Trastuzumab deruxtecan (T-DXd) is a HER2-targeting IgG1 mAb with a topoisomerase I inhibitor payload approved in the US and Europe for treating multiple solid tumors, including breast cancer, gastric or gastroesophageal junction adenocarcinoma, and NSCLC.31,73,74. The gene discussed is ERBB2; the disease is breast cancer.