G protein-coupled receptors (GPRs), also known as seven transmembrane receptors, heptahelical receptors or 7TM receptors, comprise a superfamily of proteins that play a role in many different stimulus-response pathways. GPR signaling is an evolutionarily ancient mechanism used by all eukaryotes to sense environmental stimuli and mediate cell-cell communication. G protein-coupled receptors translate extracellular signals into intracellular signals (G protein activation) and they respond to a variety of signaling molecules, such as hormones and neurotransmitters. GPR20 is a 358 amino acid membrane protein that constitutively activates G(i) proteins without ligand stimulation. Also, GPR20 may be involved in the control of intracellular cAMP levels and mitogenic signaling. Interestingly, GPR20 is expressed in liver and certain regions of the brain, including putamen, caudate and thalamus, but is not expressed in hypothalamus, pons and frontal cortex.