Nat. Biotechnol. 27, 411–414 (2009); published online 8 May 2009; corrected after print 8 July 2009

In the version of this article initially published, on p. 411, left column, last paragraph, one of the researchers' names was incorrectly given as “Martin Feller.” It should have read Martin Keller. The error has been corrected in the HTML and PDF versions of the article.