CPS1 and Hyperammonemia: This enzyme is encoded by CPS1 (MIM *608307), located on chromosome 2q35 and composed of 38 exons leading to a 1500 amino acid protein.1, 2, 3 Mutations in CPS1 can result in reduced or absent enzyme function leading to hyperammonemia and other features of CPS1 deficiency such as neonatal or late onset encephalopathy with vomiting, seizures, and coma if left untreated (CPS1D, MIM #237300).4 Diagnosis of CPS1D is based on a biochemical profile with increased plasma ammonia, decreased plasma citrulline, and a normal or low orotic acid in urine.