Nat. Biotechnol. 30, 179–183 (2012); doi:10.1038/nbt.2089; published online 15 January 2012; corrected after print 5 December 2013

In the version of this article initially published, Arvin Iracheta-Vellve's name was incorrectly spelled as Arvin Iracheta-Velle. The error has been corrected in the HTML and PDF versions of the article.