HER2 contributes to the aggressive features of breast cancers; however, rapid evolution of anti-HER2 therapies, including anti-HER2 antibodies such as trastuzumab [4], pertuzumab [5], T-DM1 [6], and DS-8201 [7] and pan-HER2 inhibitors such as neratinib [8], lapatinib [9], and tucatinib [10], has significantly improved the survival of patients with HER2-positive disease. This evidence concerns the gene ERBB2 and breast cancer.