Nat. Methods 10, 885–888 (2013); published online 4 August 2013; corrected after print 21 November 2013

In the version of this article initially published, in Figure 2e, lanes 6 and 8 should have been labeled “Fact,” not “Ffact.” In the legend for this figure, the sentence “N3 indicates cysteine...” should have read “C3 indicates cysteine....” The errors have been corrected in the HTML and PDF versions of the article.