Gal4 is a modular protein consisting broadly of a DNA-binding domain and an activation domain. The UAS to which GAL4 binds is CGG-N11-CCG, where N can be any base.[6] Although GAL4 is a yeast protein not normally present in other organisms it has been shown to work as a transcription activator in a variety of organisms such as Drosophila, and human cells, highlighting that the same mechanisms for gene expression have been conserved over the course of evolution. GAL4 is then only expressed in cells where the driver gene is usually active. In turn, GAL4 should only activate gene transcription where a UAS has been introduced. 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). However, some research has indicated that over-expression of GAL4 in Drosophila can have side-effects, probably relating to immune and stress responses to what is essentially an alien protein.