Nat. Immunol. 9, 513–521 (2008); published online 6 April 2008; corrected after print 19 May 2008

In the version of this article initially published, the key for Figure 5b is incorrect. The black bars should be 'CD4+ SP' and the gray bars should be 'DP'. The error has been corrected in the HTML and PDF versions of the article.