CCND1 and glioblastoma: Furthermore, in nearly half of anaplastic astrocytoma and almost all glioblastoma cases, the p16-cyclin-dependent kinase (CDK)4/CDK6-cyclin D1 (CCND1)/retinoblastoma (RB) pathways [9] that negatively regulate cell cycle progression are aberrant, leading to uncontrolled tumor cell proliferation.