Nat. Med. 13, 189–197 (2007); published online 4 February 2007; corrected after print 4 September 2009

In the version of this article initially published, the number of one of the grants listed in the Acknowledgments was incorrect; 'HL55993' should have been 'P01 HL81587'. The error has been corrected in the HTML and PDF versions of the article.