**2.3 Stimuli-responsive cationic lipids**

It was revealed that lipoplexes enter the cells *via* nonspecific endocytosis, which occurs after the electrostatic binding of the positively charged lipoplexes to the negatively charged components of the cell membrane (Rejman, et al., 2006; Belting et al., 2005). There are a number of obstacles, hindering the efficient cationic liposome-mediated gene transfection: DNA release from the endosomes, DNA dissociation from the lipoplexes (Escriou et al., 1998; Rolland, 1998; Zabner et al., 1995) as well as insufficient release of DNA from endosomes. The use of stimuli-responsive delivery systems offers a new opportunity for the improvement of the delivery of nucleic acid (Ganta et al., 2008). Therefore, pH and redox microenvironment can be used as biological stimuli to improve the TE of lipoplexes. To achieve a stimuli-responsive release of DNA it is necessary to design cationic lipids containing trigger-groups, which specifically react to the alteration of the pH value, or to the presence of the intracellular reducing agents.
