CDK4 and central nervous system cancer: The increased expression of tumor-suppressors (66) is common in many cancer types, including cyclin dependent kinase inhibitor p16Ink4a in glioma and HPV-induced cancers (67–69), CDK4/6 inhibitor p18Ink4c in cervical cancer and glioblastoma (70,71), and CDK2 and PCNA inhibitor p21 in many types of cancer (72).