Nat. Immunol. 15, 15–25 (2014); published online 18 December 2013; corrected after print 8 May 2017

In the version of this article initially published, the top middle portion of Figure 4a was incorrect. The MHC and TCR should be on the left, and the B7 and CD28 should be on the right (diagrams and labels for both). The error has been corrected in the HTML and PDF versions of the article.