 These genetic instructions occur in every cell of the animal, but in most cells nothing happens since that cell is not producing GAL4.In the cells that "are" producing GAL4, however, the UAS is activated, the gene next to it is turned on, and it starts producing its resulting protein.This may report to the investigator which cells are expressing GAL4, hence the term "reporter line", but genes intended to manipulate the cell behavior are often used as well.