GPR37 (cathepsin Z) is an orphan receptor that belongs to the G-protein coupled receptor 1 family. G protein-coupled receptors are a large protein family comprised of transmembrane receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. They only exist in eukaryotes, including yeast, choanoflagellates, and animals. These receptors are bonded 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 the brain and spinal cord, and at lower levels in the testis, placenta, and liver, but no detectable expression is observed in any other tissue. GPR37 may have a unique functional role in the central nervous system.