CPS1 is a mitochondrial enzyme that catalyzes synthesis of carbamoyl phosphate from ammonia and bicarbonate. This reaction is the first committed step of the urea cycle, which is important in the removal of excess urea from cells. CPS1 also represents a core mitochondrial nucleoid protein. CPS1 is primarily expressed in hepatocytes in the liver. The antigen for Hep par-1 (Hepatocyte) antibody has been identified as CPS1. This CPS1 antibody shows a similar staining profile to Hep par-1 antibody, which is highly specific and sensitive for hepatocytes and derived tumors with rare positivity in hepatoid adenocarcinomas in GI tract as well as several other types of nonhepatic tumors.