As described in Thibault et al. (2004), the Exelixis PBac{PB}, P{XP}, PBac{WH} and PBac{RB} insertion screens were designed to preserve a common isogenic genetic background. Nevertheless, problems were encountered in establishing stocks of the X chromosome insertions and most of the insertions were balanced or homozygosed using an FM7c stock that was not coisogenic. Consequently, the nonbalancer X chromosomes in all Exelixis insertion stocks share the same background, but the autosomes in most X insertion stocks are different. The P{EP} insertion stocks donated by Exelixis were generated by Rørth et al. (1996) and have a different genetic background.