Nat. Biotechnol. 30, 1117–1124 (2012); doi:10.1038/nbt.2424; published online 8 November 2012; corrected after print 7 December 2012

In the version of this article initially published, the funders were not acknowledged. The funding statement should have read “E.J.T. was funded by the National Institutes of Health/National Center for Advancing Translational Sciences TR000109.” The error has been corrected in the HTML and PDF versions of the article.