Nat. Chem. Biol. 6, 54–62 (2010); published online 29 November 2009; corrected after print 12 February 2010

In the version of this article initially published, in the key for Figure 6a, the units for two of the inhibitor concentrations were incorrectly labeled as “M”, where they should have read “μM”. The error has been corrected in the HTML and PDF versions of the article.