IDH1 and oligodendroglioma: According to the 2016 WHO classification system, grade II oligodendroglioma is now defined by an isocitrate dehydrogenase (IDH) mutation with whole-arm codeletion of chromosomal arms 1p and 19q, whereas IDH mutation combined with intact 1p and 19q chromosomal status is classified as IDH-mutant astrocytoma.