BMI1 and glioblastoma: The involvement of Notch signaling in the maintenance of the tumorigenic potential of GSC was indicated by the demonstration that treatment of glioblastoma sphere cultures with gamma-secretase inhibitors (GSIs) can deplete CD133+ GSC, downregulate putative GSC markers (CD133, nestin, BMI1, Olig2), and inhibit growth of tumor spheres and xenografts [23].