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Fig. 6. Gene targeting in Drosophila. (A) Ends-in
insertional gene targeting. The donor construct, within a P element,
contains a region of homology (the targeting construct, red) interrupted by a
restriction recognition site for the meganuclease I-SceI and flanked by
FRT recognition sites for FLP recombinase. It also contains a marker
(white+) and an appropriately located restriction
recognition site for the meganuclease I-CreI for a second round of homologous
recombination. After P element transgenesis, a linearized episome is
generated in vivo by FLP and I-SceI. Correct targeting results in
white+ expression and a tandem duplication of the locus.
This duplication can be reduced to single copy using I-CreI, resulting in loss
of white+. (B) Ends-out replacement gene targeting.
The donor construct, within a P element, contains a region of
homology interrupted by a white+ marker and is flanked by
restriction recognition sites for the meganuclease I-SceI and FRT
recognition sites for FLP recombinase. After P element transgenesis,
identified by white+, linearized targeting DNA is
generated in vivo by FLP and I-SceI. Correct targeting results in a
white+ phenotype and replacement of part of the locus.