With the fifth edition of the World Health Organization (WHO) classification of tumors of the Central Nervous System (CNS) from 2021, gliomas with isocitrate dehydrogenase (IDH) mutations are classified either as diffuse astrocytoma of the WHO CNS grade 2–4, or as oligodendroglioma of the WHO CNS grade 2–3, if a 1p/19q co-deletion is additionally present [1]. The gene discussed is IDH1; the disease is central nervous system cancer.