T-DM1 enhanced binding to HER2-positive cancer cell-derived exosomes but not to exosomes derived from HER2-negative MCF-7 cells. Treatment of SKBR-3 and EFM-192A cells with T-DM1 containing exosomes caused tumour growth inhibition and activation of caspases 3 and/or 7. The gene discussed is ERBB2; the disease is cancer.