Nature Immunology 7, 361 (2006); published online 20 March 2006; corrected after print 28 March 2006

In the version of this research highlight initially published, the reference is missing. The reference should be as follows: Proc. Natl. Acad. Sci. USA 103, 2202–2207 (2206). The error has been corrected in the HTML and PDF versions of the research highlight.