The stable genomic integration and expression of a large transgene is a major hurdle in gene therapy. We show that the modified piggyBac (PB) transposon system can be used to introduce a 207 kb genomic DNA fragment containing the RORγ/γt locus into human cells and mice. PB-mediated transgenesis results in a single copy of a stably inherited and expressed transgene. These results indicate that PB could serve as an effective high-capacity vector for functional analysis of the mammalian genome and for gene therapy in human cells.