Nat. Immunol. 1, 195–202 (2009); published online 4 January 2009; corrected after print 27 February 2015

In the version of this article initially published, the contrast in lane 4 of Figure 2c had been altered, and the top right image in Figure 5a and bottom right image Figure 6e were incorrect. The correct gel and images are now presented. The error has been corrected in the HTML and PDF versions of the article.