G protein-coupled receptors (GPRs or GPCRs), also known as seven transmembrane receptors, heptahelical receptors, or 7TM receptors, are members of the largest protein family and play a role in many different stimulus-response pathways. G-protein coupled receptors mediate extracellular signals into intracellular signals (G-protein activation). They respond to a great variety of signaling molecules, including hormones, neurotransmitters and other proteins and peptides. GPR proteins are integral seven-pass membrane proteins with some conserved amino acid regions. GPR19, an orphan receptor, shows elevated expression during embyronic development of the nervous sytem as well as in specific regions of adult mouse brain, including the olfactory bulb, the hippocampus, hypothalamic nuclei and the cerebellum. The GPR19 gene maps to a location on chromosome 12, which is a frequent target for rearrangement in cancer cells and involved in childhood acute lymphoblastic leukemia (ALL).