Adult diffuse gliomas are subdivided into three molecular subtypes that differ in their prognosis and age of onset: primary isocitrate dehydrogenase wild-type (IDHwt) glioblastoma (GB), IDH-mutant (IDHmut) astrocytoma, and IDHmut oligodendroglioma. The gene discussed is IDH2; the disease is astrocytoma (excluding glioblastoma).