Tissue-specific induction of the luciferase transgene is completely DOX-dependent. (A) Schematic representation of the tetracycline regulatory system. Restriction endonuclease recognition sites are as in Figure 1. DOX indicates doxycycline; tTA-2S, tetracycline-dependent transactivator; tetO, DNA-binding consensus for tTA-2S homodimers; pCMV, human cytomegalovirus minimal promoter; pA, polyA signal. (B) Luciferase activity expressed as relative light units (RLU) per microgram of protein extract was determined for different organs as indicated. The top bar graph shows luciferase activities of double heterozygous SCL-tTA-2S/LC-1 mice in the absence of DOX (-DOX, luciferase on). The bottom bar graph represents luciferase values obtained from double-transgenic SCL-tTA-2S/LC-1 mice that were kept from conception onwards in the presence of DOX (+DOX, luciferase off). The luciferase values in each graph are shown for a single bitransgenic mouse. A similar pattern of activity was obtained also in 2 additional independent experiments using different mice.