The transgene contains the human beta actin promoter upstream of a chimeric gene encoding an enhanced tetracycline regulated repressor protein (tTS) that comprises the C-terminal portion of the E. coli Tn10 tetracycline repressor (TetR) protein, the 6-amino acid SV40 large T-antigen nuclear localization signal and the N-terminal 121 amino acids of the human ZNF10 (KOX1) zinc finger protein, which contains the KRAB repressor domain, followed by an SV40 polyadenylation signal. The expression construct is flanked by chicken beta-globin insulators.