PEI covalently modified with the HIV-1 gp41-derived peptide HGP led to a 38-fold increase of gene expression after plasmid DNA delivery and also enhanced siRNAmediated knockdown of GAPDH by approximately 2-fold [199] . 30 % of cells incubated with PEI-HGP polyplexes showed not only the punctuate plasmid DNA staining observed with PEI polyplexes alone, but also a diffuse fluorescence throughout the cell, indicative of endosomal release of vectors. This would explain the observed increase in transfection efficiency, since the overall uptake was unaffected.