Family A receptors make up the largest group of GPCRs, are involved in almost every aspect of human physiology, and are activated by a diverse variety of ligands (e.g.-. hormones, neurotransmitters, light).
Family B GPCRs are peptide binding receptors comprised of two sub-families: the secretin-like receptors that primarily bind peptide hormones and adhesion receptors that are characterized by large extracellular domains.
Family C GPCRs function as obligate dimers with large extracellular domains, termed the Venus flytrap (VFT). Unlike other classes of GPCRs orthosteric ligand binding occurs at the VFTs which are distal to the transmembrane region.