Nat. Chem. Biol. 11, 303 (2015); published online 17 April 2015; corrected after print 17 June 2015

In the version of this article initially printed, the shorthand label in Figure 1 for the preferred chair conformation (top structure shown) was incorrectly shown as 1C4 instead of 4C1. The figure is correct in the HTML and PDF versions of the article.