These lines use regulatory elements derived from tetracycline-resistance operon in E.coli. The Tet-Off system allows expression in the absence of tetracyclines (e.g. Doxycycline). Addition of tetracyclines prevents DNA binding and transcription of any gene of interest cloned behind the Tet-regulated promoter stops. The complementary system Tet-On requires binding of tetracyclines for promoting expression of genes controlled by Tet-responsive promoters. Over the years both systems have been modified and improved.
These are the TET-expression Systems strains available via EMMA.