Exhibit 4.1



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Exhibit 4.1

COUNTERSIGNED AND REGISTERED:

AMERICAN STOCK TRANSFER & TRUST COMPANY

(NEW YORK, NY)

TRANSFER AGENT AND REGISTRAR

BY

COMMON STOCK

COMMON STOCK

SEE REVERSE FOR

CERTAIN DEFINITIONS

CUSIP 45665G 30 3

Infinity Pharmaceuticals, Inc.

INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE

THIS CERTIFIES THAT

SPECIMEN

is the owner of

FULLY PAID AND NON-ASSESSABLE SHARES OF COMMON STOCK, $0.001 PAR VALUE, OF

INFINITY PHARMACEUTICALS, INC.

transferable on the books of the corporation by such owner in person or by attorney upon surrender of this certificate duly
endorsed or assigned. This certificate and the shares represented hereby are subject to the laws of the State of Delaware and the Certificate of Incorporation and By-Laws of the corporation, as now in effect or hereinafter amended. This certificate
is not valid until countersigned and registered by the Transfer Agent and Registrar.

WITNESS the facsimile seal of the
corporation and the facsimile signatures of its duly authorized officers.

Dated:

SECRETARY

PRESIDENT

AUTHORISED SIGNATURE

AMERICAN BANK NOTE COMPANY.



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INFINITY PHARMACEUTICALS, INC.



The Corporation is authorized to issue more than one class of stock. The Corporation will furnish without charge to each
stockholder who so requests a statement of the powers, designations, preferences, and relative, participating, optional or other special rights of each class of stock or series thereof of the Corporation, and the qualifications, limitations or
restrictions of such preferences and/or rights.



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 |  |        
|TEN ENT      |  | as tenants by the entireties                                             |  |                    |  | (Cust)                                                   |  |           |  | (Minor)
|JT TEN       |  | as joint tenants with right of survivorship and not as tenants in common |  |                    |  | under Uniform Gifts to Minors Act_______________ (State)



Additional
abbreviations may also be used though not in the above list.



For value received, __________________________________________ hereby sell, assign and transfer, unto



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|PLEASE INSERT SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF ASSIGNEE              |  | 
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|Please print or typewrite name and address, including postal zip code, of assignee
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|                                                                                                            |  | Shares
|of the Common Stock represented by the within Certificate, and do hereby irrevocably constitute and appoint

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|Attorney to transfer the said stock on the books of the within-named Corporation with full power of substitution in the premises.
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|Dated ____________________________________                                                                                       

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|Signature(s) Guaranteed:                                                                                                                                                                                                                              
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|THE SIGNATURE(S) MUST 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.



KEEP THIS CERTIFICATE IN A SAFE
PLACE, IF IT IS LOST, STOLEN, MUTILATED OR DESTROYED, THE CORPORATION MAY REQUIRE A BOND OF INDEMNITY AS A CONDITION TO THE ISSUANCE OF A REPLACEMENT CERTIFICATE.



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.