CDK4 and astrocytoma (excluding glioblastoma): In fact, the presence of homozygous CDKN2A/B deletion is now a diagnostic criterion for Astrocytoma WHO grade 4 in the new WHO 2021 classification system of CNS tumors.19 Furthermore, the cell cycle regulatory axis involving CDK4 and 6 is potentially targetable with new inhibitors such as ribociclib, palbociclib, and abemaciclib.20 These inhibitors may act to replace the lost inhibition seen with CDKN2A/B co-deletion.