The understanding of molecular features of diffuse gliomas has been revolutionized by the discovery of two distinct molecular markers: complete deletion of both the short arm of chromosome 1 (1p) and the long arm of chromosome 19 (19q) (1p/19q-codeletion) in oligodendroglial tumors, as well as mutations of the isocitrate dehydrogenase (IDH) 1 or 2 genes in oligodendrogliomas and a subset of astrocytic tumors [1–3]. The gene discussed is IDH3A; the disease is oligodendroglioma.