The UAS upstream activation sequence contains varying numbers of a 17bp repeat that is specifically bound by the DNA-binding domain of the Saccharomyces cerevisiae GAL4 gene ( SGDID:S000006169 ) (PMID:3886158). GAL4 binds as a dimer to the UAS regulatory sequence, resulting in transcriptional activation of a downstream gene (reviewed in FBrf0233764). This property can be utilized to form a binary expression system to control the spatial and temporal expression of a gene of interest: a transgene or modified endogenous locus in which the target gene of interest is downstream of UAS sequences can be combined with a transgene or modified endogenous locus encoding any 'driver' that includes the GAL4 DNA-binding domain plus a transcriptional activation sequence (FBrf0064375, also reviewed in FBrf0233764, FBrf0216478).