ERBB2 and breast cancer: However, current guidelines recommend the use of adjuvant T‐DM1, an ADC consisting of trastuzumab and the cytotoxic agent emtansine (DM1), for patients with HER2‐positive early breast cancer who have residual disease after neoadjuvant therapy based on the phase 3 KATHERINE study, which showed benefit of adjuvant T‐DM1 compared to trastuzumab, regardless of HER2 status of the residual disease.23