Trastuzumab Deruxtecan, an antibody-drug conjugate (ADC) consisting of a humanized anti-ERBB2 (HER2) monoclonal antibody linked to the topoisomerase I inhibitor Deruxtecan, represents a novel therapeutic option for patients with advanced hormone receptor-positive (HR+, e.g., ER+) breast cancer. This evidence concerns the gene ESR1 and breast cancer.