ERBB2 and neoplasm: Specifically, T-DM1 carries emtansine, a microtubule inhibitor, which is potent but not membrane-permeable; while T-DXd carries deruxtecan, a topoisomerase I inhibitor, which is more potent and membrane-permeable, allowing a bystander effect by killing not only HER2-positive tumor cells but also surrounding tumor cells.