Effect of Sp1 siRNA transfection upon invasiveness of U87 tumor cells under normoxic and hypoxic conditions. A. U87 cell invasion at 4 × objectives: N: normoxic incubation, H: hypoxic incubation, U87: control cells, Sp1-DR: U87 cells expressing Sp1 siRNA. 1. U87 control cells with transfected empty vector under normoxic conditions. 2. U87 control cells subjected to hypoxic incubation. 3. Sp1-deficient U87 cells under normoxic conditions. 4. Sp1-deficient U87 cells under hypoxic conditions. B. Invasive cell number compared to normoxic control. *P < 0.05 compared to normoxic control. #P < 0.05 compared to hypoxic control.