The previously known entity of oligoastrocytoma was defined on histological grounds only and is now extinct [44], as there is robust evidence that the IDH mutations segregate either with the chromosomal codeletion of 1p/19q in oligodendrogliomas, or with a loss of function mutation in the ATRX gene (alpha thalassaemia/mental retardation syndrome X-linked) in astrocytomas [41] (Fig. 2). The gene discussed is IDH1; the disease is oligodendroglioma.