is a fusion protein of the tet (tetracycline) repressor of E. coli and the transcriptional activation domain VP16 of herpes simplex virus. This system is generally driven by the tetP promoter which is actually a minimal immediate early cytomegalovirus (CMVIE) promoter, preceded by 7 copies of tetO, the tetracycline resistance operator of transposon 10 (Tn10). In the presence of tetracycline, this system is expressed at very low level and by removal of tetracycline the gene(s) under its control (e.g., luciferase, β-galactosidase) may be expressed at three orders of magnitude higher level. The system can be used also under the control of other promoters that suit best for regulating the expression pattern of the gene of interest.  rtTA,  tetracycline,  split-hybrid system; Gossen M, Bujard H 1995 Science 268:1766.