CTLA4 and Autoimmunity: First, lineage-specific deletion of the Ctla4 gene in Treg cells but not in effector T cells is sufficient to recapitulate the autoimmune phenotype observed in mice with a germline deletion of the Ctla4 gene,26 although the onset of fatality is slower than in mice with either germline or pan-T cell deletion of the gene.44–46 While the function of Ctla4 in Foxp3− cells remains to be investigated, these data suggest that development of fatal autoimmunity in the Ctla4−/− mice does not require deletion of Ctla4 in effector T cells.