CPS1 and carbamoyl phosphate synthetase I deficiency disease: Although advocated for many years,9, 10 there are few reports on the use of NCG in CPS1D.11, 12 Results in five late‐onset CPS1D patients were encouraging,12 although a paradoxical negative biochemical effect in one patient led to propose individual CPS1 mutations‐based counselling.13 We report here a patient with genetically characterized partial but severe CPS1D due to homozygosity for the novel c.3632C>G, p.(Pro1211Arg) mutation that was controlled for a long period with merely oral NCG, arginine, and moderate protein restriction.