Exhibit
4.1


 




	
  

  	
  THIS CERTIFIES
  THAT is the owner of CUSIP DATED COUNTERSIGNED AND REGISTERED: COMPUTERSHARE
  TRUST COMPANY, N.A. TRANSFER AGENT AND REGISTRAR, FULLY-PAID AND
  NON-ASSESSABLE SHARES OF THE COMMON STOCK OF Merrimack Pharmaceuticals, Inc.
  (the “Company”), transferable on the books of the Company in person or by
  duly authorized attorney, upon surrender of this Certificate properly
  endorsed. This Certificate and the shares represented hereby are issued and
  shall be held subject to all of the provisions of the Certificate of
  Incorporation and the Bylaws of the Company, each as amended and/or restated
  from time to time (copies of which are on file with the Company and the
  Transfer Agent), to all of which each holder, by acceptance hereof, assents.
  This Certificate is not valid unless countersigned and registered by the
  Transfer Agent and Registrar. Witness the facsimile seal of the Company and
  the facsimile signatures of its duly authorized officers. COMMON STOCK $0.01
  PAR VALUE PER SHARE COMMON STOCK THIS CERTIFICATE IS TRANSFERABLE IN CANTON,
  MA AND NEW YORK, NY SEE REVERSE FOR CERTAIN DEFINITIONS Certificate Number
  Shares  MERRIMACK PHARMACEUTICALS, INC.
  INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE President Treasurer By
  AUTHORIZED SIGNATURE 016570| 003590|127C|RESTRICTED||4|057-423 590328 10 0
  <<Month Day, Year>> * * 000000* * * * * * * * * 000000* * * * * *
  * * * 000000* * * * * * * * * 000000* * * * * * * * * 000000* * ** Mr.
  Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander
  David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample
  **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.
  Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander
  David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample
  **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.
  Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander
  David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample
  **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.
  Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander
  David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample
  **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.
  Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander
  David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample
  **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.
  Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander
  David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample
  **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander
  David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample
  **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.
  Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander
  David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample
  **** Mr. Sample **** Mr. Sample
  **000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares***
  *000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****
  000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****0
  00000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****00
  0000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000
  000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****0000
  00**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****00000
  0**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000
  **Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000*
  *Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**
  Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**S
  ***ZERO HUNDRED THOUSAND ZERO HUNDRED AND ZERO*** MR. SAMPLE & MRS.
  SAMPLE & MR. SAMPLE & MRS. SAMPLE NNNNN ZQ 000000 Certificate Numbers
  1234567890/1234567890 1234567890/1234567890 1234567890/1234567890
  1234567890/1234567890 1234567890/1234567890 1234567890/1234567890 Total
  Transaction Num/No. 123456 Denom. 123456 Total 1234567 MR A SAMPLE
  DESIGNATION (IF ANY) ADD 1 ADD 2 ADD 3 ADD 4 Merrimack Pharmaceuticals, Inc.
  PO BOX 43004, Providence, RI 02940-3004 CUSIP XXXXXX XX X Holder ID
  XXXXXXXXXX Insurance Value 1,000,000.00 Number of Shares 123456 DTC 12345678
  123456789012345 Num/No. 1234567

  






 


 












	
  

  	
  The IRS
  requires that we report the cost basis of certain shares acquired after
  January 1, 2011. If your shares were covered by the legislation and you have
  sold or transferred the shares and requested a specific cost basis
  calculation method, we have processed as requested. If you did not specify a
  cost basis calculation method, we have defaulted to the first in, first out
  (FIFO) method. Please visit our website or consult your tax advisor if you
  need additional information about cost basis. If you do not keep in contact
  with us or do not have any activity in your account for the time periods
  specified by state law, your property could become subject to state unclaimed
  property laws and transferred to the appropriate state. For value received,
  hereby sell, assign and transfer unto Shares Attorney Dated: 20 Signature:  Signature: Notice: The signature to this
  assignment must correspond with the name as written upon the face of the
  certificate, in every particular, without alteration or enlargement, or any change
  whatever. PLEASE INSERT SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF
  ASSIGNEE (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING POSTAL ZIP
  CODE, OF ASSIGNEE) of the common stock represented by the within Certificate,
  and do hereby irrevocably constitute and appoint to transfer the said stock
  on the books of the within-named Company with full power of substitution in
  the premises. MERRIMACK PHARMACEUTICALS, INC. THE COMPANY WILL FURNISH
  WITHOUT CHARGE TO EACH STOCKHOLDER WHO SO REQUESTS A SUMMARY OF THE POWERS,
  DESIGNATIONS, PREFERENCES AND RELATIVE, PARTICIPATING, OPTIONAL OR OTHER
  SPECIAL RIGHTS OF EACH CLASS OF STOCK OF THE COMPANY AND THE QUALIFICATIONS,
  LIMITATIONS OR RESTRICTIONS OF SUCH PREFERENCES AND RIGHTS, AND THE
  VARIATIONS IN RIGHTS, PREFERENCES AND LIMITATIONS DETERMINED FOR EACH SERIES,
  WHICH ARE FIXED BY THE CERTIFICATE OF INCORPORATION OF THE COMPANY, AS
  AMENDED AND/OR RESTATED FROM TIME TO TIME, AND THE RESOLUTIONS OF THE BOARD
  OF DIRECTORS OF THE COMPANY, AND THE AUTHORITY OF THE BOARD OF DIRECTORS TO
  DETERMINE VARIATIONS FOR FUTURE SERIES. SUCH REQUEST MAY BE MADE TO THE
  OFFICE OF THE SECRETARY OF THE COMPANY OR TO THE TRANSFER AGENT. THE OWNER OF
  A LOST OR DESTROYED STOCK CERTIFICATE, OR HIS LEGAL REPRESENTATIVES, MAY BE
  REQUIRED TO GIVE THE COMPANY A BOND TO INDEMNIFY IT AND ITS TRANSFER AGENTS
  AND REGISTRARS AGAINST ANY CLAIM THAT MAY BE MADE AGAINST THEM ON ACCOUNT OF
  THE ALLEGED LOSS OR DESTRUCTION OF ANY SUCH CERTIFICATE. Signature(s)
  Guaranteed: Medallion Guarantee Stamp THE SIGNATURE(S) SHOULD BE GUARANTEED
  BY AN ELIGIBLE GUARANTOR INSTITUTION (Banks, Stockbrokers, Savings and Loan
  Associations and Credit Unions) WITH MEMBERSHIP IN AN APPROVED SIGNATURE
  GUARANTEE MEDALLION PROGRAM, PURSUANT TO S.E.C. RULE 17Ad-15. The following
  abbreviations, when used in the inscription on the face of this certificate,
  shall be construed as though they were written out in full according to
  applicable laws or regulations: TEN COM - as tenants in common UNIF GIFT MIN
  ACT - Custodian (Cust) (Minor) TEN ENT - as tenants by the entireties under
  Uniform Gifts to Minors Act (State) JT TEN - as joint tenants with right of
  survivorship UNIF TRF MIN ACT - Custodian (until age ) and not as tenants in
  common (Cust) under Uniform Transfers to Minors Act (Minor) (State)
  Additional abbreviations may also be used though not in the above list.