Nat. Biotechnol. 26, 235–240; published online 23 December 2007; corrected after print 19 March 2008.

In the version of this article initially published online, an author's middle initial was incorrectly given as “A”. Brigitte A Schmidt should be Brigitte F Schmidt in the author list, and B.A.S should be B.F.S. in the authors' contribution section. The error has been corrected in the HTML and PDF versions of the article.