The World Health Organization (WHO) Classification of Central Nervous System (CNS) Tumors (2021) classifies adult-type diffuse glioma based on the presence of IDH mutations and other key molecular alterations in astrocytoma IDH mutant, oligodendroglioma IDH mutant and 1p/19q-co-deleted, and glioblastoma IDH-WT [10]. The gene discussed is IDH1; the disease is oligodendroglioma.