Transposons are promising systems for somatic gene integration because they can 
not only integrate exogenous genes efficiently, but also be delivered to a 
variety of organs using a range of transfection methods. piggyBac (PB) 
transposon has a high transposability in mammalian cells in vitro, and has been 
used for genetic and preclinical studies. However, the transposability of PB in 
mammalian somatic cells in vivo has not been demonstrated yet. Here, we 
demonstrated PB-mediated sustained gene expression in adult mice. We constructed 
PB-based plasmid DNA (pDNA) containing reporter [firefly and Gaussia luciferase 
(Gluc)] genes. Mice were transfected by injection of these pDNAs using a 
hydrodynamics-based procedure, and the conditions for high-level sustained gene 
expression were examined. Consequently, gene expressions were sustained over 2 
months. Our results suggest that PB is useful for organ-selective somatic 
integration and sustained gene expression in mammals, and will contribute to 
basic genetic studies and gene therapies.