 The presence of GAL4, by itself, in these cells has little or no effect, since GAL4's main effect is to bind to a UAS region, and most cells have no (or innocuous) UAS regions.Since Gal4 by itself is not visible, and has little effect on cells, the other necessary part of this system are the "reporter lines".These are strains of flies with the special UAS region next to a desired gene.