Mitochondrial ALAS (delta-aminolevulinate synthase) catalyzes the reaction of succinyl-CoA and glycine to form delta-aminolevulinate, CoA SH and CO2. ALAS occurs in two isoforms. ALAS1 is widely expressed in the body though particularly abundant is liver. its expression can be induced by steroids and other substrates of cytochrome P450-mediated reactions, and repressed by hemin. ALAS2 is expressed at high levels in erythroid precursor cells. Both ALAS1 and ALAS2 are active as dimers, with each subunit polypeptide bound to a molecule of pyridoxal phosphate. The two proteins are encoded by separate genes; cDNAs corresponding to both have been cloned (Riddle et al. 1989).