**5. Conclusion and prospect**

Since the first clinical trial of gene therapy in 1990, 1703 gene therapy clinical trials have been completed as of March of 2011 (http://www.wiley.com/legacy/wileychi/genmed /clinical/). However, little progress has been made since the first gene therapy clinical trial, and therefore, the Food and Drug Administration of the United States has not yet approved any human gene therapy for actual patient treatments. Although current gene therapy is still in the experimental stage, *ex vivo* gene therapeutic approaches show a great potential to treat monogenic genetic diseases as shown in the clinical trial results of adenosine deaminase deficiency and familial hypercholesterolemia (Cappelli et al., 2010 & Kassim et al., 2010). The clinical trial results of these diseases were encouraging to continually pursue *ex vivo* gene therapy.

Primary cells, including adult stem cells, have limited self-renewal ability and are vulnerable to epigenetic modification (Dube & Denis, 1995; Muller-Sieburg & Sieburg, 2006; Tseng et al., 2006; Nehlin & Barington, 2009). The long-term culture of primary cells is not possible. Therefore, it is absolutely necessary to deliver therapeutic DNA molecules into isolated cells promptly with high efficiency. However, the transfer efficiency of exogenous DNA into primary cells is very low (Beyer & Da sliva, 2006; Tonti & Mannello, 2008). The gene transfer efficiency into primary cells is several fold less than those of cell lines in current gene transfer methods. This means that improvement of the transfer efficiency of exogenous DNA into primary cells is the first obstacle for the practical use of for *ex vivo* gene therapy.

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#### **6. Acknowledgement**

This Research was supported by the grant of Rural development administration, South Korea.
