X0305 4 2012-04-03 0 0001274792 MERRIMACK PHARMACEUTICALS INC MACK 0001032566 GAY ROBERT C C/O MERRIMACK PHARMACEUTICALS, INC. ONE KENDALL SQUARE, SUITE B7201 CAMBRIDGE MA 02139 1 0 0 0 Common Stock 2012-04-03 4 C 0 175316 A 175316 D Common Stock 2012-04-03 4 C 0 142857 A 318173 D Common Stock 2012-04-03 4 C 0 175316 A 493489 I See footnote Common Stock 2012-04-03 4 C 0 142857 A 636346 I See footnote Series B Convertible Preferred Stock 2012-04-03 4 C 0 113586 0 D Common Stock 175316 0 D Series D Convertible Preferred Stock 2012-04-03 4 C 0 142857 0 D Common Stock 142857 0 D Series B Convertible Preferred Stock 2012-04-03 4 C 0 113586 0 D Common Stock 175316 0 I See footnote Series D Convertible Preferred Stock 2012-04-03 4 C 0 142857 0 D Common Stock 142857 0 I See footnote Each share of Series B Convertible Preferred Stock automatically converted into approximately 1.5435 shares of Common Stock upon the closing of the issuer's initial public offering and had no expiration date. 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. These shares are held by the Robert C. Gay 1998 Family Trust, of which Dr. Gay is the grantor. Dr. Gay disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest therein, and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of any such shares for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purpose. /s/ Jeffrey A. Munsie, attorney-in-fact 2012-04-05