Although endocrine therapy utilizing tamoxifen and raloxifene (for ER) is effective in treating luminal A and luminal B subtypes of BC [25,26], HER2-enriched BCs are not responsive and thus HER2-targeted therapy using trastuzumab is developed for tamoxifen-resistant HER2-enriched BC [27]. The gene discussed is ERBB2; the disease is breast cancer.