This framework distinguishes three principal groups: (1) IDH-wildtype glioblastoma (CNS WHO grade 4), characterized by the absence of an IDH mutation and the presence of alterations such as TERT promoter mutations; (2) IDH-mutant astrocytoma (CNS WHO grade 3 or 4), defined by an IDH mutation and homozygous deletion of CDKN2A/B; and (3) IDH-mutant and 1p/19q-codeleted oligodendroglioma (CNS WHO grade 3). Here, IDH1 is linked to oligodendroglioma.