ERBB2 and breast cancer: The first foray prospectively testing this strategy was a study of trastuzumab emtansine (T-DM1), another HER2-targeted therapy FDA-approved in breast cancer, which is an antibody-drug conjugate that links trastuzumab with the cytotoxic agent DM1 (microtubule inhibitor, a maytansine derivative) [25, 26].