This group contains ASAL and related proteins. It is a member of the Lyase class I family. Members of this family for the most part catalyze similar beta-elimination reactions in which a C-N or C-O bond is cleaved with the release of fumarate as one of the products. These proteins are active as tetramers. The four active sites of the homotetrameric enzyme are each formed by residues from three different subunits. ASAL is a cytosolic enzyme which catalyzes the reversible breakdown of argininosuccinate to arginine and fumarate during arginine biosynthesis. In ureotelic species ASAL also catalyzes a reaction involved in the production of urea. Included in this group are the major soluble avian eye lens proteins from duck, delta 1 and delta 2 crystallin. Of these two isoforms only delta 2 has retained ASAL activity. These crystallins may have evolved by, gene recruitment of ASAL followed by gene duplication. In humans, mutations in ASAL result in the autosomal recessive disorder argininosuccinic aciduria.