The ADC binds HER2 on tumor cells, is internalized, and releases MMAE, which binds the vinca site on β‑tubulin to block microtubule polymerization, causing G2/M arrest and apoptosis. β‑tubulin is the intracellular payload target, not the ADC’s binding antigen; cells are killed only after HER2-mediated uptake (with possible bystander effect).