Nat. Chem. Biol. 2, 249–253 (2006); published online 26 March 2006; corrected after print 8 May 2006

In the version of this article initially published, a funding source was omitted. The following statement should be added to the end of the Acknowledgments section: “D.A. Bosco is grateful to the National Institutes of Health for a postdoctoral fellowship. We thank the National Institutes of Health (NS050636), the Skaggs Institute for Chemical Biology and the Lita Annenberg Hazen Foundation for financial support.” The error has been corrected in the PDF version of the article.