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The activities and kinetic properties of argininosuccinate synthetase in cultured skin fibroblasts of three citrullinemic patients with qualitative or quantitative abnormalities of hepatic argininosuccinate synthetase were examined. The cultured skin fibroblasts of a citrullinemic patient with a low hepatic argininosuccinate synthetase activity showed low activity and abnormal kinetics similar to those of the hepatic enzyme. This suggests a similar genetic origin of the fibroblast argininosuccinate synthetase as of the hepatic enzyme. However, we found normal argininosuccinate synthetase activity in cultured fibroblasts of two citrullinemic patients who had a low hepatic enzyme activity resulting from decrease in the amount of enzyme protein but with normal kinetic properties. This suggests that part of the quantitative abnormality of hepatic argininosuccinate synthetase of citrullinemic patients may be due to an abnormal organ-specific gene expression in the liver.