Heterotrimeric G proteins function to relay information from cell surface receptors to intracellular effectors. In mammals, G protein alpha, beta and gamma polypeptides are encoded by at least 16, 4 and 7 genes, respectively. Most interest in G proteins has been focused on their alpha subunits, since these proteins bind and hydrolyze GTP and most obviously regulate the activity of the best studied effectors. Several Galpha GTPase activating proteins (GAPs) have been identified and are designated RGS1, RGS2, RGS4, RGS7, RGS9, RGS10 and GAIP (Galpha-interacting protein). Each of these proteins has been shown to deactivate specific Galpha isoforms by increasing the rate at which they convert GTP to GDP. RGS2 has been shown to be an inhibitor of Galpha q function. RGS9 expression is restricted to photoreceptor cells and RGS9 has been shown to regulate Galpha t.