Relaxins are part of a family of peptide hormones that diverged from insulin in early vertebrate evolution to form the insulin-like peptides and relaxins, now often referred to as the relaxin peptide family. All are heterodimers; both chains are cleaved from a common propeptide and cross-linked by 2 disulphide bonds. Humans have 3 true relaxins encoded by 3 related genes, plus several more distantly related insulin-like peptide genes. Non-primates have 2 relaxin genes. The major circulating form of relaxin in humans is relaxin-2, equivalent to relaxin-1 in non-primates. Relaxin-3 is very highly conserved. Little is known about human relaxin-1; several of the insulin-like peptides have no known receptor or function.
There are 4 human G-protein coupled receptors for relaxin family peptides. Relaxin receptor 1 (RXFP1) and relaxin receptor 2 (RXFP2) are also known as LGR7 and LGR8 respectively, members of the LRR-containing G protein-coupled receptors (LGRs). Relaxin-3 receptor 1 (RXFP3) and Relaxin-3 receptor 2 (RXFP4) are unrelated, having more homology with small peptide receptors such as the somatostatin receptors.