X0303 4 2010-07-26 0 0001373671 Isilon Systems, Inc. ISLN 0001381752 McIlwain Matthew S 1000 SECOND AVENUE SUITE 3700 SEATTLE WA 98104 1 0 1 0 Common Stock 2010-07-26 4 J 0 25541 A 60501 D Common Stock 2010-07-26 4 J 0 1200015 D 5823722 I See footnote Distribution of shares by Madrona Venture Fund I-A, LP, Madrona Venture Fund I-B, LP and Madrona Managing Director Fund, LLC to their respective limited partners and members. The reporting person is a managing director and member of various entities affiliated with Madrona Venture Group, LLC. Not applicable. 4,000 shares are held through reporting person's 401k plan. Distribution of shares by Madrona Venture Fund I-A, LP, Madrona Venture Fund I-B, LP and Madrona Managing Director Fund, LLC to their respective limited partners and members. The reporting person is a managing director and member of various entities affiliated with Madrona Venture Group, LLC. The reporting person does not have sole voting or dispositive authority over these shares and disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein. Represents 4,695,625 shares held by Madrona Venture Fund I-A, LP, 511,049 shares held by Madrona Venture Fund I-B, LP and 617,048 shares held by Madrona Managing Director Fund, LLC. The reporting person is a managing director and member of various entities affiliated with Madrona Venture Group, LLC. The reporting person does not have sole voting or dispositive authority over these shares and disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein. Troy Cichos, Attorney-in-Fact 2010-07-28