X0306 5 2012-12-31 0 0 0 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001094201 CROCKER GARY L C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 1 0 0 0 Common Stock 2012-12-19 5 G 0 39000 0 D 2492331 I By Crocker Ventures, LLC Common Stock 2012-12-19 5 G 0 39000 0 A 2531331 I By Wife Common Stock 2012-12-19 5 G 0 90090 0 D 2441241 I By Wife Common Stock 2012-12-19 5 G 0 15015 0 A 2456256 I By Daughter Common Stock 2012-12-19 5 G 0 15015 0 A 2471271 I By Son This transaction involved the gift of securities by Crocker Ventures, LLC to the reporting person's wife. This transaction involved the gift of securities by the reporting person's wife to, among others, the reporting person's daughter and son, who share the reporting person's household. /s/ Jeffrey A. Munsie, attorney-in-fact 2013-02-13