Glutamate decarboxylase or glutamic acid decarboxylase (GAD) is an enzyme that catalyzes the decarboxylation of glutamate to gamma-aminobutyric acid (GABA), the major inhibitory transmitter in higher brain regions, and putative paracrine hormone in pancreatic islets. GAD uses pyridoxal-phosphate (PLP) as a cofactor.Two molecular forms of GAD (65 kDa and 67 kDa) are highly conserved and both are expressed in the CNS, pancreatic islet cells, testis, oviduct and ovary. The isoforms are regionally distributed cytoplasmically in the brains of rats and mice. GAD65 is an ampiphilic, membrane-anchored protein (585 a.a.), and is responsible for vesicular GABA production. GAD67 is cytoplasmic (594 a.a.), and seems to be responsible for significant cytoplasmic GABA production.