Relaxin is a protein hormone of about 6000 Da first described in 1926 by Frederick Hisaw.The relaxin-like peptide family belongs in the insulin superfamily and consists of 7 peptides of high structural but low sequence similarity; relaxin-1 (RLN1), 2 (RLN2) and 3 (RLN3), and the insulin-like (INSL) peptides, INSL3, INSL4, INSL5 and INSL6. The functions of relaxin-3, INSL4, INSL5, INSL6 remain uncharacterised.