Nat. Immunol. 10, 273–280 (2009); published online 8 February 2009; corrected after print 18 May 2009

In the version of this article initially published, the GenBank accession number for the Indian human immunodeficiency virus 2 isolate is incorrect. The correct accession number is DQ307022. The error has been corrected in the HTML and PDF versions of the article.