Nat. Med. 15, 741–745 (2009); published online 5 June; corrected after print 4 September 2009

In the version of this article initially published, the fourth condition from the top in the key to the bar graphs in Figure 2c was mislabeled as 'mC8'. The correct label is 'hC8'. The error has been corrected in the HTML and PDF versions of the article.