CDK4 and breast carcinoma: The development of selective of CDK4/6 inhibitors has markedly changed the perception of CDKs as therapeutic targets in cancer and, based on favorable data from pivotal phase III trials, palbociclib, ribociclib, and abemaciclib, are now approved both in the United States and Europe for women with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer.