pTet-Off expresses the tet-responsive transcriptional activator (tTA) from the strong immediate early promoter of cytomegalovirus (PCMV). tTA is a fusion of amino acids 1–207 of the tet repressor (TetR) and the negatively charged C-terminal activation domain (130 amino acids) of the VP16 protein of herpes simplex virus. pTet-Off was originally described as pUHD15-1neo by Resnitzky et al. (1994) and was created by insertion of a neomycin resistance gene into pUHD15-1 (Gossen & Bujard,1992). pTet-Off can be distinguished from pTet-On by digestion with Hind III.