Initial Statement of Beneficial Ownership (3)

FORM 3	UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES	OMB APPROVAL OMB Number: 3235-0104 Estimated average burden hours per response... 0.5
	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 ^*

Bain Capital Life Sciences Investors, LLC

2. Date of Event Requiring Statement (MM/DD/YYYY) 9/19/2017

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)

(Street)

BOSTON, MA 02116

(City) (State) (Zip)

5. If Amendment, Date Original Filed (MM/DD/YYYY)

6. Individual or Joint/Group Filing (Check Applicable Line)

___ Form filed by One Reporting Person
_ X _ Form filed by More than One Reporting Person

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	5250000	I	See Footnotes ⁽¹⁾ ⁽²⁾ ⁽³⁾

*Table II - Derivative Securities Beneficially Owned ( e.g.* , puts, calls, warrants, options, convertible securities)**
1. Title of Derivate Security (Instr. 4)	2. Date Exercisable and Expiration Date (MM/DD/YYYY)		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)
1. Title of Derivate Security (Instr. 4)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	4. Conversion or Exercise Price of Derivative Security		6. Nature of Indirect Beneficial Ownership (Instr. 5)

Explanation of Responses:
(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.

Reporting Owners
Reporting Owner Name / Address	Relationships
Reporting Owner Name / Address	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		9/28/2017
^** 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		9/28/2017
^** 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		9/28/2017
^** 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		9/28/2017
^** Signature of Reporting Person		Date
/s/ Jeffrey Schwartz		9/28/2017
^** Signature of Reporting Person		Date
/s/ Adam Koppel		9/28/2017
^** Signature of Reporting Person		Date

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
*	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).
Note:	File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
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