The WHO 2021 CNS update incorporates many additional genomic alterations and defines IDH-mutant (Mut) astrocytoma, IDH-Mut oligodendroglioma, and H3K27-altered diffuse midline glioma, among others, as separate entities from IDH-WT glioblastoma, recognizing their distinct clinical and histopathological features.14 The gene discussed is IDH1; the disease is oligodendroglioma.