Nat. Methods 12, 203–206 (2015); published online 5 January 2015; corrected after print 10 April 2017

In the version of this article initially published, the unit (nM) for the dUTP concentration described in the “Wild-type template preparation” section of the Online Methods was incorrect. The correct unit should be μM. The error has been corrected in the HTML and PDF versions of the article as of 10 April 2017.