The tetracycline-controlled transcriptional activator (tTA) is used in Tet-off system to control gene expression. In the absence of doxycycline (Dox) , tTA binds to the Tet Response Element (TRE) and activates downstream gene expression. When Dox is added to a cell, tTA is release from TRE, leading to the inactivation of the TRE regulated gene expression. tTA lentivirus (CMV, Puro) expresses an advanced version of tTA under the control of the CMV promoter, with puromycin as a selection marker.