**4. Cationic polymers**

DNA, when combined with sufficient amounts of cationic polymers, will condense into discrete entities which are called as polyplexes [43]. The polyplexes are compact nanoparticles formed through electrostatic interactions between the positive charges of amines and the negative charges of DNA phosphates. The strength of DNA binding to the polymers is related to the N:P ratio.

The most common cationic polymers used as nonviral gene-delivery vectors include chitosan, PLL, polyethylenimine (PEI), poly(amido amine) (PAMAM) dendrimers and select polypeptides [24, 44, 45].
