FOXP3 and diabetes mellitus: Immunodysregulation, Polyendocrinopathy Enteropathy X-linked syndrome is due to a mutation in FOXP3 (Forkhead Box P3, a master regulator of Treg function) and is characterized by a triad of intractable diarrhea, type 1 diabetes mellitus (T1DM), and eczema, as well as other autoimmune features (4).