Polypeptides with therapeutic benefit and those for which a cellular and humoral immune response is desirable can be produced by injection of plasmid DNA into muscle (for review, see ref. [1 ]). The safety and efficacy of the DNA-based approach has resulted in several clinical trials with plasmids encoding cytokines, alloantigens, cancer antigens, and antigens from pathogens (2 -8 ), but in most cases the results from these studies have been less than inspiring. Data collected to date would suggest that the success of this method requires higher levels of transfection and sustained expression than are achievable with plasmid in the absence of a delivery system. Furthermore, the efficacy of DNA vaccines will depend on the presentation of DNA encoded antigen by activated professional antigen-presenting cells (APCs) such as Langerhans cells, macrophages, and dendritic cells (DCs) (1 ).