The GAL4::GCN4 entry in FlyBase represents an artificial transcriptional activator ('driver') that includes the DNA-binding domain from the Saccharomyces cerevisiae GAL4 gene ( SGDID:S000006169 ) and a transcriptional activation domain from the Saccharomyces cerevisiae GCN4 gene ( SGDID:S000000735 ). The GAL4 DNA-binding domain binds specifically to the UAS regulatory sequence, and thus UAS and GAL4::GCN4 form a binary expression system that can be used to control the spatial and temporal expression of a gene of interest: a transgene or modified endogenous locus carrying the target gene of interest downstream of UAS sequences is combined with a second transgene or modified endogenous locus encoding the GAL4::GCN4 driver. The exact sequence of the driver may differ depending on the particular transgene or modified endogenous locus being used. In particular, the number of GCN4 activation domains present may vary, and/or the GCN4 activation domains may contain mutations that alter the strength of transcriptional activation.