CPS1 and carbamoyl phosphate synthetase I deficiency disease: Carbamoyl phosphate synthetase 1 deficiency (CPS1D), a rare autosomal recessive inborn error of the urea cycle, is due to loss‐of‐function CPS1 gene mutations.1 In CPS1D, hyperammonemic crises compromise life and cognitive status, requiring prompt treatment by protein withdrawal, prevention of catabolism, ammonia scavengers, arginine or citrulline, and hemofiltration.2 Maintenance treatment aims at preventing decompensations by protein restriction, oral ammonia scavengers plus arginine or citrulline, and eventually, curative liver transplantation.2