ERBB2 and breast cancer: The prototype ADC, ado-trastuzumab emtansine (T-DM1) is comprised of a humanized, monoclonal human epidermal growth factor receptor 2 (HER2)-directed antibody, trastuzumab, linked to the cytotoxic agent, DM1, and is used for the treatment of early-stage and advanced HER2-positive breast cancer.