The fifth edition of the World Health Organization (WHO) Classification of Tumors of the Central Nervous System (CNS) released in 2021 has categorized adult-type diffuse gliomas into three types: (1) astrocytoma, isocitrate dehydrogenase (IDH)-mutant, (2) oligodendroglioma, IDH-mutant, and 1p/19q-codeleted, and (3) glioblastoma, IDH-wildtype (short for A, O, and GBM)2. The gene discussed is IDH2; the disease is oligodendroglioma.