IDH1 and astrocytoma (excluding glioblastoma): High-grade IDH-mutant astrocytomas often show cell cycle deregulation due to homozygous CDKN2A/B deletions or amplifications of CDK4 or CDK6. However, these were only rare events in ASTRO PNCs with only a single case showing homozygous CDKN2A/B deletion (n = 1/51, 2%) and three cases with CDK4 amplification (n = 3/51, 6%).