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Nature Immunology 10, 1143–1144 (1 November 2009) | doi:10.1038/ni1109-1143

Positively selecting peptides: their job does not end in the thymus

Kai W Wucherpfennig & Etienne Gagnon

Although peptides responsible for positively selecting major histocompatibility complex (MHC) class I–restricted thymocytes have been identified, peptides able to positively select MHC class II–restricted T cells have remained elusive. In this issue of Nature Immunology, groups led by Allen and Davis report the identification of self peptides that induce positive selection of CD4+ T cells in the thymus.