ERBB2 and breast cancer: As an erbB2-targeted therapy, trastuzumab (also known as Herceptin, a humanized monoclonal antibody (Ab) against erbB2) has been approved by FDA and demonstrated significant activity in the treatment of breast cancer patients with erbB2-overexpressing (erbB2+) tumors [4-6]; however, both primary (de novo) and acquired resistances to trastuzumab are common and currently represent a significant clinical problem [7-9].