Knockdown of P-gp in 4T1-R (top panel), SKBR-3 (middle panel) and MCF-7 (bottom panel) breast cancer cells with/without aptamer-labeled F31 nanoparticles compared to lipofectamine transfection. In both cases of lipofectamine and aptamer-labeled nanoparticle transfection, the cells were transfected with 100 pmol siRNA for 24h, and then the cells were scraped by using trypsin-EDTA, washed with PBS, pelleted and the expression of P-gp was measured by Western blot analysis.