Nat. Biotechnol. 31, 697–703 (2013); doi:10.1038/nbt.2653; published online 8 August 2013; corrected after print 10 September 2013

In the version of this article initially published, the name of the company BDO should have been given as BDO USA (Chicago) and not BDO (Brussels). The error has been corrected in the HTML and PDF versions of the article.