The insulin receptor related receptor (IRR) is a heterotetrameric transmembrane receptor composed of two alpha and 2 beta chains linked by disulfide bonds. The alpha chains contribute to the formation of the ligand-binding domain, while the beta chains carry the kinase domain. Member of the insulin RTK family, IRR shares high homology with the insulin (IR) and the insulin-like growth factor-1 receptor (IGF-1R), but doesn’t bind any of IR and IGF-1R known ligands. In contrast to the widespread patterns of expression to IR and IGF-1R, IRR demonstrates a very restricted cellular distribution in a subset of tissues of neuronal origin and its biological functions are still unknown.