The expression of P-gp as assessed by western blot analysis. The levels of β-actin protein were also examined and served as a loading control. The expression of P-gp was upregulated in MDR1-transfected 4T1 cells. The bar graphs represent the quantification and comparison of the signal intensity of the bands on the immunoblots. P < 0.05** vs. control cells. This experiment was repeated at least 3 times with the same results.