Much data has shown that human epidermal growth factor receptor 2 (HER2) gene amplification and/or protein overexpression occurs in ~25 to 30% of all breast cancers and closely contributes to a poor prognosis as well as an encouragingly good response to HER2-targeting agents, such as anti-HER2 monoclonal antibodies and antibody–drug conjugates (e.g., trastuzumab, pertuzumab, and trastuzumab-emtansine) (1–3). Here, ERBB2 is linked to breast cancer.