IDH1 and oligodendroglioma: Based on clinical and molecular pathology, the 2021 World Health Organization (WHO) Classification of Tumors of the Central Nervous System (5th edition) has classified adult-type diffuse gliomas into three types, including astrocytoma, IDH-mutant; oligodendroglioma, IDH-mutant and 1p/19q-deleted; and glioblastoma, IDH-wildtype (3).