If a future prospective study could show that the patients with tumors with the highest levels of HER2 biomarkers fail on trastuzumab and/or pertuzumab but not T-DXd, this subgroup of patients with the highest levels of HER2 gene amplification or highest levels of ERBB2 mRNA could be offered T-DXd as an initial treatment for early HER2-positive breast cancer. Here, ERBB2 is linked to breast cancer.