Nat. Chem. Biol. 6, 821–828 (2010); published online 3 October 2010; corrected after print 18 April 2011

In the version of this article initially published, the authors incorrectly acknowledged funding from the US National Institutes of Health, GM64742. The correct grant number should be GM072701. The error has been corrected in the HTML and PDF versions of the article.