CDK4 and cancer: PROTACs utilize the UPS by linking a target protein with an E3 ubiquitin ligase to artificially induce the target’s ubiquitination and have been studied as inhibitors of cyclin-dependent kinase 4 (CDK4), CDK6, and histone deacetylase 6 (HDAC6)—all of which promote cell cycle progression in GBM and numerous other oncogenic proteins in other cancers [55].