The small molecules used to control the RNA switch can be any cellular biomolecule, including metabolites, siRNA, miRNA, or proteins. In fact, it has been possible to engineer the ribozyme as a sensor to recognize a specific cellular ligand, and turn on a transgene according to the levels of the cellular ligand. We have great interest in exploring the engineering principles required to create sophisticated RNA switches that function as biosensors in vivo. Such biosensors would provide spatial as well as temporal information regarding the levels of specific ligands in disease, and the input information can be used to regulate cellular behavior for achieving therapeutic goals. For example, an RNA biosensor can be engineered to recognize glucose as its ligand, and in response, regulates the expression of an engineered insulin protein to modulate the glucose levels in diabetic patients (Figure 2). Similar biosensor platforms can also function as safety switches. For example, a biosensor can be engineered to detect the presence of a cancer biomarker in stem cells. When a normal stem cell erroneously transforms into a cancer cell, the biosensor would switch on a suicidal gene for self-destruction.