The lexA::VP16 entry in FlyBase represents an artificial transcriptional activator ('driver') that includes the DNA-binding domain from the Escherichia coli lexA gene (ECOGENE:EG10533) and transcriptional activation domain sequences from the Herpes simplex virus Type 1 VP16 gene (UniProtKB:P06492). In Escherichia coli the lexA protein acts as a transcriptional repressor, binding specifically to the lexAop DNA sequence via its DNA-binding domain (reviewed in PMID:18726173). By fusing the lexA DNA-binding domain to a transcriptional activation domain, the resulting lexA::VP16 protein is converted into a transcriptional activator that specifically binds lexAop. lexAop and lexA::VP16 thus 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 lexAop sequences is combined with a second transgene or modified endogenous locus encoding the lexA::VP16 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 VP16 activation domains present may vary, and/or the VP16 activation domains may contain mutations that alter the strength of transcriptional activation (see FBrf0211848 for examples).