Nat. Immunol. 14, 389–395 (2013); published online 3 March 2013, corrected after print 21 March 2013

In the version of this article initially published, the key to Figure 1c was labeled incorrectly. The correct labeling is as follows: dotted line, Isotype; shaded box, T-bet. The error has been corrected in the HTML and PDF versions of the article.