The Editors of Blood retract the 24 February 2011 paper cited above. Concerns regarding the data underlying Figures 3A, 3B, 3C, 4A, and 5C were brought to the attention of Stanford University. The university investigated the issue, conducting a search for any original sources of these data. The search was unsuccessful; the experiments, data, and figure preparation for these figures were overseen by Holbrook E. Kohrt, who died before the university became aware of the concerns. As a result, the data underlying these figures cannot be validated. Holbrook E. Kohrt is deceased and cannot consent to the retraction. Kipp Weiskopf, Ash A. Alizadeh, Josh Brody, Debra Czerwinski, Steven Coutre, Mark P. Chao, Thomas F. Tedder, and Ronald Levy approve the retraction. Roch Houot, Matthew J. Goldstein, Antonia Müller, Russell Pachynski, and Lieping Chen could not be reached for a response.