ERBB2 and breast cancer: The mechanism of action associated with the ADC is that T-DM1 targets HER2 overexpressed on the cell surface of breast cancers via trastuzumab, and subsequently T-DM1/HER2 complexes are internalized into lysosomes where antibody component of T-DM1 is degraded followed by the release of Lys-MCC-DM1 into the cytoplasm3,4.