ERBB2 and breast cancer: Treatment with the CDK4/6 inhibitor ribociclib plus ET significantly improved overall survival (OS) relative to ET alone in the landmark phase III MONALEESA-2, MONALEESA-3, and MONALEESA-7 trials, which represented the spectrum of premenopausal and postmenopausal patients with HR-positive, HER2-negative advanced breast cancer.1–3