Nature Medicine 12, 1198–1202 (2006); published online 20 August, corrected after print 4 October 2006

In the version of the article initially published, the key in Figure 4a,b identifying the bars in the histograms is incorrect. The white bar should be identified as peptide and the black bar should be identified as peptide + anti–PD-L1. The error has been corrected in the HTML and PDF versions of the article.