The classification under the 2021 update is dependent largely on isocitrate dehydrogenase (IDH1/2) mutation status and 1p/19q codeletion status, resulting in three primary disease groups: IDH-mutant, 1p/19q co-deleted oligodendroglioma; IDH-mutant, non-co-deleted astrocytoma; and IDH-wildtype glioblastoma (GBM). The gene discussed is IDH1; the disease is oligodendroglioma.