Deoxyribonucleic acid (DNA) has long been envisioned as a drug for both therapeutic (gene therapy) and prophylactic (vaccination) use. The idea of gene-based medicine is simple and appealing: If a disease is caused by a deficient gene, delivery of a good copy of the gene should correct the defect, making DNA a therapeutic drug. Similarly, delivery of an antigen gene should elicit a specific immune response against the protein encoded by the delivered gene, making DNA a prophylactic vaccine. (Note that delivery of genes is much safer than delivery of proteins because DNA molecules themselves do not elicit immune reactions.) However, after more than a decade of intense research and more than 600 clinical trials, no truly successful gene therapy protocol has been reported, and no DNA vaccination has been approved. The reason is the lack of an efficient and safe gene delivery system.