X0306 5 2012-12-31 0 1 1 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001531563 Mulroy Robert J. C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 1 1 0 0 President and CEO Common Stock 2012-04-03 4 C 0 L 142850 A 748669 I By Trust Common Stock 2012-04-03 4 C 0 L 17142 A 765811 I By Trust Common Stock 2012-06-15 5 M 0 75000 2.19 A 840811 D Series D Convertible Preferred Stock 3 Common Stock 142850 199993 I By Trust Series G Convertible Preferred Stock 3 Common Stock 17142 17142 I By Trust Series D Convertible Preferred Stock 2012-04-03 4 C 0 L 142850 0 D Common Stock 142850 0 I By Trust Series G Convertible Preferred Stock 2012-04-03 4 C 0 L 17142 0 D Common Stock 17142 0 I By Trust 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. 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. 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. 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. 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. /s/ Jeffrey A. Munsie, attorney-in-fact 2013-02-13