Transposons are mobile genetic elements that can be used to integrate transgenes into host cell genomes. The piggyBac transposon system has been used for transgenesis of insects and for germline mutagenesis in mice. We compared transposition activity of piggyBac with Sleeping Beauty (SB), a widely used transposon system for preclinical gene therapy studies. An engineered piggyBac transposon with minimal length 5' and 3' terminal repeats exhibited greater transposition activity in transfected cultured human cells than a well-characterized hyperactive SB system. PiggyBac excision was very precise as evidenced by the typical absence of "footprint" mutations at the site of transposon excision. We mapped 575 piggyBac integration sites in human cells to determine site selectivity of genomic integration. PiggyBac demonstrated non-random integration site selectivity that differed from that previously reported for SB, including a higher preference for integrations in regions surrounding transcriptional start sites and within long terminal repeat elements. Importantly, overproduction inhibition was not observed with piggyBac, a major limitation of the SB system. This permitted the generation of combination "helper-independent" piggyBac transposase-transposon vectors that exhibited a 2-fold increase of transposition activity in human cells as compared with cells transfected with separate transposon and transposase plasmids. We conclude that piggyBac is a transposon system with certain properties, including high efficiency and lack of overproduction inhibition that are advantageous in preclinical development of transposon-based gene therapy.