Notably, palbociclib (IBRANCE®), a dual CDK4/6 inhibitor, recently received accelerated approval by the Food and Drug Administration (FDA) for clinical breast cancer treatment due to its potent and selective inhibitory effect on estrogen receptor (ER) positive/human epidermal growth factor receptor 2 (HER2) negative breast cancer [15–18]. Here, CDK4 is linked to breast cancer.