CPS1 and hyperinsulinemic hypoglycemia, familial, 4: CPS1 activation by NAG1, 2 has paramount importance, as shown by human NAG synthase deficiency3, 4, which is indistinguishable clinically from CPS1 deficiency but which is successfully treated by giving to the patient the NAG analog and orphan drug N-carbamoyl-L-glutamate3, 4.