Nat. Med. 14, 448–453 (2008); published online 16 March 2008; corrected after print 18 April 2008

In the version of this article initially published, the affiliation of Rebecca A. Stockton was incorrect. Her correct affiliation is affiliation 5: the Department of Medicine, University of California, San Diego, 9500 Gilman Dr., La Jolla, California 92093-0726, USA. The error has been corrected in the HTML and PDF versions of the article.