Ownership Submission
FORM 3
 UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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(Print or Type Responses)
1. Name and Address of Reporting Person *
  Bain Capital Life Sciences Investors, LLC
 2. Date of Event Requiring Statement (Month/Day/Year)
09/19/2017		 3. Issuer Name and Ticker or Trading Symbol
MARINUS PHARMACEUTICALS INC [MRNS]
(Last)
(First)
(Middle)
200 CLARENDON STREET		 4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
_____ Director __X__ 10% Owner
_____ Officer (give title below) _____ Other (specify below)
5. If Amendment, Date Original Filed(Month/Day/Year)
(Street)

BOSTON, MA 02116		 6. Individual or Joint/Group Filing(Check Applicable Line)
___ Form filed by One Reporting Person
_X_ Form filed by More than One Reporting Person
(City)
(State)
(Zip)
Table I - Non-Derivative Securities Beneficially Owned
1.Title of Security
(Instr. 4)		 2. Amount of Securities Beneficially Owned
(Instr. 4)		 3. Ownership Form: Direct (D) or Indirect (I)
(Instr. 5)		 4. Nature of Indirect Beneficial Ownership
(Instr. 5)
Common Stock 5,250,000
I
See Footnotes (1) (2) (3)

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly. SEC 1473 (7-02)
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Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security
(Instr. 4)		 2. Date Exercisable and Expiration Date
(Month/Day/Year)		 3. Title and Amount of Securities Underlying Derivative Security
(Instr. 4)		 4. Conversion or Exercise Price of Derivative Security 5. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 5)		 6. Nature of Indirect Beneficial Ownership
(Instr. 5)
Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners

Reporting Owner Name / Address Relationships
Director 10% Owner Officer Other
Bain Capital Life Sciences Investors, LLC
200 CLARENDON STREET
BOSTON, MA 02116		     X    
Bain Capital Life Sciences Partners, LP
200 CLARENDON STREET
BOSTON, MA 02116		     X    
Bain Capital Life Sciences Fund, L.P.
200 CLARENDON STREET
BOSTON, MA 02116		     X    
BCIP Life Sciences Associates, LP
200 CLARENDON STREET
BOSTON, MA 02116		     X    
Schwartz Jeffrey Lawrence
200 CLARENDON STREET
BOSTON, MA 02116		     X    
Koppel Adam
200 CLARENDON STREET
BOSTON, MA 02116		     X    

Signatures

BAIN CAPITAL LIFE SCIENCES INVESTORS, LLC, By: /s/ Adam Koppel, Name: Adam Koppel, Title: Managing Director 09/28/2017
**Signature of Reporting Person Date
BAIN CAPITAL LIFE SCIENCES PARTNERS, LP, By: Bain Capital Life Sciences Investors, LLC, its general partner, By: /s/ Adam Koppel, Name: Adam Koppel, Title: Managing Director 09/28/2017
**Signature of Reporting Person Date
BAIN CAPITAL LIFE SCIENCES FUND, L.P., By: Bain Capital Life Sciences Partners, LP, its general partner, By: Bain Capital Life Sciences Investors, LLC, its general partner, By: /s/ Adam Koppel, Name: Adam Koppel, Title: Managing Director 09/28/2017
**Signature of Reporting Person Date
BCIP LIFE SCIENCES ASSOCIATES, LP, By: Boylston Coinvestors, LLC, its general partner, By: /s/ Adam Koppel, Name: Adam Koppel, Title: Authorized Signatory 09/28/2017
**Signature of Reporting Person Date
/s/ Jeffrey Schwartz 09/28/2017
**Signature of Reporting Person Date
/s/ Adam Koppel 09/28/2017
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 5(b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1) Bain Capital Life Sciences Investors, LLC ("BCI LS") is the general partner of Bain Capital Life Sciences Partners, LP ("BC LS P"), which is the general partner of Bain Capital Life Sciences Fund, L.P. ("BC LS"). As a result, BC LS P may be deemed to share voting and dispositive power with respect to the securities held by BC LS. BC LS holds 4,762,511 shares of Common Stock.
(2) BCIP Life Sciences Associates, LP (together with BC LS, the "Bain Life Sciences Entities"), whose general partner is Boylston Coinvestors, LLC, holds 487,489 shares of Common Stock.
(3) The governance, investment strategy and decision-making process with respect to the investments held by the Bain Life Sciences Entities is directed by BCI LS, whose managers are Jeffrey Schwartz and Adam Koppel. As a result, BCI LS, Mr. Schwartz and Dr. Koppel may each be deemed to share voting and dispositive power with respect to the securities held by the Bain Life Sciences Entities. BCI LS, Mr. Schwartz and Dr. Koppel each disclaim beneficial ownership of such securities except to the extent of their pecuniary interest therein.

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