Nat. Struct. Mol. Biol. 14, 754–761 (2007); published online 15 July 2007; corrected after print 24 November 2008

In the version of this article initially published, one of the US National Institutes of Health grant numbers was incorrect. The correct acknowledgment should be: “US National Institutes of Health to T.d.L. (GM49046 and AG016642).” The error has been corrected in the HTML and PDF versions of the article.