G protein-coupled receptors (GPCRs), also known as seven transmembrane receptors, 7TM receptors, and heptahelical receptors, are a protein family of transmembrane receptors that transduce an extracellular signal (ligand binding) into an intracellular signal (G protein activation). The GPCRs are the largest protein family known, members of which are involved in all types of stimulus-response pathways, from intercellular communication to physiological senses. The diversity of functions is matched by the wide range of ligands recognized by members of the family, from photons (rhodopsin, the archetypal GPCR) to small molecules (in the case of the histamine receptors) to proteins (for example, chemokine receptors). This pervasive involvement in normal biological processes has the consequence of involving GPCRs in many pathological conditions, which has led to GPCRs being the target of 40 to 50% of modern medicinal drugs

(source: Wikipedia)