X0206 3/A 2017-07-26 2017-07-26 0 0001656328 Sienna Biopharmaceuticals, Inc. SNNA 0001616165 ARCH Venture Partners VIII, LLC C/O ARCH VENTURE PARTNERS 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 Series A-3 Preferred Stock Common Stock 2538641 I See Footnote This amendment is filed solely to include a copy of a power of attorney form which was inadvertently omitted from the original filing. One line in the original filing has been restated without restating the original footnotes in order to gain access to the filing system. None of the information in the body of the original filing is incorrect or being amended via this amendment. 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 Managing Director of ARCH Venture Partners VIII LLC, the GP of ARCH Venture Partners VIII, L.P., the GP of ARCH Venture Fu 2017-07-31 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. 2017-07-31 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. 2017-07-31 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 2017-07-31 /s/ Mark McDonnell, as Attorney-in-Fact for Keith Crandell 2017-07-31 /s/ Mark McDonnell, as Attorney-in-Fact for Clinton Bybee 2017-07-31