Argininosuccinate Synthase (ASS1) is an urea cycle enzyme with a tetrameric structure composed of identical subunits. ASS1 is involved in the synthesis of arginine and catalyzes that condensation of citrulline and aspartate to argininosuccinate using ATP. ASS1 is important to the urea cycle as it catalyzes the important second last step in the arginine biosynthetic pathway. ASS1 mainly expressed in periportal hepatocytes, but also in most other body tissues. A deficiency of ASS1 causes citrullinemia (CTLN1), an autosomal recessive disease which is characterized by severe vomiting spells and mental retardation.