Although our present study has some limitations, because the T-DM1-resistant cells in this study were established from cells naïve for both TRAS and PER despite the clinical use of T-DM1 after the regimen including TRAS and PER, our results showed the potential of TRAS + PER + chemotherapy as third-line combination therapy after T-DM1 resistance if cancer cells remain dependent on HER2. The gene discussed is ERBB2; the disease is cancer.