Nat. Chem. Biol. 5, 581–583 (2009); published online 28 June 2009; corrected after print 18 August 2009

In the version of this article initially published, reference 10 was incorrect. The correct reference is: Olah, M. et al. in Chemical Biology: from Small Molecules to Systems Biology and Drug Design (eds. Schreiber, S.L., Kapoor, T.M. & Wess, G.) 760–786 (Wiley-VCH, Weinheim, Germany, 2007). The error has been corrected in the HTML and PDF versions of the article.