CDK4 and breast cancer: Since the first report of efficacy of the cyclin dependent kinase 4/6 (CDK4/6) inhibitor palbociclib in hormone receptor (HR)-positive advanced breast cancer (ABC) [5], multiple large randomized phase three trials have established the efficacy of CDK4/6 inhibitors (palbociclib, ribociclib and abemaciclib) in the first-line setting in both pre-menopausal and post-menopausal women in combination with ET [6,7,8,9,10,11].