ERBB2 and breast cancer: T-DM1 had substantial anti-cancer efficacy in preclinical models of HER2-positive breast cancer [1, 6], and after demonstration of its efficacy and safety in clinical trials [7, 8], the U.S. Food and Drug Administration (FDA) approved T-DM1 in 2013 as monotherapy for the treatment of patients with HER2-positive advanced breast cancer who had previously received trastuzumab and a taxane.