The PiggyBac (PB) transposon from the cabbage looper moth Trichoplusia ni was engineered to be highly active in mammalian cells, and is capable of genome-wide mutagenesis. Transposons contained both PB and Sleeping Beauty inverted repeats, in order to be recognized by both transposases and increase the frequency of transposition. In addition, the transposon contained promoter and enhancer elements, a splice donor and acceptors to allow gain- or loss-of-function mutations depending on the transposons orientation, and bidirectional polyadenylation signals. The transposons were transformed into mouse cells in vitro' and mutants containing tumours were analyzed. The mechanism of the mutation leading to tumour formation determined if the gene was classified as an oncogene or a tumour-suppressor gene. Oncogenes tended to be characterized by insertions in regions leading to overexpression of a gene, whereas tumour-suppressor genes were classified as such based on loss-of-function mutations. Since the mouse is a model organism for the study of human physiology and disease, this research will help lead to an increased understanding of cancer-causing genes and potential therapeutic targets.