The rtTA driver encodes a conditional transcriptional activator that contains a DNA-binding domain derived from the tetracycline repressor (tetR) gene of the transposon Tn10-encoded operon of Escherichia coli fused to the transcriptional activation domain from the Herpes simplex virus Type 1 VP16 gene. The tetR DNA-binding domain can bind specifically to the tetO DNA sequence and thus tetO and rtTA form a conditional 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 tetO sequences is combined with a second transgene or modified endogenous locus encoding the rtTA driver. The tetR DNA-binding domain in the rtTA driver has been mutated such that binding to tetO, and subsequent activation of transcription only occurs in the presence of tetracycline or related derivatives (PMID:7792603, PMID:9251018).