Among the genetically driven autoimmune diseases, immunodysregulation polyendocrinopathy enteropathy X-linked (IPEX) syndrome is a classic example of a monogenic autoimmune disease in which the mutated forkhead box P3 (FOXP3) gene disrupts the transcription of Treg signature genes, such as interleukin 2 receptor subunit alpha (IL-2RA) and cytotoxic T-lymphocyte associated protein 4 (CTLA4), thus affecting the development and function of Tregs and ultimately resulting in several autoimmune disorders, such as type 1 diabetes mellitus (T1D) and severe bowel disease (66). The gene discussed is FOXP3; the disease is type 1 diabetes mellitus.