A living laser could be used to activate cancer-treating drugs using photodynamic therapy. Doctors could inject light-sensitive compounds into a patient's bloodstream to seek out tumors and diseased cells. Normally, such compounds are activated externally, but if both the drugs and the light itself were internal, treatment would be more precise. For now, though, Yun is primarily interested in the possibility of using his human laser to detect slight changes in cells. The intracavity light passes through the cells thousands or millions of times before exiting as a laser beam. Yun says that scientists could use the ricocheting light to monitor cell behavior with unprecedented sensitivity, similar to an intracellular high-speed camera. And yes, he says, his process could one day allow people to shoot laser beams from their eyes, though it would be more flashlight than death ray. "If a light source was implanted in the eye, it might be possible to control it with brain signals."