IDH1 and glioblastoma: The updated 2021 WHO classification of CNS tumors further emphasizes the clinical importance of the IDH1/2 mutations by reducing the number of types of adult diffuse glioma to three (astrocytoma, oligodendroglioma, and GBM), with astrocytoma and oligodendroglioma now requiring the presence of an IDH1/2 mutation for diagnosis.54