They are present in all oligodendrogliomas by definition (oligodendroglioma, IDH-mutant and 1p/19q-codeleted), in most low-grade diffuse astrocytomas in adults and in a subset of glioblastomas that are now renamed astrocytoma, IDH-mutant, grade 4 (2). The gene discussed is IDH2; the disease is oligodendroglioma.