There were two key components of this vector system: (i) the
rtTA2sM2 reverse tetracycline transactivator sequence under the
control of the PGK promoter, and (ii) theGFPmarker gene downstream of the miniCMV promoter, which is activated by rtTA2sM2
in cooperation with tetracycline. The initial version of this vector
system consisted of two different retrovirus vectors each of which
carried one of the two components mentioned above. This
study, however, simplified the experimental procedure by combining the two components into a single vector containing all the elements in a single cassette (Fig. 1). Among farm animals, the first
successful application of this single vector system was reported
in a pig model.