We demonstrate that vectors incorporating components from Epstein-Barr virus (EBV) for retention and from human genomic DNA for replication greatly enhance the level and duration of marker gene expression in dividing cultured cells. The same types of vectors were tested in vivo by high-pressure tail vein injection of naked DNA in mice, resulting in liver delivery and expression. The therapeutic gene was a human factor IX (hFIX) minigene comprising genomically derived 5', 3', and intronic sequences that provided relatively good gene expression in vivo. a demonstrated hat addition of the EBV and its family of repeats binding sites provided a 10- to 100-fold increase in prolonged hFIX expression in mouse liver. A single 25-mug dose of vector DNA generated normal (>5 mug/mL) levels of hFIX throughout the 8 month duration of the experiment. Vector DNA. with or without the EBV urns retained in liver cells, and vector replication was not a factor in these nondividing liver cells. Instead, it appears that enhancement of stable hFIX expression by the EBV components was responsible for the increased level and duration of therapeutic gene expression. The EBV also significantly enhanced stable expression. of a vector carrying the full genomic hFIX gene delivered to mouse liver. These results underline the crucial importance of appropriate gene expression signals on gene therapy vectors and the utility of EBV sequences in particular for increasing stable gene expression.