IDH1 and oligodendroglioma: Among the diffusely infiltrating gliomas in adults, the WHO classification distinguishes three major tumor types, namely the IDH-mutant astrocytomas of CNS WHO grades 2, 3, or 4, the IDH-mutant and 1p/19q-codeleted oligodendrogliomas of CNS WHO grades 2 or 3, as well as the IDH-wildtype glioblastomas of CNS WHO grade 4.