Sec Form 4 Filing - Burow Kristina @ 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 Burow Kristina 2. Issuer Name and Ticker or Trading Symbol Sienna Biopharmaceuticals, Inc. [ SNNA] 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 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) __ X __ Form filed by One Reporting Person _____ 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 Burow Kristina C/O ARCH VENTURE PARTNERS 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X Signatures /s/ Timothy K. Andrews, attorney-in-fact for Kristina Burow 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. The Reporting Person has an interest in the GPLP and the GPLLC, but does not have voting or investment control over the shares held by ARCH VIII. The 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. The Reporting Person has an interest in the GPLLC, but does not have voting or investment control over the shares held by ARCH Overage. The 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.

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 4(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).

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