Nat. Med. 18, 221–223 (2012); published online 22 January 2012; corrected after print 6 July 2012

In the version of this article initially published, due to an oversight by the authors, the first and last names of one the authors, Somasekar Seshagiri, were incorrectly transposed. The error has been corrected in the HTML and PDF versions of the article.