Df(1)BSC857 was isolated as a FLP recombinase-induced recombination event involving P{XP}yld10478 and PBac{WH}Flo-2f04495. The deletion was isolated as a chromosome lacking miniwhite markers in progeny of P{XP}yld10478/PBac{WH}Flo-2f04495; MKRS, P{hsFLP}86E/+ females crossed to Binsinscy/Y males. These females were heat shocked as larvae as described in Parks et al., Nature Genetics 36: 288-292, 2004 (FBrf0175003). This cross and crosses in preceding and succeeding generations maintained the original genetic background of the Exelixis insertion stocks (Thibault et al., Nature Genetics 36: 283-287, 2004; FBrf0175002). The recombination event generated the genetic element P+PBac{XP5.WH5}BSC857 from the segment of P{XP}yld10478 to the left of its FRT site and the segment of PBac{WH}Flo-2f04495 to the right of its FRT site. Its presence was verified using the PCR methods and primers described in Parks et al. The breakpoints of Df(1)BSC857 predicted from the Release 5 genomic coordinates of the transposable element insertion sites are  X:14087266 ;14819627 and the cytological breakpoints predicted from these coordinates are 12E3;13A1. It failed to complement mud4 and na1.