Recently, studies have suggested that at least genetically, the majority of these “mixed histology” tumors contain genetic events representative of either astrocytomas, namely mutations in IDH1, ATRX, and TP53, or oligodendrogliomas, namely chr 1p/19q LOH and mutations in CIC and/or FUBP1 [9, 11, 18, 19]. This evidence concerns the gene IDH1 and oligodendroglioma.