IDH1 and oligodendroglioma: We divide the gliomas discussed herein into 5 main histologic/molecular categories: IDH-mutant lower-grade astrocytoma (WHO grades II and III), IDH-mutant 1p/19q-codeleted oligodendroglioma (WHO grades II and III), IDH-mutant GBM (WHO grade IV), IDH-wildtype lower-grade astrocytoma (WHO grades II and III), and IDH-wildtype GBM (WHO grade IV), as well as various additional assorted gliomas not elsewhere classified.