Fig. 4. Long-term silencing by PRE requires its continuous presence in the transgene. X-gal stainings of wing imaginal discs from larvae that carry the transgenes indicated on the left. Larvae were either not heat-shocked (no hs) or were heat-shocked for 1 hour to induce flp expression and then allowed to develop for the indicated number of hours before analysis (hs + xy hours). Note the loss of expression after flp-mediated excision of the PRE fragment during larval development. (Bottom) Time-course experiment; no X-gal signal is detected 8 hours after flp induction but robust X-gal staining is observed 12 hours after flp induction. Note that flp-mediated excision only occurs in a fraction of the cells, and that the number of X-gal positive cells increases over time due to cell division.