Nat. Biotechnol. 26, 1379–1382 (2008); published online 23 November 2008; corrected after print 9 February 2009

In the version of this article initially published, on page 1379, column 2, line 7, a parenthesis was misplaced. “...the passenger (often the sense strand)” should have read, “...the passenger (often the sense) strand”. The error has been corrected in the HTML and PDF versions of the article.