G protein-coupled receptors (GPCRs) are also known as 7-transmembrane domain receptors. More than 1000 genes encoding GPCRs have been identified. GPCRs transduce external signals to intracellular G proteins by binding with ligands and thus activate or inhibit many second messenger systems in cells.
GPCRs are classified on the basis of their ligands. The ligands of some GPCRs are endogenous such as neurotransmitters, hormones, and chemokines. Some GPCRs are sense receptors involved in vision, odor, and taste. The others (approximately 150 types) are orphan receptors whose endogenous ligands have not yet been identified. The functions of GPCRs physiologically important. GPCRs are involved in many diseases and are the targets of medicinal drugs.