Nat. Immunol. 8, 1173–1178 (2007); published online 19 October 2007; corrected after print 19 October 2007

In the version of this article initially published, the citations for Figure 1 and Figure 2 are incorrect. The citations for Figure 2a, 2b and 2c should be 1a, 1b and 1c; the citations for Figure 1 should be Figure 2. The error has been corrected in the HTML and PDF versions of the article.