Nat. Med. 15, 84–90 (2009); published online 21 December 2008; corrected after print 7 November 2011

In the version of this article initially published, the labels in the key to the bar graph in Figure 2c were incorrect. The gray bars should have been labeled 'Double cortex', and the white bars should have been 'P0 rescue'. The error has been corrected in the HTML and PDF versions of the article.