A defect in the next step of the urea cycle leads to NAGSD—the rarest of the UCDs—with an estimated incidence of less than 1:2 000 000 in the United States and Europe.2 NAG is synthesized by the mitochondrial enzyme NAGS and activates CPS1. This evidence concerns the gene CPS1 and hyperammonemia due to N-acetylglutamate synthase deficiency.