Argininosuccinate synthetase 1 catalyzes the penultimate step of the arginine biosynthetic pathway. There are approximately 10 to 14 copies of this gene including the pseudogenes scattered across the human genome, among which the one located on chromosome 9 appears to be the only functional gene for argininosuccinate synthetase. The ASS1 gene encodes ardininosuccinate synthetase-1, a cytosolic urea cycle enzyme mainly expressed in periportal hepatocytes, but also in most other body tissues. The enzyme is a homotetrameric protein composed of 45kD monomers and is involved in the synthesis of arginine and catalyzes that condensation of citrulline and aspartate to argininosuccinate using ATP.