Nat. Med. 18, 1369–1377 (2012); published online 2 September 2012; corrected after print 5 October 2012

In the version of this article initially published, in Figure 6f the left-hand graph was an inadvertent duplication of the right-hand graph. The error does not alter the overall conclusions of the paper. The figure has been corrected in the HTML and PDF versions of the article.