Nat. Biotechnol. 31, 892–893 (2013); doi:10.1038/nbt.2715; published online 8 October 2013; corrected after print 8 November 2013

In the version of this article initially published, on p. 892, right column, 9 lines from the bottom, the number “27” was inadvertently set as superscript. The error has been corrected in the HTML and PDF versions of the article.