GAD-65 and GAD-67, glutamate decarboxylases, function to catalyze the production of GABA (gamma-aminobutyric acid). In the central nervous system GABA functions as the main inhibitory transmitter by increasing a Cl- conductance that inhibits neuronal firing. GABA has been shown to activate both ionotropic (GABAA) and metabotropic (GABAB) receptors as well as a third class of receptors called GABAC. Both GABAA and GABAC are ligand-gated ion channels; however, they are structurally and functionally distinct. GABAC receptors (GABAC Rr) mediate rapid inhibitory neurotransmission in retina. Three human genes, r1 (GABRR1), r2 (GABRR2) and r3 (GABRR3), encode the three polypeptides that comprise this receptor. GABRR1 and GABRR2 are located close together, in a region of chromosome 6q that contains loci for inherited disorders of the eye, but GABRR3 maps to chromosome 3q11-q13.3. The r polypeptide genes, which are thought to share a common ancestor with GABA(A) receptor subunit genes, diverged at an early stage in the evolution of this gene family. The expression of GABAC Rr subunits is not restricted to the retina, but significant expression can also be detected in many other brain regions, especially in those belonging to the visual pathways.