Nat. Chem. Biol. 7, 885–887 (2011); published online 16 October 2011; corrected after print 3 July 2012

In the version of this article initially published, one of the institute affiliations of co–first author Xu Zhao and coauthors Ying Yang and Yun-Gui Yang was not included. The affiliations statement has been corrected in the HTML and PDF versions of the article.