Last but not the least, since the promising clinical use of Trastuzumab deruxtecan (T-DXd) in patients with IHC-based HER2-Low (1+, and 2+/FISH−) metastatic breast cancer was revealed by the Destiny-Breast04 study [19], the identification of a subset of “HER2-Low” breast cancers has attracted pathologists attention. The gene discussed is ERBB2; the disease is breast carcinoma.