The UCDs caused by enzyme deficiencies include proximal disorders, associated with altered or inhibited functioning of the mitochondrial enzymes: N‐acetylglutamate synthase (NAGS) deficiency (NAGSD; OMIM #237310), carbamoyl phosphate synthetase 1 (CPS1) deficiency (CPS1D; OMIM #237300), and ornithine transcarbamylase (OTC) deficiency (OTCD; OMIM #311250). The gene discussed is OTC; the disease is hyperinsulinemic hypoglycemia, familial, 4.