X0306 4 2019-03-13 0 0001651235 Acacia Communications, Inc. ACIA 0001657394 Reiss Stan J C/O MATRIX PARTNERS VIII, L.P. 101 MAIN STREET, 17TH FLOOR CAMBRIDGE MA 02142 1 0 0 0 Common Stock 2019-03-13 4 J 0 1000000 0.00 D 0 I By Matrix Partners VIII, L.P. Common Stock 2019-03-13 4 J 0 552 0.00 D 0 I By Weston & Co. VIII LLC Common Stock 2019-03-13 4 J 0 552 0.00 A 47467 I By Matrix VIII US Management Co., L.L.C. Common Stock 2019-03-13 4 J 0 283326 0.00 A 330793 I By Matrix VIII US Management Co., L.L.C. Common Stock 2019-03-13 4 J 0 279343 0.00 D 51450 I By Matrix VIII US Management Co., L.L.C. Common Stock 2019-03-13 4 J 0 2365 0.00 A 26411 I By The Reiss Family Irrevocable Trust Common Stock 2019-03-13 4 J 0 18452 0.00 A 216583 D Common Stock 2019-03-14 4 S 0 3409 53.667 D 23002 I By The Reiss Family Irrevocable Trust Common Stock 2019-03-14 4 S 0 26591 53.487 D 189992 D Represents a pro-rata distribution, and not a purchase or sale, without additional consideration by Matrix Partners VIII, L.P. ("Matrix Partners VIII"), to its partners. The shares are owned directly by Matrix Partners VIII. Matrix VIII US Management Co., L.L.C. ("Matrix VIII US Management Co.") is the General Partner of Matrix Partners VIII and disclaims beneficial ownership of the shares owned by Matrix Partners VIII except to the extent of its proportionate pecuniary interest therein. Mr. Reiss, a member of the Board of Directors of the Issuer and a managing member of Matrix VIII US Management Co., has sole voting and dispositive power with respect to these shares. Mr. Reiss disclaims beneficial ownership of such shares, except to the extent of his pecuniary interest therein. Re-registration of shares by Weston & Co. VIII L.L.C. ("Weston VIII"), without consideration, in the name of Matrix VIII US Management Co., the beneficial owner of those shares. Matrix VIII US Management Co. is the beneficial owner of the shares reported herein as being held of record by Weston VIII, and disclaims beneficial ownership of the shares owned by Weston VIII except to the extent of its proportionate pecuniary interest therein. Mr. Reiss, a member of the Board of Directors of the Issuer and a managing member of Matrix VIII US Management Co., has sole voting and dispositive power with respect to these shares. Mr. Reiss disclaims beneficial ownership of such shares, except to the extent of his pecuniary interest therein. Represents the shares received in connection with the pro-rata distribution by Matrix Partners VIII, without consideration to its partners. Represents pro-rata distribution by Matrix VIII US Management Co., without consideration, to its members. Shares held by The Reiss Family Irrevocable Trust. Mr. Reiss is a trustee and beneficiary of the trust. Represents the shares received in connection with the pro-rata distribution by Matrix VIII US Management Co., without consideration, as described in this Form 4. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $53.39 to $53.83, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (10) to this Form 4. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $53.38 to $53.85, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (11) to this Form 4. /s/ Stan Reiss 2019-03-15