Nat. Biotechnol. 31, 875–877 (2013); doi:10.1038/nbt.2710; published online 8 October 2013; corrected after print 8 November 2013

In the version of this article initially published, in the Acknowledgments, the name Devyin Smith should have been Devyn Smith. An email address for the corresponding author has also been added. The errors have been corrected in the HTML and PDF versions of the article.