Trastuzumab deruxtecan (T-DXd, formerly DS-8201a), an antibody drug conjugate of trastuzumab with a novel topoisomerase I inhibitor payload, has been recently approved in the US for the treatment of adult patients with unresectable or metastatic HER2-positive breast cancer who have received two or more prior anti-HER2-based regimens in the metastatic setting [27]. The gene discussed is ERBB2; the disease is breast cancer.