The construct contains a nuclear localization signal, the beta-galactosidase structural gene, an intron, and a poly(A) signal. Expression of beta-galactosidase is controlled by a tetracycline-responsive promoter element (TRE) consisting of the human cytomegalovirus early promoter fused to tet operator sequences. No beta-galactosidase activity was observed in the absence of a tTA transgene. (J:92587)