Somatic cell gene transfer has permitted inducible gene expression in vivo 
through coinfection of multiple viruses. We hypothesized that the highly 
efficient plasmid-based piggyBac transposon system would enable long-term 
inducible gene expression in mice in vivo. We used a multiple-transposon 
delivery strategy to create a tetracycline-inducible expression system in vitro 
in human cells by delivering the two genes on separate transposons for inducible 
reporter gene expression along with a separate selectable transposon marker. 
Evaluation of stable cell lines revealed 100% of selected clones exhibited 
inducible expression via stable expression from three separate transposons 
simultaneously. We next tested and found that piggyBac-mediated gene transfer to 
liver or lung could achieve stable reporter gene expression in mice in vivo in 
either immunocompetent or immune deficient animals. A single injection of 
piggyBac transposons could achieve long-term inducible gene expression in the 
livers of mice in vivo, confirming our multiple-transposon strategy used in 
cultured cells. The plasmid-based piggyBac transposon system enables 
constitutive or inducible gene expression in vivo for potential therapeutic and 
biological applications without using viral vectors.