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The ability to control the expression of a therapeutic gene or a transgene in mammalian cells is crucial for safe and efficacious gene and cell therapy, as well as for elucidating the function of a specific gene product. Yet current mammalian gene regulation systems either evoke harmful immune responses in hosts or lack the required regulatory efficiency. Here we describe a highly responsive RNA-based molecular switch, the pA regulator, that harnesses the power of polyA signal cleavage within the 5’ UTR to control mammalian gene expression. The pA regulator is governed by a ‘dual mechanism’ to ensure maximal control of gene expression: (1) aptamer clamping of polyA signal via drug binding and (2) drug-induced alternative splicing that removes the polyA signal. The metholology achieves an induction efficiency up to 900-fold with an EC50of 0.5μg/ml Tetracycline, a drug concentration that falls well within the FDA-approved dose range. The pA regulator circumvents the immune responses that plague other systems by eliminating the use of a regulatory foreign protein and the need to change transgene coding sequences. Furthermore, it is not dependent on any specific promoter, therefore the system is simple to implement in a single non-viral or viral vector. In a mouse study using AAV-mediated gene transfer, we showed that the pA regulator controlled transgene expression in a “dose-dependent’ and “reversible” manner and exhibited long-term stabilityin vivo, in which both features are crucial for effective therapeutics. The pA regulator is the first non-immunogenic system that demonstrates an EC50at a drug concentration approved by FDA, making it a clinically relevant gene regulation system that could open a new window of opportunity in clinical applications as well as biological studies.