The tTA driver encodes a conditional transcriptional activator that contains the DNA-binding domain from the tetracyclin 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 binds specifically to the tetO DNA sequence, and thus tetO and tTA 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 tetO sequences is combined with a second transgene or modified endogenous locus encoding the tTA driver. The activity of the tTA driver can be suppressed by tetracycline: in the presence of tetracycline the tetR DNA-binding domain cannot bind the tetO sequence and thus the tTA driver cannot activate transcription (PMID:1319065, PMID:9251018).