Sec Form 4 Filing - ARCH Venture Partners VIII, LLC @ Sienna Biopharmaceuticals, Inc. - 2017-08-01

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 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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1. Name and Address of Reporting Person ARCH Venture Partners VIII, LLC 2. Issuer Name and Ticker or Trading Symbol Sienna Biopharmaceuticals, Inc. [ SNNA] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O ARCH VENTURE PARTNERS, 8755 W. HIGGINS ROAD, SUITE 1025 3. Date of Earliest Transaction (MM/DD/YY) 08/01/2017 (Street) CHICAGO, IL60631 4. If Amendment, Date Original Filed (MM/DD/YY) 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 Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction 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 08/01/2017 C 2,538,641 A 2,538,641 I See Footnote ( 2 ) Common Stock 08/01/2017 C 211,934 A 2,750,575 I See Footnote ( 2 ) Common Stock 08/01/2017 C 563,072 A 563,072 I See Footnote ( 3 ) Common Stock 08/01/2017 C 319,421 A 882,493 I See Footnote ( 3 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction 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 (MM/DD/YY) 7. Title and Amount of Underlying Securities (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 Series A-3 Preferred Stock ( 1 ) 08/01/2017 C 2,538,641 ( 1 ) ( 4 ) Common Stock 2,538,641 ( 1 ) 0 I See Footnote ( 2 ) Series A-3 Preferred Stock ( 1 ) 08/01/2017 C 563,072 ( 1 ) ( 4 ) Common Stock 563,072 ( 1 ) 0 I See Footnote ( 3 ) Series B Preferred Stock ( 1 ) 08/01/2017 C 211,934 ( 1 ) ( 4 ) Common Stock 211,934 ( 1 ) 0 I See Footnote ( 2 ) Series B Preferred Stock ( 1 ) 08/01/2017 C 319,421 ( 1 ) ( 4 ) Common Stock 319,421 ( 1 ) 0 I See Footnote ( 3 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other ARCH Venture Partners VIII, LLC C/O ARCH VENTURE PARTNERS 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X ARCH Venture Partners VIII, L.P. C/O ARCH VENTURE PARTNERS VIII, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X ARCH Venture Fund VIII, L.P. C/O ARCH VENTURE PARTNERS VIII, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X ARCH Venture Fund VIII Overage, L.P. C/O ARCH VENTURE PARTNERS VIII, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X BYBEE CLINTON C/O ARCH VENTURE PARTNERS VIII, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X CRANDELL KEITH C/O ARCH VENTURE PARTNERS VIII, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X Signatures ARCH Venture Fund VIII, L.P. By: ARCH Venture Partners VIII, L.P. By: /s/Mark McDonnell, as Attorney-in-Fact for Keith Crandell, the Mgng Dir of ARCH Venture Partners VIII LLC, the GP of ARCH Venture Partners VIII, L.P., the GP of ARCH Venture Fund VIII 08/01/2017 Signature of Reporting Person Date ARCH Venture Fund VIII Overage, L.P. By: ARCH Venture Partners VIII, LLC By: /s/ Mark McDonnell, as Attorney-in-Fact for Keith Crandell, the Managing Director of ARCH Venture Partners VIII LLC, the GP of ARCH Venture Fund VIII Overage, L.P. 08/01/2017 Signature of Reporting Person Date ARCH Venture Partners VIII, L.P. By: ARCH Venture Partners VIII, LLC By: /s/ Mark McDonnell as Attorney-in-Fact for Keith Crandell, the Managing Director of ARCH Venture Partners VIII LLC, the GP of ARCH Venture Partners VIII, L.P. 08/01/2017 Signature of Reporting Person Date ARCH Venture Partners VIII, LLC By: /s/ Mark McDonnell as Attorney-in-Fact for Keith Crandell, the Managing Director of ARCH Venture Partners VIII, LLC 08/01/2017 Signature of Reporting Person Date /s/ Mark McDonnell, as Attorney-in-Fact for Keith Crandell 08/01/2017 Signature of Reporting Person Date /s/ Mark McDonnell, as Attorney-in-Fact for Clinton Bybee 08/01/2017 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The shares of the Issuer's Preferred Stock automatically converted into shares of the Issuer's Common Stock, for no additional consideration, on a 1:1 basis immediately prior to the consummation of the Issuer's initial public offering. ( 2 )The shares are directly held by ARCH Venture Fund VIII, L.P. ("ARCH VIII"). ARCH Venture Partners VIII, L.P. (the "GPLP"), as the sole general partner of ARCH VIII, may be deemed to beneficially own certain of the shares held by ARCH VIII. ARCH Venture Partners VIII, LLC ("GPLLC"), as the sole general partner of GPLP, may be deemed to beneficially own the shares held by ARCH VIII. Keith Crandell and Clinton Bybee are managing d irectors of GPLLC, and as such may be deemed to beneficially own the shares held by ARCH VIII. Each Reporting Person disclaims beneficial ownership of such shares except to the extent of any pecuniary interest therein. ( 3 )The shares are directly held by ARCH Venture Fund VIII Overage, L.P. ("ARCH Overage"). GPLLC, as the sole general partner of ARCH Overage, may be deemed to beneficially own the shares held by ARCH Overage. Keith Crandell and Clinton Bybee are managing directors of GPLLC, and as such may be deemed to beneficially own the shares held by ARCH Overage. Each Reporting Person disclaims beneficial ownership of such shares except to the extent of any pecuniary interest therein. ( 4 )The expiration date is not relevant to the conversion of these securities.

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