Nat. Immunol. 13, 223–228 (2012); published online 12 February 2012; corrected after print 15 January 2013

In the version of this article initially published, the Author Contributions statement was incomplete. The correct statement should include the following: “C.T. raised the original idea that Vpx may increase the amount of nucleotides.” The error has been corrected in the HTML and PDF versions of the article.