OLIG2 and brain neoplasm: Of significant importance to the lineage hierarchy and heterogeneity of GBMs is that the Ascl1-OE phenotype, combined with the Olig2-CKO phenotype, offers two important insights into the functional interactions between ASCL1 and OLIG2 and the significance of their roles in regulating opposing NSC/astrocyte-like versus OPC/oligodendrocyte-like cell types in brain tumors, respectively.