Besides technical difficulties arising from the syntheses of conjugates consisting of short cationic or hydrophobic peptides and highly negatively charged siRNAs, Dowdy and his group [92] present a rather critical point of view referring to previous studies with CPP-siRNA-conjugates. They claim that the successful delivery described therein is solely the result of excess free peptide, which leads to additional complexation, and thereby cellular import of the siRNA. This is in accordance with Turner et al. [93] , who were the first to observe that careful purification of CPP-antisense-conjugates abrogates their biological effect. Among other things, this might be the reason why most of the studies reporting on successful peptide-mediated delivery of siRNAs use a noncovalent complexation approach (confer Table 2 ).