ERBB2 and breast cancer: Meanwhile, recent clinical trial studies have shown that some novel anti-HER2 therapeutic agents, especially new generation antibody–drug conjugates (ADCs) [6–9], achieved an objective response rate (ORR) of approximately 40% in patients with HER2-low expressing metastatic BC (BCs with HER2 IHC 1+ or 2+/ISH negative) [10].