pTet-On expresses the reverse tet-responsive transcriptional activator （rtTA） 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-On is similar to pTet-Off except for four amino acid changes that convert the TetR to a rTetR, and consequently, tTA to rtTA. （pTet-Off also contains several silent mutations.） pTet-On was originally described as pUHD17-1neo by Gossen et al. pTet-On can be distinguished from pTet-Off by digestion with Hind III.