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 a subunits, since these proteins bind and hydrolyze GTP and most obviously regulate the activity of the best studied effectors. Four distinct classes of G(alpha) subunits have been identified; these include Gs, Gi, Gq and Ga 12/13. The Gi class comprises all the known a subunits that are susceptible to pertussis toxin modifications, including Ga i-1, Ga i-2, Ga i-3, Ga o, Ga t1, Ga t2, Ga z and Ga gust. Of these, the three Ga i subtypes function to open atrial potassium channels. Ga 16 is a member of the Gq subfamily and is expressed specifically in hematopoietic cells.