Recently, HER2-low breast cancer, defined as IHC 1+ or 2+ with negative in situ hybridization, has gained attention, with approximately 60% of previously defined HER2-negative tumors being reclassified as HER2-low.1) Based on the results of the DESTINY-Breast04 trial,2) T-DXd was approved in Japan on March 27, 2023, for patients with unresectable or recurrent HER2-low breast cancer following chemotherapy.3) The gene discussed is ERBB2; the disease is breast cancer.