ERBB2 and breast cancer: reported HER2 amplification, impaired access to the binding site, augmented signalling through other ErbB family receptors and their ligands, activation of HER2 targets by alternate heterodimers, signalling triggered by downstream members, altered expression of cell cycle and apoptotic regulators, hormone receptor status, resistance to ADCC (FcγR polymorphism), and altered miRNA expression signatures as possible trastuzumab resistance biomarkers in BC (103).