Ownership

Top Holders   Shares Held % O/S Share Change Filing Date RS Investments 2,905,756 14.9 542,892 03/31/13 Wellington Management Company, LLP 2,697,318 13.9 121,934 03/31/13 HealthCor Management, L.P. 2,252,119 11.6 -97,881 03/31/13 BlackRock Institutional Trust Company, N.A. 1,843,962 9.5 279,971 03/31/13 T. Rowe Price Associates, Inc. 1,114,900 5.7 -7,200 03/31/13 Turner Investment Partners, Inc. 896,695 4.6 896,695 03/31/13 The Vanguard Group, Inc. 878,259 4.5 -298,660 03/31/13 JP Morgan Asset Management 605,626 3.1 43,116 03/31/13 Tiverton Asset Management LLC 550,000 2.8 0 03/31/13 Dimensional Fund Advisors, LP 457,000 2.4 17,863 03/31/13

Ownership data based on most recent publicly available data according to Thomson Reuters.