CDK4 and glioblastoma: The above studies showed that GBM cells were arrested in cell cycle G1 phase after USP5 knockdown, so we wondered whether USP5 regulated key proteins driving cell cycle G1 to S transition, including CDK2, CDK4, CDK6, CyclinD1, and CyclinE1 (Malumbres and Barbacid, 2009; Musgrove et al., 2011).