ALDH18A1 and Hyperammonemia: Indeed, the first patient reported with recessive mutations in ALDH18A1 causing P5CS deficiency had a clear biochemical phenotype with low levels of plasma ornithine, citrulline, arginine, proline, and fasting hyperammonemia possibly reflecting the need of de novo-synthetized ornithine for ureagenesis under fasting conditions (8, 9, 28).