IDH-mutant and 1p/19q-co-deleted oligodendrogliomas are the second tumor classified as diffuse lower-grade adult glioma and are characterized by the coexistence of an IDH1 or IDH2 mutation and the deletion of the entire 1p and 19q arms, as well as a characteristic morphological appearance. The gene discussed is IDH2; the disease is oligodendroglioma.