 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.The GAL4-UAS system has also been employed to study gene expression in organisms besides "Drosophila" such as the African clawed frog "Xenopus" and zebrafish.The GAL4/UAS system is also utilized in Two-Hybrid Screening, a method of identifying interactions between two proteins or a protein with DNA.