ASS1 (argininosuccinate synthase or citrulline-aspartate ligase) belongs to the argininosuccinate synthase family. ASS1 is a urea cycle enzyme that is essential in the conversion of nitrogen from ammonia and aspartate to urea. A deficiency of ASS1 causes citrullinemia (CTLN1), an autosomal recessive disease which is characterized by severe vomiting spells and mental retardation. Cells deficient in ASS1 must absorb the arginine they need for growth from circulating blood.