G protein-coupled receptors (GPCRs), also designated seven transmembrane (7TM) receptors and heptahelical receptors, are a protein family which interact with G proteins (heterotrimeric GTPases) to synthesize intracellular second messengers such as diacylglycerol, cyclic AMP, inositol phosphates, and calcium ions. Their diverse biological functions range from vision and olfaction to neuronal and endocrine signaling and are involved in many pathological conditions. G protein receptor 128 (GPR128), a member of the secretin family of GCPRs with a GPS domain in its N-terminal domain, may mediate signaling processes to the interior of the cell via activation of G proteins. GPR128 represents an allopeptide which may be involved in T cell mediated transplant rejection as it is able to stimulate 2.102 T cells.