ERBB2 and breast cancer: Trastuzumab emtansine (T-DM1) was approved for marketing applications in 2013 for the treatment of HER2 + breast cancer patients,[6] but its low membrane permeability prevented the cytotoxic drug from reaching the neighboring HER2 low-expressing cells and exhibited bystander killing,[5] which prompted the development of next-generation antibody-drug conjugate (ADC) trastuzumab deruxtecan (T-DXd).