Astrocytoma, isocitrate dehydrogenase (IDH)-mutant (A_IDH_mut) and oligodendroglioma, IDH-mutant and 1p/19q-codeleted (O_IDH_mut) are the 2nd and 3rd most common diffuse infiltrating gliomas in adults after glioblastoma, IDH-wildtype (GBM). The gene discussed is IDH2; the disease is oligodendroglioma.