**3. Non-degradable cationic polymers as non-viral gene delivery vectors**

This section reviews typical non-degradable cationic polymers as non-viral vectors for gene delivery. Although these polymer systems normally have low transfection capability *in vitro* and/or high cytotoxicity, from the studies on these systems, a few fundamentals on gene delivery properties have been well understood, which are valuable in the design of safe and potent polymeric gene delivery vectors.
