IDH1 and oligodendroglioma: The overall patterns of genetic alterations that we have noted in GBM-O suggest a similar conclusion and indicate that these tumors can be divided into three discrete molecular classes: 1) those that are IDH mutant and 1p/19q co-deleted have the molecular signature of a high grade oligodendroglioma; 2) those that are IDH mutant and not co-deleted have features of secondary GBM; and 3) those that are IDHwt have the molecular signature of primary GBMs.