Mutagenesis by transposon-mediated imprecise excision is the most extensively 
used technique for mutagenesis in Drosophila. Although P-element is the most 
widely used transposon in Drosophila to generate deletion mutants, it is limited 
by the insertion coldspots in the genome where P-elements are rarely found. The 
piggyBac transposon was developed as an alternative mutagenic vector for 
mutagenesis of non-P-element targeted genes in Drosophila because the piggyBac 
transposon can more randomly integrate into the genome. Previous studies 
suggested that the piggyBac transposon always excises precisely from the 
insertion site without initiating a deletion or leaving behind an additional 
footprint. This unique characteristic of the piggyBac transposon facilitates 
reversible gene-transfer in several studies, such as the generation of induced 
pluripotent stem (iPS) cells from fibroblasts. However, it also raised a 
potential limitation of its utility in generating deletion mutants in 
Drosophila. In this study, we report multiple imprecise excisions of the 
piggyBac transposon at the sepiapterin reductase (SR) locus in Drosophila. 
Through imprecise excision of the piggyBac transposon inserted in the 5'-UTR of 
the SR gene, we generated a hypomorphic mutant allele of the SR gene which 
showed markedly decreased levels of SR expression. Our finding suggests that it 
is possible to generate deletion mutants by piggyBac transposon-mediated 
imprecise excision in Drosophila. However, it also suggests a limitation of 
piggyBac transposon-mediated reversible gene transfer for the generation of 
induced pluripotent stem (iPS) cells.