PGP and breast cancer: Silencing P-gp expression by RNAi with reduction-sensitive linear cationic click polymer nanoparticles (RCPNs) loaded with plasmid iMDR1-pDNA for gene delivery revealed higher transfection efficiency and lower cytotoxicity than PEI/DNA nanoparticles against human breast cancer MCF-7 cells and drug-resistant MCF-7/ADR cells (Gao et al., 2011).