Fig. 8. Model for molecular mechanism underlying the activation of preformed ErbB receptor dimers by ligand binding. The extracellular domain of the ErbB receptor consists of four sub-domains denoted by 1-4. N and C indicate the N and C lobes of the intracellular kinase domain, which is followed by the C-terminal tail containing five major tyrosine residues for autophosphorylation. The top two molecules are inactive dimers with a closed/tethered form (left) and open/extended/untethered form (right). The bottom two molecules are ligand-bound active dimers, in which unphosphorylated tyrosine (Y) and phosphorylated (pY) are shown.