FoxP3 is the master transcription factor for Treg cells, and inherited defects in the FoxP3 gene lead to the failure of Treg development, which results in fatal autoimmunity in mice [10] and Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked (IPEX) syndrome in humans [11]. The gene discussed is FOXP3; the disease is Autoimmunity.