The GAL4::VP16 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 Herpes simplex virus Type 1 VP16 gene (UniProtKB:P06492) (PMID:3047590). The GAL4 DNA-binding domain binds specifically to the UAS regulatory sequence, and thus UAS and GAL4::VP16 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::VP16 driver. The exact sequence of the driver may differ depending on the particular transgene or modified endogenous locus being used. In particular, the VP16 activation domain may contain mutation(s) that alter its activity, changing the strength of an individual driver.