IDH1 and oligodendroglioma: The 2021 revision of the World Health Organization (WHO) classification of central nervous system tumors reclassified adult diffuse gliomas into the following three types: oligodendrogliomas, isocitrate dehydrogenase mutant (IDH-mut), and 1p/19q co-deleted; astrocytomas, IDH-mut; and glioblastomas, IDH-wildtype (wt).