The identification of mutations in the gene encoding Foxp3 as the cause of aggressive autoimmunity in human patients with IPEX syndrome (immunodysregulation, polyendocrinopathy, enteropathy, X-linked syndrome) and in the mutant mouse strain scurfy and the subsequent discovery of the essential function of Foxp3 in the development of Treg have provided a genetic foundation for the phenomenon of Treg-mediated dominant tolerance [8,9]. The gene discussed is FOXP3; the disease is Autoimmunity.