UCDs caused by mitochondrial enzyme deficiencies induce an arginine deficiency, so patients with deficiencies in N-acetylglutamate synthase (NAGS), carbamoyl phosphate synthetase 1 (CPS1), ornithine transcarbamylase (OTC), or ornithine translocase (ORNT1) require supplementation with either arginine or its precursor, citrulline [2]. Here, OTC is linked to hyperinsulinemic hypoglycemia, familial, 4.