SEC FORM 3 SEC Form 3
FORM 3 UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940
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1. Name and Address of Reporting Person*
Nielsen Ulrik B.

(Last) (First) (Middle)
C/O MERRIMACK PHARMACEUTICALS, INC.
ONE KENDALL SQUARE, SUITE B7201

(Street)
CAMBRIDGE MA 02139

(City) (State) (Zip)
2. Date of Event Requiring Statement (Month/Day/Year)
01/31/2012
3. Issuer Name and Ticker or Trading Symbol
MERRIMACK PHARMACEUTICALS INC [ MACK ]
4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
Director 10% Owner
X Officer (give title below) Other (specify below)
SVP and CSO
5. If Amendment, Date of Original Filed (Month/Day/Year)
6. Individual or Joint/Group Filing (Check Applicable Line)
X Form filed by One Reporting Person
Form filed by More than One Reporting Person
Table I - Non-Derivative Securities Beneficially Owned
1. Title of Security (Instr. 4) 2. Amount of Securities Beneficially Owned (Instr. 4) 3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5) 4. Nature of Indirect Beneficial Ownership (Instr. 5)
Common Stock 247,443 D
Table II - Derivative Securities Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4) 2. Date Exercisable and Expiration Date (Month/Day/Year) 3. Title and Amount of Securities Underlying Derivative Security (Instr. 4) 4. Conversion or Exercise Price of Derivative Security 5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5) 6. Nature of Indirect Beneficial Ownership (Instr. 5)
Date Exercisable Expiration Date Title Amount or Number of Shares
Stock Option (right to buy) (1) 08/02/2012 Common Stock 4,368 2.19 D
Stock Option (right to buy) (1) 05/08/2013 Common Stock 10,483 2.19 D
Stock Option (right to buy) (1) 08/30/2014 Common Stock 150,000 1.25 D
Stock Option (right to buy) (1) 08/03/2015 Common Stock 82,977 1.71 D
Stock Option (right to buy) (1) 08/03/2015 Common Stock 17,023 1.71 D
Stock Option (right to buy) (1) 10/03/2016 Common Stock 48,175 2.47 D
Stock Option (right to buy) (1) 10/03/2016 Common Stock 26,825 2.47 D
Stock Option (right to buy) (1) 10/04/2017 Common Stock 53,378 2.59 D
Stock Option (right to buy) (1) 10/04/2017 Common Stock 146,622 2.59 D
Stock Option (right to buy) (1) 09/21/2018 Common Stock 250,000 1.81 D
Stock Option (right to buy) (2) 11/04/2019 Common Stock 180,000 2.12 D
Stock Option (right to buy) (3) 01/31/2020 Common Stock 100,000 2.12 D
Stock Option (right to buy) (4) 10/14/2020 Common Stock 60,000 2.69 D
Stock Option (right to buy) (5) 12/21/2020 Common Stock 50,000 2.69 D
Stock Option (right to buy) (6) 05/02/2021 Common Stock 100,000 5.54 D
Explanation of Responses:
1. This option is fully vested.
2. This option vested as to 1/12th of the shares on each of November 5, 2009 and February 1, 2010 and vests in equal quarterly installments thereafter until August 1, 2012.
3. This option vested as to 1/12th of the shares on April 1, 2010 and vests in equal quarterly installments thereafter until January 1, 2013.
4. This option vested as to 1/12th of the shares on each of October 15, 2010 and January 1, 2011 and vests in equal quarterly installments thereafter until July 1, 2013.
5. This option vested as to 1/6th of the shares on January 1, 2011 and vests in equal quarterly installments thereafter until July 1, 2013.
6. This option vested as to 1/12th of the shares on August 1, 2011 and vests in equal quarterly installments thereafter until May 1, 2014.
/s/ Jeffrey A. Munsie, attorney-in-fact 02/01/2012
** Signature of Reporting Person Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 5 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.
LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

       Know all by these presents, that the undersigned hereby 
makes, constitutes and appoints each of William A. Sullivan, 
Jeffrey A. Munsie, Ian R. Kaminski and Christopher J. 
Frankenfield, signing singly and each acting individually, as the 
undersigned's true and lawful attorney-in-fact with full power 
and authority as hereinafter described to:
       (1)        execute for and on behalf of the undersigned, in 
the undersigned's capacity as an Officer and a Director of 
Merrimack Pharmaceuticals, Inc. (the "Company"), Forms 3, 4, and 
5 (including any amendments thereto) in accordance with 
Section 16(a) of the Securities Exchange Act of 1934 and the 
rules thereunder (the "Exchange Act");
       (2)        do and perform any and all acts for and on 
behalf of the undersigned which may be necessary or desirable to 
prepare, complete and execute any such Form 3, 4, or 5, prepare, 
complete and execute any amendment or amendments thereto, and 
timely deliver and file such form with the United States 
Securities and Exchange Commission (the "SEC") and any stock 
exchange or similar authority, including without limitation the 
filing of a Form ID or any other application materials to enable 
the undersigned to gain
 or maintain access to the Electronic Data 
Gathering, Analysis and Retrieval system of the SEC;
       (3)        seek or obtain, as the undersigned's 
representative and on the undersigned's behalf, information 
regarding transactions in the Company's securities from any third 
party, including brokers, employee benefit plan administrators 
and trustees, and the undersigned hereby authorizes any such 
third party to release any such information to the herein 
appointed attorney-in-fact and approves and ratifies any such 
release of information; and
       (4)        take any other action of any type whatsoever in 
connection with the foregoing which, in the opinion of such 
attorney-in-fact, may be of benefit to, in the best interest of, 
or legally required by, the undersigned, it being understood that 
the documents executed by such attorney-in-fact on behalf of the 
undersigned pursuant to this Power of Attorney shall be in such 
form and shall contain such terms and conditions as such 
attorney-in-fact may approve in such attorney-in-fact's 
discretion.
       The undersigned hereby grants to each such attorney-in-fact 
full power and authority to do and perform any and every act and 
thing whatsoever requisite, necessary, or proper to be done in 
the exercise of any of the rights and powers herein granted, as 
fully to all intents and purposes as the undersigned might or 
could do if personally present, with full power of substitution 
or revocation, hereby ratifying and confirming all that such 
attorney-in-fact, or such attorney-in-fact's substitute or 
substitutes, shall lawfully do or cause to be done by virtue of 
this power of attorney and the rights and powers herein granted.  
The undersigned acknowledges that the foregoing 
attorneys-in-fact, in serving in such capacity at the request of 
the undersigned, are not assuming nor relieving, nor is the 
Company assuming nor relieving, any of the undersigned's 
responsibilities to comply with Section 16 of the Exchange Act.  
The undersigned acknowledges that neither the Company nor the 
foregoing attorneys-in-fact assume (i) any liability for the 
undersigned's responsibility to comply with the requirement of 
the Exchange Act, (ii) any liability of the undersigned for any 
failure to comply with such requirements, or (iii) any obligation 
or liability of the undersigned for profit disgorgement under 
Section 16(b) of the Exchange Act.
       This Power of Attorney shall remain in full force and 
effect until the undersigned is no longer required to file 
Forms 3, 4, and 5 with the SEC with respect to the undersigned's 
holdings of and transactions in securities issued by the Company, 
unless earlier revoked by the undersigned in a signed writing 
delivered to the foregoing attorneys-in-fact.
       IN WITNESS WHEREOF, the undersigned has caused this Power 
of Attorney to be executed as of this 4th day of October, 
2011.

/s/ Ulrik B. Nielsen, Ph.D.	
	
Ulrik B. Nielsen, Ph.D.