Trastuzumab deruxtecan (DS-8201) has previously been approved for the treatment of advanced/metastatic HER2+ (score 3+) breast cancer, advanced/metastatic HER2-low (defined by scores 1+ or 2+ without HER2/neu gene amplification) breast cancer, HER2+ advanced gastric cancer, and HER2-mutated non-small cell lung cancer (NSCLC) [18–21]. The gene discussed is ERBB2; the disease is breast cancer.