Since the first description of the helper-free retrovirus vector by Mann et al. (1983), many improvements have been introduced to the system to increase titer, or to achieve better expression of the transduced genes in cells of different lineage. The typical form of recombinant retrovirus vector utilizes its 5' long terminal repeat (LTR) as a promoter unit for the transcription of the inserted gene(s), which allows only the constitutive expression of the inserted gene(s) in target cells. Although constitutive expression of the delivered gene(s) in target cells may be sufficient for some purposes, controlled expression of gene(s) in target cells or tissues would be favorable in many basic and clinical applications. To circumvent these problems, we developed a new retroviral vector that allows controlled expression of the inserted genes. With these new retroviral vectors, the expression level of the inserted genes can be controlled up to 200-fold in the presence of a minimum amount of tetracycline. We think these new retroviral vectors will be useful in regulating the expression of the genes delivered in vivo.