Nat. Immunol. 10, 333–339 (2009); published online 19 March 2009; corrected after print 18 May 2009

In the version of this article initially published, the second funding attribution in Acknowledgments is incorrect. The correct attribution is as follows: “Supported by Howard Hughes Medical Institute (J.S.B. and R.M.) and Damon Runyon Cancer Research Foundation (J.S.B).” The error has been corrected in the HTML and PDF versions of the article.