X0303 4 2010-05-03 0 0001373671 Isilon Systems, Inc. ISLN 0001381752 McIlwain Matthew S 1000 SECOND AVENUE SUITE 3700 SEATTLE WA 98104 1 0 1 0 Common Stock 2010-05-03 4 S 0 50000 13.57 D 7073737 I See footnote Common Stock 2010-05-04 4 S 0 50000 13.8808 D 7023737 I See footnote Common Stock 34960 D Represents a weighted-average price. These shares were sold in multiple transactions at prices ranging from $12.97 to $13.83, inclusive. For all transactions reported in this Form 4 utilizing a weighted-average price, the reporting person will provide to the issuer, any security holder of the issuer, or the SEC staff, upon request, information regarding the number of shares sold at each price within the range. Represents 5,703,488 shares held by Madrona Venture Fund I-A, LP, 620,763 shares held by Madrona Venture Fund I-B, LP and 749,486 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. Represents a weighted-average price. These shares were sold in multiple transactions at prices ranging from $13.44 to $14.29, inclusive. For all transactions reported in this Form 4 utilizing a weighted-average price, the reporting person will provide to the issuer, any security holder of the issuer, or the SEC staff, upon request, information regarding the number of shares sold at each price within the range. Represents 5,663,173 shares held by Madrona Venture Fund I-A, LP, 616,375 shares held by Madrona Venture Fund I-B, LP and 744,189 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. 4,000 shares were acquired through reporting person's 401k plan. Keenan Conder, Attorney-in-Fact 2010-05-05