Scientists use a confocal microscope to track Reactive Oxygen Species (ROS) production in liver cells. The microscope acts like a CAT scanner for single cells, enabling scientists to follow ROS production in real time. The liver cells were treated with a chemical that emits light when the amount of ROS increases. The blue color represents low levels of light (low ROS), green and yellow represent intermediate levels of light (intermediate ROS) and red represents high ROS.