The human genome has several copies of the ASS1 gene spread across 11 chromosomes. Out of these copies, the functional one is localized to the long arm of chromosome 9. It encodes an enzyme named argininosuccinate synthetase, which is involved in the urea cycle and biosynthesis of arginine.
Mutations in the chromosome 9 copy of ASS1 gene are associated with type I citrullinemia. These mutations affect the structure of argininosuccinate synthetase enzyme or may lead to its deficiency resulting in the accumulation of ammonia and other toxic byproducts of the urea cycle in the bloodstream.