X0303 4 2010-12-17 1 0001373671 Isilon Systems, Inc. ISLN 0001381752 McIlwain Matthew S 1000 SECOND AVENUE SUITE 3700 SEATTLE WA 98104 1 0 0 0 Common Stock 2010-12-17 2010-12-17 4 U 0 5823722 33.85 D 0 I See footnote Common Stock 2010-12-17 2010-12-17 4 U 0 48001 33.85 D 0 D 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. Mr. McIlwain is a director of Issuer. /s/ Matthew S. McIlwain, By Troy Cichos as attorney-in-fact 2010-12-21