CDK4 and neoplasm: Activated Akt promotes G1/S transition primarily by increasing Cyclin D1 expression and stability and by enhancing CDK4/6 activity, which leads to phosphorylation and functional inactivation of Rb, thereby liberating E2F transcription factors that drive S-phase gene expression (37, 44), and promoting ribosomal biosynthesis and energy metabolism by activating mTORC1, thereby providing the biomolecules required for tumor cell proliferation (45).