Tetracycline-Controlled Transcriptional Activation is a method of inducible gene expression where transcription is reversibly turned on or off in the presence of the antibiotic tetracycline or one of its derivatives (e.g. doxycycline). In nature, the P_tet promoter expresses TetR, the repressor, and TetA, the protein that pumps tetracycline antibiotic out of the cell.