After unearthing that it is indeed those cationic polyethyleneimine (PEI) chains free in the mixture of polymeric non-viral vector and DNA that promote the gene transfection,we have tried in the last several years to understand the captioned question by using a combination of chemical synthesis,physical characterization and molecular biology.We have studied the dynamics of the complexation between different PEI chains and plasmid DNA (pDNA) and their subsequent in-vitro gene transfection under different conditions.