Fig. 7. Targeted mutagenesis of Su(var)3-7. (A) The Su(var)3-7 locus and neighboring genes. (B) Donor construct using 2.6 kb of homology upstream of Su(var)3-7 (red) and 4 kb downstream (blue). (C) Combination of the donor transgenic lines with an I-SceI and FLP-producing strain leads to a double-strand break in the middle of the downstream region (blue) and can give rise to homologous recombination. (D) Use of homologous recombinant bearing a 1033 bp deletion 5' of the I-SceI site displaces the equilibrium towards a reduction in the 5' region of Su(var)3-7. (E,F) Crosses of recombinants with a strain expressing the I-CreI restriction enzyme lead to a double-strand break permitting many different reduction events. (F) Repair of the break in the 5' region of Su(var)3-7 results in deletion of the gene. B, BamHI; H, HindIII; R, EcoRI; X, XbaI.