Nat. Med. 17, 110–116 (2011); published online 19 December 2010; corrected online 21 December 2010; corrected after print 7 September 2011

In the first paragraph on page 112, the authors made a typographical error: “constitutes part the active histidine dyad” should have been “is proximate to the active histidine dyad.” The error has been corrected in the HTML and PDF versions of the article.