For in vivo gene transfer, synthetic (nonviral) vectors are thought to have several advantages compared with viral vectors: they can accommodate largesize DNA molecules (therapeutic genes including their endogenous regulatory regions), be used under reduced confinement conditions, be administered repeatedly, and be modified with appropriate ligands that allow specific cell targeting. Until now, many different types of organic compounds have been tested as synthetic gene delivery vectors in animal models by various routes of administration (reviewed in refs. 1 –13 ).