Glutamic Acid Decarboxylase (GAD) catalyzes the formation of gamma-aminobutyric acid (GABA), a major inhibitory neurotransmitter in the brain. Two isoforms, GAD1/GAD67 and GAD2/GAD65, have been identified. They are expressed in the brain, pancreas, ovary, and oviduct. Autoantibody against GAD2/GAD65 is seen in pre-diabetics and insulin-dependent (Type I) diabetes mellitus.