FORM 4 [ ] Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).         
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES
                                                                                  
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Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940
                       
1. Name and Address of Reporting Person * DAWES KAREN A 2. Issuer Name and Ticker or Trading Symbol REPLIGEN CORP [ RGEN ] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable)__X__ Director                    _____ 10% Owner
_____ Officer (give title below)    _____ Other (specify below)
(Last)         (First)         (Middle)
C/O REPLIGEN CORPORATION, 41 SEYON ST., BLDG 1, STE 100		 3. Date of Earliest Transaction (MM/DD/YYYY)
4/20/2020
(Street)
WALTHAM, MA 02453
(City)       (State)       (Zip)
 4. If Amendment, Date Original Filed (MM/DD/YYYY)
 
 6. Individual or Joint/Group Filing (Check Applicable Line) _X _ Form filed by One Reporting Person
___ Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3)		 2. Trans. Date 2A. Deemed Execution Date, if any 3. Trans. Code
(Instr. 8)		 4. Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4 and 5)		 5. Amount of Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 3 and 4)		 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4)
Code V Amount (A) or (D) Price
Common Stock  4/20/2020    M    8200  A $3.30  121234  D   
Common Stock  4/20/2020    S(1)    3202  D $104.98 (2) 118032  D   
Common Stock  4/20/2020    S(1)    8998  D $104.42 (3) 109034  D   
Common Stock  4/21/2020    M    11800  A $3.30  120834  D   

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivate Security
(Instr. 3)		 2. Conversion or Exercise Price of Derivative Security 3. Trans. Date 3A. Deemed Execution Date, if any 4. Trans. Code
(Instr. 8)		 5. Number of Derivative Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4 and 5)		 6. Date Exercisable and Expiration Date 7. Title and Amount of Securities Underlying Derivative Security
(Instr. 3 and 4)		 8. Price of Derivative Security
(Instr. 5)		 9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4)
Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares
Stock Option (Right to Buy)  $3.30  4/20/2020    M        8200    (4) 9/16/2020  Common Stock  8200  $0.00  11800  D   
Stock Option (Right to Buy)  $3.30  4/21/2020    M        11800    (4) 9/16/2020  Common Stock  11800  $0.00  0  D   

Explanation of Responses:
(1)  Sale of common stock was effected pursuant to a 10(b)5-1 trading plan adopted on February 19, 2020.
(2)  $104.98 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $104.73 to $105.39, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range.
(3)  $104.42 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $103.72 to $104.70, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range.
(4)  This option is currently exercisable

Reporting Owners
Reporting Owner Name / Address
Relationships
Director 10% Owner Officer Other
DAWES KAREN A
C/O REPLIGEN CORPORATION
41 SEYON ST., BLDG 1, STE 100
WALTHAM, MA 02453		 X



Signatures
/s/ Jeffrey P. Leduc (Attorney in Fact) 4/22/2020
**Signature of Reporting Person Date