The lexAop DNA sequence is specifically bound by the DNA-binding domain of the lexA gene (ECOGENE:EG10533) from Escherichia coli. 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 lexAop sequences can be combined with a transgene or modified endogenous locus encoding any 'driver' that contains the lexA DNA-binding domain plus a transcriptional activation sequence (FBrf0191281).