For instance, any ADC processing-related or trastuzumab-dependent mechanisms of resistance may also promote resistance to the second-generation ADC trastuzumab deruxtecan (T-DXd), which is used in the treatment of HER2-positive breast, gastric and other solid tumours, HER2-low breast cancer and HER2-mutant non-small cell lung cancer [40–44]. The gene discussed is ERBB2; the disease is breast cancer.