IlGF_like family, relaxin_like subgroup, specific to vertebrates. Members include a number of active peptides including (pro)relaxin, mammalian Leydig cell-specific insulin-like peptide (gene INSL3), early placenta insulin-like peptide (ELIP; gene INSL4), and insulin-like peptides 5 (INSL5) and 6 (INSL6). Members of this subgroup are widely expressed in testes (INSL3, INSL6), decidua, placenta, prostate, corpus luteum, brain (various relaxins), GI tract, and kidney (INSL5) where they serve a variety of functions in parturition and development. Typically, the active forms of these peptide hormones are composed of two chains (A and B) linked by two disulfide bonds; the arrangement of four cysteines is conserved in the "A" chain: Cys1 is linked by a disulfide bond to Cys3, Cys2 and Cys4 are linked by interchain disulfide bonds to cysteines in the "B" chain. This alignment contains both chains, plus the intervening linker region, arranged as found in the propeptide form. Propeptides are cleaved to yield two separate chains linked covalently by the two disulfide bonds.