Sustained transgene expression will be required for the vast majority of genetic diseases being considered for gene therapy. The initially high levels of expression attained with plasmid DNA (pDNA) vectors containing viral promoters, such as that from cytomegalovirus (CMV), decline precipitously to near background levels within 2 to 3 weeks. We have constructed pDNA vectors containing the human cellular ubiquitin B (Ub) promoter and evaluated their expression in the mouse lung. Cationic lipid-pDNA complexes were instilled intranasally (IN) or injected intravenously (IV) into immunodeficient BALB/c mice. Chloramphenicol acetyltransferase (CAT) reporter gene expression from the Ub promoter was initially very low at day 2 post-administration but by day 35 exceeded the level of expression attained from a CMV promoter vector by 4- to 9-fold. Appending a portion of the CMV enhancer 5' of the Ub promoter (CMV-Ub) increased CAT expression to nearly that of the CMV promoter and expression persisted in the lung for at least three months, with 50% of day 2 levels remaining at day 84. In the liver, expression from the CMV-Ub hybrid promoter was sustained for 42 days. Since previous studies have shown that eliminating immunostimulatory CpG motifs in pDNA vectors reduces their toxicity, we constructed a CpG deficient version of the CMV-Ub vector expressing alpha-galactosidase A, the enzyme that is deficient in Fabry disease, a lysosomal storage disorder. After IN or IV administration, levels of alpha-galactosidase A from this vector were not only undiminished but increased 500% to 1500% by day 35. These results suggest that CpG-reduced plasmid vectors containing a CMV-Ub hybrid promoter may provide the long-term expression and efficacy required for a practical gene therapeutic.