 For example, by fusing a gene encoding a visible marker like GFP (Green Fluorescent Protein) the expression pattern of the driver genes can be determined.GAL4 and the UAS are very useful for studying gene expression in "Drosophila" as they are not normally present and their expression does not interfere with other processes in the cell.For example, GAL4/UAS-regulated transgenes in "Drosophila" have been used to alter glial expression to produce arrhythmic behavior in a known rhythmic circadian output called pigment dispersing factor (PDF).