The functionally diverse group of G-protein-coupled receptors (GPCRs) is a superfamily of membrane receptors. They include receptors for many different signaling molecules, such as peptide and nonpeptide hormones, neurotransmitters, chemokines, prostanoids, and proteinases. The principal function of GPCRs is to transmit information about the extracellular environment to the interior of the cell by interacting with the heterotrimeric G-proteins and, thereby, participate in regulation of many cellular functions. In view of their major importance, it is not surprising that GPCR-mediated responses are subject to dynamic regulation by a number of mechanisms. These regulatory mechanisms have important roles in fine-tuning signals from multiple receptor signaling pathways. Multiple mechanisms contribute to the regulation of GPCRs and their transmembrane signaling. Posttranslational modifications of the receptors, such as phosphorylation, may modulate receptor function; in addition, changes in receptor gene expression can lead to alterations in sensitivity and responsiveness of cells to various signaling molecules (1–3).