However, there is still controversy about the boundary between HER2-zero and HER2-ultralow and questions about the mechanism of action leading to the antitumor activity of anti-HER2 drugs in HER2-low breast cancers, perhaps related to the targeted delivery of cytotoxic molecules of ADC drugs, but not to the blockade of the HER2 pathway make the concept of HER2 low- and ultralow-expression limited application in the clinical setting (118). Here, ERBB2 is linked to breast cancer.