Nat. Med. 17, 1242–1250 (2011); published online 25 September 2011; corrected after print 7 November 2011

In the version of this article initially published, the affiliations of the author Nathalie Sabaa were misidentified as affiliations 3 and 4. Her correct affiliations are 6 and 7. The error has been corrected in the HTML and PDF versions of the article.