The early evidence that IPEX was associated with T-cell abnormalities (1) and the similarities between the autoimmunity caused by depletion of CD4+CD25+ regulatory T cells (Tregs) and the phenotype of foxp3 mutants led to the discovery that FOXP3 dictates Treg function (8, 9). This evidence concerns the gene FOXP3 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.