ERBB2 and breast cancer: Despite the clinical breakthrough trastuzumab brought to the treatment of HER2-overexpressing BC, the majority of metastatic BC (MBC) does not respond to trastuzumab monotherapy, and controversy remains over the optimal therapeutic strategy after disease progression (Cobleigh et al, 1999; Vogel et al, 2002; Montemurro et al, 2006).