This subgroup contains mainly eukaryotic proteins similar to ASL, 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. ASL catalyzes two steps in the de novo purine biosynthesis: the conversion of 5-aminoimidazole-(N-succinylocarboxamide) ribotide (SAICAR) into 5-aminoimidazole-4-carboxamide ribotide (AICAR) and, the conversion of adenylsuccinate (SAMP) into adenosine monophosphate (AMP). ASL deficiency has been linked to several pathologies including psychomotor retardation with autistic features, epilepsy and muscle wasting.