Trastuzumab deruxtecan (T-DXd) is a HER2-directed antibody-drug conjugate (ADC) that has demonstrated significant efficacy in treating HER2-positive and HER2-low cancers, including metastatic breast cancer, gastric cancer, and non-small-cell lung cancer (NSCLC) [1,2]. This evidence concerns the gene ERBB2 and cancer.