Glutamic acid decarboxylase (GAD) is the enzyme responsible for the conversion of glutamic acid to gamma-aminobutyric acid (GABA), the major inhibitory transmitter in higher brain regions, and putative paracrine hormone in pancreatic islets. Two molecular forms of GAD (65kD and 67kD, 64% amino acid identity between forms) are highly conserved and both forms are expressed in the CNS, pancreatic islet cells, testis, oviduct and ovary. GAD67 is cytoplasmic (594aa), encoded on chromosome 2, and seems to be responsible for significant cytoplasmic GABA production. GAD67 expression seems to be particularly plastic and can change in response to experimental manipulation or disease progression.