CTLA4 and hyperinsulinemic hypoglycemia, familial, 4: Monoallelic germline mutations in the Cytotoxic T-lymphocyte antigen-4 (CTLA4) gene result in CTLA-4 deficiency (CTLA-4d), a complex IEI characterized by a broad and multifaceted clinical spectrum, including various autoimmune manifestations, lymphoproliferation, and recurrent infections [1].