Trastuzumab deruxtecan (T-DXd) is a HER2-directed antibody-drug conjugate that was approved in the US in December 2019 for the treatment of patients with HER2-positive metastatic or unresectable BC who have received 2 or more prior anti-HER2–based regimens in the metastatic setting. This evidence concerns the gene ERBB2 and breast cancer.