G protein-coupled receptors (GPCRs) are a large superfamily of cell surface receptors characterized by 7 helical transmembrane domains, together with N-terminal extracellular and C-terminal intracellular domains.[supplied by OMIM, Mar 2008] see less G protein-coupled receptors (GPCRs) are a large superfamily of cell surface receptors characterized by 7 helical transmembrane domains, together with N-terminal extracellular and C-terminal intracellular domains.[supplied by OMIM, Mar 2008]