G protein-coupled receptors (GPCRs), also designated seven transmembrane (7TM) receptors and heptahelical receptors, are a protein family which interacts with G proteins (heterotrimeric GTPases) to synthesize intracellular second messengers such as diacylglycerol, cyclic AMP, inositol phosphates and calcium ions. Their diverse biological functions range from vision and olfaction to neuronal and endocrine signaling, along with involvement in many pathological conditions. GPR31 (G-protein coupled receptor 31) is a 319 amino acid orphan receptor that localizes to the cell membrane. GPR31 shares 25-33% homology with members of the chemokine, purino and somatostatin receptor gene families.