Nat. Chem. Biol. 6, 117–124 (2010); published online 10 January 2010; corrected after print 12 February 2010

In the version of this article initially published, Pingda Ren was inadvertently left off the author list. The error has been corrected in the author list, author contributions and conflict of financial interest declaration of the HTML and PDF versions of the article.