**4.1 Transfection efficiency of chitosan**

Tong et al. (2009) describes seven steps that should be overcome before the expression of exogenous DNA. They are complexation, *in vivo* administration, endocytosis, escape from endolysosome, release of DNA, trafficking through cytoplasm and finally importation of DNA into the nucleus. The transfection efficiency of chitosan itself is; however, relatively low, when compared to lipoplex or other methods. But this aminopolysaccharide can be modified for ease of DNA delivery, as well as for target gene delivery, which currently attracts many researchers to use chitosan and its modifications for gene delivery. Chitosan can be modified by ligand conjugation, such as transferrin-, folate- (folate and transferrin are over expressed in cancer cells), mannose- (target dendritic cells in tumor) and galactose (target Kupffer cells of the liver) conjugated chitosan, which can improve transfection efficiency of the targeted cells via receptor–mediated endocytosis (Duceppe & Tabrizian, 2010; Mao et al., 2010).
