Isolation and characterization of Df(2R)BSC261 Stacey Christensen and Kevin Cook Bloomington Stock Center Indiana University Df(2R)BSC261 was isolated as a FLP recombinase-induced recombination event involving PBac{RB}Epace01145 and P{XP}d06988. The deletion was isolated as a chromosome lacking miniwhite markers in progeny of P{hsFLP}1, y1 w1118; PBac{RB}Epace01145/P{XP}d06988 males crossed to w1118; P{hs-hid}2, wgSp-1/CyO females. These males 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.RB3}BSC261 from the segment of PBac{RB}Epace01145 to the left of its FRT site and the segment of P{XP}d06988 to the right of its FRT site. Its presence was verified using the PCR methods and primers described in Parks et al. with the substitution of the primer 5'-CCAATGCGTTTATTTCAGGTCACG-3' for the RB3' plus or RB3' minus primer in the Hybrid PCR protocol in the Supplementary Methods. Exelixis, Inc. determined the insertion site of P{XP}d06988 to be at Release 3 genomic coordinate 2085948 on chromosome arm 2R. This corresponds to 42E5 on Release 3 and 4 genome map2. The predicted position of PBac{RB}Epace01145 on the Release 4 map is 42D1. Consequently, the cytological breakpoints of Df(2R)BSC261 are predicted to be 42D1;42E5. It failed to complement Df(2R)Exel6051.