Nat. Chem. Biol. 9, 826–833 (2013); published online 13 October 2013; corrected after print 11 December 2013

In the version of this article initially published, the US Department of Energy Office of Basic Energy Sciences, Energy Frontier Research Center grant number was incorrect. The correct number is DE-SC 0001035. The error has been corrected in the HTML and PDF versions of the article.