This deacylase‐resistant analogue of N‐acetyl‐l‐glutamate (NAG), the essential activator of CPS1,5 is used in primary NAG synthase (NAGS) deficiency2 and the hyperammonemia of organic acidemias.6 It could be valuable in partial CPS1D if NAG fails to saturate CPS1 in vivo, which might be common in urea cycle disorders if liver glutamate levels (needed for NAG synthesis; NAGS has a high Km for glutamate7) are low due to therapeutic protein restriction and to increased glutamate conversion to glutamine. Here, CPS1 is linked to Hyperammonemia.