Nat. Immunol. 9, 176–185 (2008); published online 13 January 2008; corrected after print 16 March 2008

In the version of this article initially published, the citation for Supplementary Figure 2c on page 179 and the citation for Supplementary Figure 3a on page 180 are incorrect. These should be “Supplementary Fig. 2a–c” and “Supplementary Fig. 3,” respectively. The errors have been corrected in the HTML and PDF versions of the article.