Fig. 5. (A) The Gal4 UAS system. A construct that contains a known promoter or enhancer coupled to the Gal4 gene drives region-specific expression of Gal4. Gal4, in turn, stimulates the transcription from a construct that links the UAS sequence to a chosen coding sequence. (B) The Gal4 enhancer trap. When the enhancer trap construct falls near an active endogenous enhancer, that enhancer drives Gal4 expression. (C) The EP construct. In the presence of Gal4, the UAS and promoter in the inserted UAS-EP construct drive the expression of neighboring genes. (D) A late third instar wing disc showing the region-specific expression of UAS-GFP (green) driven by a ptc^Gal4 enhancer trap. Strong anti-Ci (Cubitus interruptus) staining (red) shows the anterior compartment. (E) In situ hybridization of a late third instar wing disc showing the ectopic expression of an endogenous gene driven using an EP insertion. ptc^Gal4 drives the expression of crossveinless 2 (cv-2), which is located near the EP(2)1103 insertion. Reproduced, with permission, from Conley et al. (Conley et al., 2000). (F) Generating dsRNA using a UAS-hairpin construct. Gal4 drives the expression of a UAS construct containing two inverted coding regions placed head to head (red, arrows), separated by a short spacer sequence. The mRNA produced by the construct folds back upon itself to form dsRNA, which will interfere with expression of the corresponding endogenous gene.