The phenotype is highly variable (Fig 6), ranging from asymptomatic adults to profound immunodeficiency causing early death or necessitating HSCT in childhood; the clinical features overlap those of other primary immunodeficiencies, such as cytotoxic T lymphocyte–associated antigen 4 (CTLA4) and LPS-responsive beige-like anchor protein (LRBA) deficiency. The gene discussed is CTLA4; the disease is hyperinsulinemic hypoglycemia, familial, 4.