Ownership Submission
FORM 5
Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
Form 3 Holdings Reported
Form 4 Transactions Reported
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

ANNUAL STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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1. Name and Address of Reporting Person *
Mulroy Robert J.
 2. Issuer Name and Ticker or Trading Symbol
MERRIMACK PHARMACEUTICALS INC [MACK] 		5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
__X__ Director _____ 10% Owner
__X__ Officer (give title below) _____ Other (specify below)
President and CEO
(Last)
(First)
(Middle)

C/O MERRIMACK PHARMACEUTICALS, INC., ONE KENDALL SQUARE, SUITE B7201		 3. Statement for Issuer's Fiscal Year Ended (Month/Day/Year)
12/31/2012
(Street)


CAMBRIDGE, MA 02139		 4. If Amendment, Date Original Filed(Month/Day/Year)
 6. Individual or Joint/Group Reporting
(check applicable line)

_X_ Form Filed by One Reporting Person
___ Form Filed by More than One Reporting Person
(City)
(State)
(Zip)
Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3)		 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code
(Instr. 8)		 4. Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4 and 5)		 5. Amount of Securities Beneficially Owned at end of Issuer's Fiscal Year
(Instr. 3 and 4)		 6. Ownership Form: Direct (D) or Indirect (I)
(Instr. 4)		 7. Nature of Indirect Beneficial Ownership
(Instr. 4)
Amount (A) or (D) Price
Common Stock (4) 04/03/2012   C4 142,850 A $ (1) 748,669 I By Trust
Common Stock (4) 04/03/2012   C4 17,142 A $ (2) 765,811 I By Trust
Common Stock (5) 06/15/2012   M 75,000 A $ 2.19 840,811 D  

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly. 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 control number. SEC 2270 (9-02)

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security
(Instr. 3)		 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (Month/Day/Year) 3A. Deemed Execution Date, if any (Month/Day/Year) 4. Transaction Code
(Instr. 8)		 5. Number of Derivative Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4, and 5)		 6. Date Exercisable and Expiration Date
(Month/Day/Year)		 7. Title and Amount of Underlying Securities
(Instr. 3 and 4)		 8. Price of Derivative Security
(Instr. 5)		 9. Number of Derivative Securities Beneficially Owned at End of Issuer's Fiscal Year
(Instr. 4)		 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 4)		 11. Nature of Indirect Beneficial Ownership
(Instr. 4)
(A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares
Series D Convertible Preferred Stock (3)       3       (1)   (1) Common Stock
142,850
  199,993
I
By Trust
Series G Convertible Preferred Stock (3)       3       (2)   (2) Common Stock
17,142
  17,142
I
By Trust
Series D Convertible Preferred Stock (4)   04/03/2012   C4   142,850   (1)   (1) Common Stock
142,850
$ 0 0
I
By Trust
Series G Convertible Preferred Stock (4)   04/03/2012   C4   17,142   (2)   (2) Common Stock
17,142
$ 0 0
I
By Trust

Reporting Owners

Reporting Owner Name / Address Relationships
Director 10% Owner Officer Other
Mulroy Robert J.
C/O MERRIMACK PHARMACEUTICALS, INC.
ONE KENDALL SQUARE, SUITE B7201
CAMBRIDGE, MA 02139		   X     President and CEO  

Signatures

/s/ Jeffrey A. Munsie, attorney-in-fact 02/13/2013
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 4(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).
(1) Each share of Series D Convertible Preferred Stock automatically converted into one share of Common Stock upon the closing of the issuer's initial public offering and had no expiration date.
(2) Each share of Series G Convertible Preferred Stock automatically converted into one share of Common Stock upon the closing of the issuer's initial public offering and had no expiration date.
(3) These shares are held by a family trust of which the reporting person is a trustee. Such shares were inadvertently omitted from the reporting person's Form 3 filed on February 1, 2012.
(4) These shares are held by a family trust of which the reporting person is a trustee. This transaction was inadvertently omitted from the reporting person's Form 4 filed on April 5, 2012.
(5) This transaction was initially reported on a Form 4 filed by the reporting person on June 19, 2012 and is being amended to correct the amount of securities beneficially owned following the reported transaction.

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