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. 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. GPR18 (G protein-coupled receptor 18), also known as GPCRW, is a 331 amino acid multi-pass membrane protein that belongs to the G-protein coupled receptor family. Expressed abundantly in spleen and testis, GPR18 functions as a receptor for N-arachidonyl glycine and is thought to contribute to the regulation of the immune system. GPR18 activity is mediated by G proteins that specifically inhibit adenylyl cyclase.