tetO regulatory sequences and an Hsp70 core promoter are fused upstream of Mhc intron, the syn21 5'UTR and sequence encoding the tTA driver (the Mhc and syn21 sequences act as translational enhancers). In the absence of tetracycline, the expressed tTA protein binds the tetO regulatory sequences, establishing a positive feedback loop and resulting in lethal levels of the tTA protein (the protein is toxic at high levels). In the presence of tetracycline, tTA can no longer bind the tetO sequences and only basal, sub-lethal levels of tTA are expressed from the Hsp70 core promoter.