Nat. Chem. Biol. 9, 674–676 (2013); published online 29 September 2013; corrected after print 11 December 2013

In the version of this article initially published, the name of contributing author Jean-Luc Guerquin-Kern was misspelled as Jean-Luc Gerquin-Kern. The error has been corrected in the HTML and PDF versions of the article.