IDH1 and oligodendroglioma: As per the 2021 fifth edition of the WHO Classification of Tumors of the Central Nervous System, adult diffuse gliomas are categorized as “astrocytoma, IDH-mutant,” “oligodendroglioma, IDH-mutant, 1p19q-codeletion,” or “glioblastoma, IDH-wildtype” (Louis et al., 2021).