After internalization, DNA complexes must escape from the formed intracellular vacuoles. Viruses that do not fuse at the plasma membrane exploit endosome acidification as an escape signal, by means of sophisticated conformational changes of fusion proteins. In the case of cationic lipids or polymer/DNA complexes, most particles probably remain trapped within the vacuoles. DNA being protected, this may even be regarded as an effective ‘slow release’ process (as an example, cell sorting of GFP-transfected cells after 24 h showed that 14% of the nontransfected cells heavily expressed GFP after 6 days [28]).