Insulin family <p> The insulin family of proteins groups together several evolutionarily related active peptides [<cite idref="PUB00003973"/>]: these include insulin [<cite idref="PUB00003972"/>, <cite idref="PUB00003970"/>], relaxin [<cite idref="PUB00053639"/>, <cite idref="PUB00053640"/>], insect prothoracicotropic hormone (bombyxin) [<cite idref="PUB00053641"/>], insulin-like growth factors (IGF1 and IGF2) [<cite idref="PUB00023078"/>, <cite idref="PUB00053642"/>], mammalian Leydig cell-specific insulin-like peptide (gene INSL3), early placenta insulin-like peptide (ELIP) (gene INSL4), locust insulin-related peptide (LIRP), molluscan insulin-related peptides (MIP), and Caenorhabditis elegans insulin-like peptides. The 3D structures of a number of family members have been determined [<cite idref="PUB00023078"/>, <cite idref="PUB00053642"/>, <cite idref="PUB00037375"/>]. The fold comprises two polypeptide chains (A and B) linked by two disulphide bonds: all share a conserved arrangement of 4 cysteines in their A chain, the first of which is linked by a disulphide bond to the third, while the second and fourth are linked by interchain disulphide bonds to cysteines in the B chain. </p>