However, the following three groups of adult-diffuse gliomas are recognized in the 2021 WHO classification: (1) astrocytoma, IDH-mutant, graded 2–4; (2) oligodendroglioma, IDH-mutant and 1p/19q-codeleted; and (3) glioblastoma, IDH-wildtype. Here, IDH1 is linked to oligodendroglioma.