Components of the commercially available T-REx (TM) system were used to create two types of transgenic mice. The first contained the tetracycline-repressor transgene under the control of the CMV promoter/enhancer; the second type contained a green fluorescent protein (GFP) reporter transgene under the control of the CMV promoter/enhancer with a tetracycline repressor operator sequence. Transgene expression was unpredictable in animals containing the individual transgenes. Animals with the reporter transgene expressed GFP in only some tissues (e.g., pancreas, kidney), and one line of reporter transgenic animals developed kidney disease, presumably due to expression of the transgene. The two types of transgenic animals were crossbred to produce double-transgenic animals with the object of regulating the expression of the reporter in vivo. When a similar double-transgenic system was constructed in cultured cells, the repressor protein suppressed the transcription of the reporter transgene. The presence of the repressor in double-transgenic animals had no effect on the expression of the reporter; double transgenic animals; developed the same kidney disease that was seen in singly transgenic mice with the reporter. Our results indicate that transgenes under the control of the CMV promoter in the T-REx system express somewhat unpredictably and in only a limited number of tissues, making the use of this system for the development of in vivo models problematical.