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5. The EPg P element is a germline-competent GAL4-inducible expression element; 8500 insertion lines were generated as described (4). For the screen EPg males from each line were crossed to w; slbo 1 slboGAL4/CyO ; nanosGAL4:VP16 virgins and ovaries from female progeny carrying all transgenes were analyzed. Abnormal migration was identified by visual inspection of egg chambers stained with X-Gal to reveal a border cell–specific lacZ -enhancer trap (Fig. 1A) (2). nanosGAL4:VP16 drives expression in the germline tissue and slboGAL4 in the border cells as well as in centripetal cells later on. Positive lines were retested with the individual GAL4 drivers. Egg chamber morphology was analyzed by 4′ 6′-diamidino-2-phenylindole (DAPI) and phalloidin staining to highlight DNA and F-actin respectively. Sequencing of a plasmid rescue fragment showed EPg35521 to be inserted into the 5′ untranslated region of vein.