Cytosolic argininosuccinate lyase (ASL) catalyzes the reversible reaction of argininosuccinate to form fumarate and arginine. The enzyme is a homotetramer (Turner et al. 1997). The function of the human enzyme in vivo is inferred from the defective argininosuccinate lyase enzyme activity observed in patients with mutant forms of the ASL gene (e.g., Walker et al. 1990).