ERBB2 and breast cancer: Endocrine therapy with ovarian suppression plus a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor is a recommended first-line treatment for premenopausal women with hormone-receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2−) advanced breast cancer (ABC)1,2; however, treatment resistance and disease progression ultimately occur.