Regulatable Transcription Cassettes or the "SIN-RetroTet Vector"

Regulatable expression constructs are provided, which allow for reversible transcription of a gene of interest in response to tetracycline (tet). The expression cassette may be introduced into a eukaryotic host and integrated into the chromosome with a retroviral vector, or by homologous recombination, conveniently using recA, as bare DNA for random integration or for extrachromosomal maintenance as plasmids, or the like.