In March 2023, trastuzumab deruxtecan (T-DXd) was indicated in Japan for treatment of patients with unresectable or metastatic HER2-low breast cancer who have previously received chemotherapy for metastatic disease, and the Japanese Breast Cancer Society (JBCS) Clinical Practice Guidelines Committee released a new recommendation in the web revision of the JBCS Clinical Practice Guidelines 2022 edition (June 2023) [1]. This evidence concerns the gene ERBB2 and breast cancer.