IDH1 and oligodendroglioma: In 2021, the World Health Organization (WHO) published the fifth version of the Classification of Tumors of the Central Nervous System, the adult-type diffuse gliomas were divided into glioblastoma (IDH-wildtype), oligodendroglioma (IDH-mutant, and 1p/19q-codeleted), and astrocytoma (IDH-mutant) [2].