On page 2434 in the March 12, 2009, issue, the name of the university, Karolinska Institutet, was omitted. The correct affiliation line should read as shown. The error was corrected in the online version, which now differs from the print version.

1Department of Microbiology, Tumor and Cell Biology, Strategic Research Center for Studies of Integrative Recognition in the Immune System (IRIS), Karolinska Institutet, Stockholm, Sweden

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