Glutamic Acid Decarboxylase (GAD) catalyzes the conversion of L glutamate to g-aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the brain, and a putative paracrine signal molecule in pancreatic islets. GAD has a restricted tissue distribution. It is highly expressed in the cytoplasm of GABAergic neurons in the central nervous system (CNS) and pancreatic beta cells. It is also present in other non-neuronal tissues such as testis, oviduct and ovary. GAD is also transiently expressed in non-GABAergic cells of the embryonic and adult nervous system, suggesting its involvement in development and plasticity.