Table 2.

Response of rtTA and of the new rtTA variants toward Dox

Dox, ng/ml	Response in arbitrary light units of rtTA alleles
	—	rtTA	S2	19/56R	M1	M2
0	0.67  ± 0.28	1.24  ± 0.41	0.62  ± 0.48	2.54  ± 2.05	1.02  ± 1.16	3.02  ± 1.26
5	0.52  ± 0.06	57.27  ± 6.66	1.26  ± 0.17	3.24  ± 2.72	1.24  ± 1.54	17.07  ± 5.58
50	0.64  ± 0.55	229.72  ± 35.34	26.52  ± 12.4	0.91  ± 0.32	30.94  ± 25.75	1,517.37  ± 169.94
500	0.94  ± 0.63	696.27  ± 70.3	1,726.75  ± 110.64	112.68  ± 6.98	2,428.19  ± 254.42	10,746.33  ± 1,272.97

Dox-dependent gene activation of various rtTAs was measured in HeLa X1/6 cells that were transiently transfected with plasmids containing the genes for the transactivators rtTA and rtTA-S2, -19/56R, -M1, and -M2, all of which were controlled by P_CMV. For standardization of transfection efficiencies, each transfection mixture contained also a vector that constitutively produces β-galactosidase. Cells were grown in absence of or in presence of 5, 50, and 500 ng/ml Dox. Twenty-four hours post transfection, luciferase activity was measured from cell extracts and normalized to β-galactosidase activity. Values given are arbitrary light units.