ERBB2 and breast cancer: In this regard, nearly 60% of breast cancer patients are diagnosed with HER2-low expression, defined as HER2 1+ or HER2 2+ by immunohistochemistry (IHC) and no gene amplification at reflex molecular testing.45 DESTINY-Breast04 proved that targeting low level of HER2 with T-DXd was a superior therapeutic approach to standard chemotherapy in patients with unresectable or metastatic HER2-low breast cancer after prior treatment in the metastatic setting.46 Based on DESTINY-Breast04, T-DXd received FDA and EMA approval for advanced, pre-treated HER2-low, breast cancer.