Nat. Methods 9, 345–350 (2012); published online 27 March 2012; corrected after print 10 April 2012.

In the version of this article initially published, the names of two authors, Fiona S.L. Brinkman and Robert E.W. Hancock, were incorrectly listed without middle initials. The error has been corrected in the HTML and PDF versions of the article.