Ado‑trastuzumab emtansine (T‑DM1) is a HER2‑targeted antibody–drug conjugate in which trastuzumab delivers the microtubule inhibitor DM1 to HER2‑overexpressing tumor cells. After HER2 binding and internalization, lysosomal processing releases DM1 to inhibit microtubules, causing mitotic arrest and apoptosis; the trastuzumab moiety also inhibits HER2 signaling and can mediate ADCC.