Cells contain large and often diverse complements of receptors that bind circulating hormones, paracrine factors, and neurotransmitters and, in so doing, initiate events that culminate in the regulation of cell function. The largest family of cell-surface receptors are those coupled to guanosine triphosphate (GTP)-binding regulatory proteins (G proteins). These receptors, or GPCRs (G protein-coupled receptors), exhibit a characteristic seven-transmembrane-domain motif. The binding of an agonist to a GPCR induces a change in structure which, if GTP is present, culminates in the activation of one or more G proteins. Activated G proteins, in turn, regulate enzymes and channels responsible for the control of intracellular second messengers.