CDK4 and breast cancer: Currently, the CDK4/6 inhibitors, ribociclib, palbociclib and abemaciclib, have been approved by the U.S. Food and Drug Administration (FDA) for the treatment of breast cancer that is hormone receptor-positive and human epidermal growth factor receptor 2-negative, advanced or metastatic (Kwapisz, 2017; Tripathy et al., 2017) In cell-free assays, all three compounds are potent inhibitors of CDK4 and CDK6 (IC50 values from 2 to 39 nM) and ribociclib and abemaciclib inhibit CDK9 (IC50 of 1.51 μM and 57 nM, respectively) (Tripathy et al., 2017).