GPR37 (cathepsin Z) is an orphan receptor which belongs to the G-protein coupled receptor 1 family. G protein coupled receptors is a large protein family comprised by transmembrane receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. They only exists in eukaryotes, including yeast, choanoflagellates, and animals. These receptors are binded and activated by light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters. These ligands vary in size from small molecules to peptides to large proteins. G protein-coupled receptors are involved in many diseases, and are also the target of approximately 40% of all modern medicinal drugs. GPR37 is expressed in brain and spinal cord, and at lower levels in testis, placenta and liver, but no detectable expression observed in any other tissue. GPR37 may have a unique functional role in the central nervous system.