Nat. Chem. Biol. 8, 941–948 (2012); published online 23 September 2012; corrected after print 19 April 2013

In the version of this article initially published, the units (kJ mol−1) associated with the ΔH values reported in Table 1 were inadvertently omitted. The error has been corrected in the HTML and PDF versions of the article.