Citrullinemia type I results from deficiency of the enzyme argininosuccinate synthase (ASS), the third step in the urea cycle in which citrulline is condensed with aspartate to form argininosuccinic acid.> Untreated individuals with the severe form of citrullinemia type I have hyperammonemia (plasma ammonia concentration 1000- 3000 pmol/L).> Plasma quantitative amino acid analysis shows absence of argininosuccinic acid and concentration of citrulline usually greater than 1000 pmol/L (normal: Argininosuccinate synthase enzyme activity, measured in fibroblasts, liver, and in all tissues in which ASS is expressed, is decreased.> ASS1 is the only gene in which mutation is known to cause citrullinemia type I.">