Figure 3: IGF receptor signalling: IGF-1R is a tetramer of two identical α-subunits and two identical β-subunits. IGF-1R and IR can hybridize to form a heterodimer composed of one α-subunit and one β-subunit of each receptor. Formation of hybrid receptors is explained with different colour code scheme. IGF-IIR, the mannose-6-phosphate (M6P) receptor, has high affinity for binding the IGF-II ligand but is a nonsignalling receptor. The biological activities of the IGF ligands are mediated by IGF-IR, but the IGF-IIR is considered to function as a “sink” that controls the local bioavailability of IGF ligands for binding to the IGF-IR.