ERBB2 and breast cancer: Human epidermal growth factor receptor 2 (HER2) plays an important role in the progression of breast cancers, and its overexpression was reported in >20% of patients with breast cancer.(1,2) HER2 overexpression is associated with poor clinical outcomes, and the monoclonal antibodies (mAbs) trastuzumab(3) and pertuzumab,(4) the tyrosine kinase inhibitor lapatinib,(5) and the antibody–drug conjugate (ADC) of trastuzumab and the maytansinoid microtubule assembly inhibitor trastuzumab emtansine(6) have been approved for the treatment of HER2-positive breast cancers.