ERBB2 and breast cancer: The prototype ADC, ado-trastuzumab (T-DM1), was approved by the U.S. Food and Drug Administration (FDA) for treatment of human epidermal growth factor receptor 2 positive (HER2+) metastatic breast cancer (MBC) in 2013 and HER2+ early breast cancer (EBC) in 2019 (1, 2).