pTet-On-Advanced expresses an improved version of the reverse Tet (tetracycline)-controlled transactivator protein (rtTA), called rtTA-Advanced (1–4). It is more sensitive to doxycycline (Dox) and yields lower background expression than the original rtTA used in the Tet-On® System (2). The rtTA-Advanced protein is a fusion of amino acids 1–207 of a mutant Tet repressor (TetR) and 39 amino acids containing three minimal "F"-type transcriptional activation domains from the VP16 protein of herpes simplex virus. It is fully synthetic, lacks cryptic splice sites, and is codon-optimized for stable expression in mammalian cells.