It has been observed that blocking ABCB1 or ABCG2 with the specific exogenous siRNAs can reverse multidrug resistance in diverse cancer cells [384,385]; even more, the concomitant application of both siRNAs using a nanoparticle-facilitated delivery showed a synergic effect in breast cancer cells [386]. The gene discussed is ABCG2; the disease is breast cancer.