Recently, however, phase III results for the novel antibody–drug conjugate trastuzumab deruxtecan (T-Dxd) showed significantly improved survival in patients with metastatic HER2-low BC—defined with reference to prevailing recommendations for HER2 testing as an immunohistochemical score of 1+ or 2+ without detectable gene amplification [2, 3]. Here, ERBB2 is linked to breast cancer.