deemed equivalent to notice. Attendance of
a person at a meeting shall constitute a waiver of notice of such meeting,
except when the person attends a meeting for the express purpose of objecting at
the beginning of the meeting, to the transaction of any business because the
meeting is not lawfully called or convened. Neither the business to be
transacted at, nor the purpose of, any regular or special meeting of the
stockholders need be specified in any written waiver of notice or any waiver by
electronic transmission unless so required by the certificate of incorporation
or these bylaws.

                ARTICLE VIII - NOTICE BY ELECTRONIC TRANSMISSION

     8.1 NOTICE BY ELECTRONIC TRANSMISSION.

     Without limiting the manner by which notice otherwise may be given
effectively to stockholders pursuant to the DGCL, the certificate of
incorporation or these bylaws, any notice to stockholders given by the
corporation under any provision of the DGCL, the certificate of incorporation or
these bylaws shall be effective if given by a form of electronic transmission
consented to by the stockholder to whom the notice is given. Any such consent
shall be revocable by the stockholder by written notice to the corporation. Any
such consent shall be deemed revoked if:

          (i) the corporation is unable to deliver by electronic transmission
two consecutive notices given by the corporation in accordance with such
consent; and

          (ii) such inability becomes known to the secretary or an assistant
secretary of the corporation or to the transfer agent, or other person
responsible for the giving of notice.

However, the inadvertent failure to treat such inability as a revocation shall
not invalidate any meeting or other action.

     Any notice given pursuant to the preceding paragraph shall be deemed given:

          (i) if by facsimile telecommunication, when directed to a number at
               which the stockholder has consented to receive notice;

          (ii) if by electronic mail, when directed to an electronic mail
               address at which the stockholder has consented to receive notice;


                                      -15-

<PAGE>


          (iii) if by a posting on an electronic network together with separate
               notice to the stockholder of such specific posting, upon the
               later of (A) such posting and (B) the giving of such separate
               notice; and

          (iv) if by any other form of electronic transmission, when directed to
               the stockholder.

     An affidavit of the secretary or an assistant secretary or of the transfer
agent or other agent of the corporation that the notice has been given by a form
of electronic transmission shall, in the absence of fraud, be prima facie
evidence of the facts stated therein.

     8.2 DEFINITION OF ELECTRONIC TRANSMISSION.

     An "electronic transmission" means any form of communication, not directly
involving the physical transmission of paper, that creates a record that may be
retained, retrieved, and reviewed by a recipient thereof, and that may be
directly reproduced in paper form by such a recipient through an automated
process.

     8.3 INAPPLICABILITY.

     Notice by a form of electronic transmission shall not apply to Sections
164, 296, 311, 312 or 324 of the DGCL.

                          ARTICLE IX - INDEMNIFICATION

     9.1 INDEMNIFICATION OF DIRECTORS AND OFFICERS

     The corporation shall indemnify and hold harmless, to the fullest extent
permitted by the DGCL as it presently exists or may hereafter be amended, any
director or officer of the corporation who was or is made or is threatened to be
made a party or is otherwise involved in any action, suit or proceeding, whether
civil, criminal, administrative or investigative (a "PROCEEDING") by reason of
the fact that he or she, or a person for whom he or she is the legal
representative, is or was a director, officer, employee or agent of the
corporation or is or was serving at the request of the corporation as a
director, officer, employee or agent of another corporation or of a partnership,
joint venture, trust, enterprise or non-profit entity, including service with
respect to employee benefit plans, against all liability and loss suffered and
expenses reasonably incurred by such person in connection with any such
Proceeding. The corporation shall be required to indemnify a person in
connection with a Proceeding initiated by such person only if the Proceeding was
authorized by the Board.

     9.2 INDEMNIFICATION OF OTHERS

     The corporation shall have the power to indemnify and hold harmless, to the
extent permitted by applicable law as it presently exists or may hereafter be
amended, any employee or agent of the corporation who was or is made or is
threatened to be made a party or is otherwise involved in any Proceeding by
reason of the fact that he or she, or a person for whom he or she is the legal
representative, is or was an employee or agent of the corporation or is or was
serving at the request of the corporation as a director, officer, employee or
agent of another corporation or of a partnership, joint venture, trust,
enterprise or non-profit entity, including service with respect to employee
benefit plans, against all liability and loss suffered and expenses reasonably
incurred by such person in connection with any such Proceeding.


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     9.3 PREPAYMENT OF EXPENSES

     The corporation shall pay the expenses incurred by any officer or director
of the corporation, and may pay the expenses incurred by any employee or agent
of the corporation, in defending any Proceeding in advance of its final
disposition; provided, however, that the payment of expenses incurred by a
person in advance of the final disposition of the Proceeding shall be made only
upon receipt of an undertaking by the person to repay all amounts advanced if it
should be ultimately determined that the person is not entitled to be
indemnified under this Article IX or otherwise.

     9.4 DETERMINATION; CLAIM

     If a claim for indemnification or payment of expenses under this Article IX
is not paid in full within sixty days after a written claim therefor has been
received by the corporation the claimant may file suit to recover the unpaid
amount of such claim and, if successful in whole or in part, shall be entitled
to be paid the expense of prosecuting such claim. In any such action the
corporation shall have the burden of proving that the claimant was not entitled
to the requested indemnification or payment of expenses under applicable law.

     9.5 NON-EXCLUSIVITY OF RIGHTS

     The rights conferred on any person by this Article IX shall not be
exclusive of any other rights which such person may have or hereafter acquire
under any statute, provision of the certificate of incorporation, these bylaws,
agreement, vote of stockholders or disinterested directors or otherwise.

     9.6 INSURANCE

     The corporation may purchase and maintain insurance on behalf of any person
who is or was a director, officer, employee or agent of the corporation, or is
or was serving at the request of the corporation as a director, officer,
employee or agent of another corporation, partnership, joint venture, trust or
other enterprise against any liability asserted against him or her and incurred
by him or her in any such capacity, or arising out of his or her status as such,
whether or not the corporation would have the power to indemnify him or her
against such liability under the provisions of the DGCL.

     9.7 OTHER INDEMNIFICATION

     The corporation's obligation, if any, to indemnify any person who was or is
serving at its request as a director, officer, employee or agent of another
corporation, partnership, joint venture, trust, enterprise or non-profit entity
shall be reduced by any amount such person may collect as indemnification from
such other corporation, partnership, joint venture, trust, enterprise or
non-profit enterprise.

     9.8 AMENDMENT OR REPEAL

     Any repeal or modification of the foregoing provisions of this Article IX
shall not adversely affect any right or protection hereunder of any person in
respect of any act or omission occurring prior to the time of such repeal or
modification."


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                             ARTICLE X - AMENDMENTS

     These bylaws may be adopted, amended or repealed by the stockholders
entitled to vote. However, the corporation may, in its certificate of
incorporation, confer the power to adopt, amend or repeal bylaws upon the
directors. The fact that such power has been so conferred upon the directors
shall not divest the stockholders of the power, nor limit their power to adopt,
amend or repeal bylaws.


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                          TRUBION PHARMACEUTICALS, INC.

                        CERTIFICATE OF ADOPTION OF BYLAWS

     The undersigned hereby certifies that he or she is the duly elected,
qualified, and acting Secretary or Assistant Secretary of Trubion
Pharmaceuticals, Inc., a Delaware corporation and that the foregoing bylaws,
comprising 18 pages, were adopted on April 26, 2006 by the corporation's board
of directors to become effective upon the closing of the Company's initial
public offering.

     IN WITNESS WHEREOF, the undersigned has hereunto set his or her hand this
26th day of April, 2006.


                                        /s/ Patrick J. Schultheis
                                        ----------------------------------------