ERBB2 and breast cancer: Nonetheless, in two phases 2 trials testing the efficacy and safety of T-DM1 in HER2-positive metastatic BC patients previously treated with at least trastuzumab, retrospective, exploratory analyses according to central laboratory assessment of HER2 status found poor clinical activity of T-DM1 among patients with HER2-normal BC compared to patients with HER2-positive BC (Table 1) [35,36].