Nat. Biotechnol. 34, 458–461 (2016); published online 6 May 2016; corrected after print 27 July 2016.

In the version of this article initially published, on p.460, columns 1 and 2, the number following “ACE-” was given as “2454” instead of “2494.”

The errors have been corrected in the HTML and PDF versions of the article.