Knock down of HIF-1α reduces the ability of murine glioma cells to form tumor spheres. Primary tumor sphere forming capacity of GL261 shHIF cells in comparison with control cells. Three independent experiments were performed. (A) Bar graph represents the % of the total number of tumor spheres in shHIF cells versus the control. Knockdown of HIF-1α significantly reduces the ability of glioma cells to form tumor spheres. The shHIF cells formed 60% fewer tumor spheres compared with control cells (asterisk denotes p = 0.02). (B) Bar graph represents the average diameter (μm) of tumor spheres in shHIF cells versus the control. Lack of HIF-1α significantly reduces the average diameter of the spheres formed (167.4 ± 4.1 μm) compared with the size of those derived from control cells (240.6 ± 2.8 μm) (three asterisks denotes p < 0.0001). (C) Representative images of tumor spheres derived from either control or shHIF cells.