G protein-coupled receptors (GPRs) are a protein family of transmembrane receptors that transmit an extracellular signal (ligand binding) into an intracellular signal (G protein activation). GPR signaling is an evolutionarily ancient mechanism used by all eukaryotes to sense environmental stimuli and mediate cell-cell communication. All of the receptors have seven membrane-spanning domains and the extracellular parts of the receptor can be glycosylated. These extracellular loops also contain two highly conserved cysteine residues which create disulfide bonds to stabilize the receptor structure. Relaxin Receptor 4, also known as Relaxin/insulin-like family peptide receptor 4, RXFP4, RLN3R2, GPCR142 or GPR100, is a G protein-coupled receptor that binds Relaxin 3 and is specifically expressed in peripheral tissues, particularly in the colon.