Relaxin is a protein hormone 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.
In the female, it is produced by the corpus luteum of the ovary, the breast and, during pregnancy, also by the placenta, chorion, and decidua.
In the male, it is produced in the prostate and is present in human semen.
Structurally, relaxin is a heterodimer of two peptide chains of 24 and 29 amino acids linked by disulfide bridges, and it appears related to insulin.
Relaxin is produced from its prohormone, "prorelaxin", by splitting off one additional peptide chain.
In males, relaxin enhances motility of sperm in semen. In females relaxin is produced mainly by the corpus luteum, in both pregnant and nonpregnant females; it rises to a peak within approximately 14 days of ovulation, and then declines in the absence of pregnancy, resulting in menstruation. During the first trimester of pregnancy,