Gliomas are reclassified by molecular similarity beyond histological boundaries: diffuse astrocytoma [isocitrate dehydrogenase (IDH) mutant, wild-type, or not otherwise specified (NOS)], gemistocytic astrocytoma (IDH mutant), anaplastic astrocytoma (IDH mutant, wild-type, or NOS), glioblastoma (IDH mutant, wild-type, or NOS), diffuse midline glioma (H3K27M mutant), oligodendroglioma (IDH mutant and 1p/19q codeleted or NOS), anaplastic oligodendroglioma (IDH mutant and 1p/19q codeleted or NOS), oligoastrocytoma (NOS), and anaplastic oligoastrocytoma (NOS) (4, 5). The gene discussed is IDH1; the disease is anaplastic oligodendroglioma.