  Recently, structural analysis of the human transferrin and growth hormone
(GH) amino acid sequences has unravelled that they harbor a motif identical to
a pattern found in viral oncoproteins known to bind the primarily nuclear tumor
suppressor retinoblastoma protein (RB). Since related signatures had previously
been identified also in insulin and the two insulin-like growth factors (IGFs),
the aim of the current study has been to investigate whether further hints
substantiating these reported homologies can be found in silico. Here,
additional similarities are presented supporting the notion of an insulin
superfamily of growth-promoting proteins with dual localization in the
extracellular environment and the intracellular space, particularly in the
nucleus, as well as characterized by a tropism for RB.
