RB1 and cancer: Therefore, CDK4/6 enzymes are considered promising targets in cancer therapy (Hanahan and Weinberg, 2011; Otto and Sicinski, 2017), and the anti-tumor effect of small molecule CDK4/6 inhibitors is based on blocking the phosphorylation of the tumor suppressor retinoblastoma (Rb), and induction of G1/S arrest in tumor cells (Knudsen et al., 2019) (Luo et al., 1998).