Nat. Med. 15, 267–276 (2009), published online 1 March 2009; corrected online 7 July 2010

In the version of this article initially published online, Figure 1b was a copy of Figure 4b, and Figure 3d did not show all the correct bands. Both figures have been corrected for the PDF and HTML versions of this article. The legends and interpretation of the data remain the same.