Insulin is only one member of a family of peptide hormones and growth factors that comprizes 10 members in humans: insulin, insulin-like growth factors I and II (IGF-I and II) and seven peptides related to relaxin (Fig. 1).

They result from successive duplications of an ancestral gene that appeared early in animal evolution. Invertebrates also have many insulin-like peptides, e.g. 37 in the worm C. elegans and seven in the fruit fly Drosophila Melanogaster (1).

They play an important role in metabolism, growth, reproduction and longevity.