Infiltrating lower-grade gliomas (LGG, grade 2/3), i.e., astrocyte- and oligodendrocyte-derived tumors, currently include isocytrate dehydrogenase (IDH) mutant 1p/19q-codeleted (IDHmut1p19qcod) oligodendroglioma, IDH-mutant 1p/19q-intact (IDHmut1p19qint) astrocytoma, and IDH-wildtype (IDHwt) astrocytoma. The gene discussed is IDH1; the disease is oligodendroglioma.