Researchers have created a molecular switch

"Researchers have created a molecular switch that can reversibly turn any mammalian gene on or off and control its level of expression."

http://www.the-scientist.com/news/home/53431/

"When the switch is off, the system expresses a repressor protein that keeps the target gene turned off. A second brake is provided by a short hairpin RNA (shRNA), which blocks any mRNA that leaks through. "