Exhibit 4.1





Exhibit 4.1

LOGO



COMMON STOCK

COMMON STOCK

PanacosTM

PANACOS PHARMACEUTICALS, INC.

INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE

SEE REVERSE SIDE FOR CERTAIN DEFINITIONS

CUSIP 69811Q 10 6

THIS CERTIFIES that

is the owner of

FULLY PAID AND NON-ASSESSABLE SHARES OF COMMON STOCK OF PAR VALUE OF $.01 PER SHARE OF

PANACOS PHARMACEUTICALS, INC. transferable on the books of the Company by the owner hereof in person or by duly authorized
attorney upon surrender of this certificate properly endorsed. This certificate and the shares represented hereby are issued subject to the laws of the State of Delaware and all the provisions of the Certificate of Incorporation and By-laws of the
Company, each as now in effect or hereafter amended.

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.

Dated:

SECRETARY

PRESIDENT

COUNTERSIGNED AND REGISTERED:

AMERICAN STOCK TRANSFER & TRUST COMPANY

(New York, New York)

TRANSFER AGENT AND REGISTRAR

By

AUTHORIZED SIGNATURE



---

The Company is authorized to issue more than one class and series of stock. The Company will furnish
without charge to each stockholder who so requests in writing a statement of the powers, designations, preferences and relative, participating, optional or other special rights of each class of stock or series thereof and the qualifications,
limitations or restrictions of such preferences and/or rights. Such request may be addressed to the Secretary of the Company or the Transfer Agent and Registrar named on the face of this certificate.



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(s), assign(s), and transfer(s) unto



|                                                                      |  |  |  | 
|--------------------------------------------------------------------- |  |  |  | 
|PLEASE INSERT SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF ASSIGNEE |  |  |  | 
|                                                                      |  |  |  | 



___________________________________________________________________________________________________________

(PLEASE PRINT OR
TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE OF ASSIGNEE)



___________________________________________________________________________________________________________



___________________________________________________________________________________________________________



Shares of the Common Stock represented by the within certificate, and do(es) hereby irrevocably constitute and appoint



Attorney to
transfer the said shares on the books of the within named Company with full power of substitution in the premises.



Dated



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| |  | NOTICE: |  | THE SIGNATURE ON 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.



|                         |  |                                                                                                                                                                                                                                                          |  | 
|------------------------ |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |  | 
|SIGNATURE(S) GUARANTEED: |  |                                                                                                                                                                                                                                                          |  | 
|                         |  | THE SIGNATURE(S) SHOULD BE GURANTEED 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. |  |