The Insulin/Relaxin superfamily includes Insulin, IGF-I, IGF-II, Relaxins, Relaxin-like Factor (RLF), Early Placenta Insulin-like Peptide 4 (INSL4), INSL5, and INSL6. Three human Relaxin genes (Relaxins-1 to -3) have been identified. Relaxin-2 is secreted and circulates in the blood, whereas Relaxin-1 is expressed in decidua, placenta, and prostate, and Relaxin-3 is expressed in the brain. Human Relaxin-2 is a peptide hormone that plays an important role in female reproduction.