Alteration of the cyclin-dependent kinase 4–6 (CDK4-6) pathway is common in several types of cancers, including GBM; a phase II trial evaluated palbociclib, an oral inhibitor of CDK4–6, in rGBM patients with RB1 (Retinoblastoma) proficiency in IHC. This evidence concerns the gene CDK4 and glioblastoma.