ERBB2 and breast cancer: Abemaciclib, a CDK4/6 inhibitor, has been approved by the United States Food and Drug Administration as monotherapy and in combination with endocrine therapy for women with hormone receptor–positive, human epidermal growth factor receptor-2–negative metastatic breast cancer across various treatment lines and settings and in patients with high-risk hormone receptor–positive, human epidermal growth factor receptor-2–negative early breast cancer (11–16).