Nat. Chem. Biol. 5, 543–550 (2009); published online 20 July 2009; corrected after print 20 July 2009

In the version of this article initially published, in the first line in the right-hand column of Box 2, the sentence began “The distance ro...,” whereas it should read “The tunneling distance....” Also, reference 47 was omitted from the end of the second-to-last sentence of the legend of Figure 3. These errors have been corrected in the HTML and PDF versions of the article.