Nat. Med. 20, 904–910 (2014); 10.1038/nm.3583; published online 6 July 2014; corrected after print 13 March 2015

In the version of this article initially published, the acknowledgment that Daniel S. Perrien was supported by a Career Development Award from the US Department of Veterans Affairs was omitted. The error has been corrected in the HTML and PDF versions of the article.