Relaxin is an insulin-like peptide consisting of two separate chains (A and B) joined by two inter- and one intrachain disulfide bonds. Binding to its receptor requires an Arg–X– X–X–Arg–X–X–Ile motif in the B-chain. A relatedmember of the insulin superfamily, INSL3, has a tertiary structure that is predicted to be similar to relaxin. It also possesses an Arg–X–X–X–Arg motif within its B-chain, although this is displaced by four amino acids towards the C-terminus from the corresponding position within relaxin
