Nat. Biotechnol. 27, 378–386 (2009); published online 5 April 2009; corrected after print 9 September 2009

In the version of this article initially published, in Methods, p. 385, line 5, the concentration of MgCl2, given as 0.5 M, is incorrect. The correct concentration is 0.5 mM MgCl2. The error has been corrected in the HTML and PDF versions of the article.