Redox reactions are also used for gated delivery systems. Within cells and the bloodstream there are several reducing agents that can be used to trigger drug release in gated systems. The most common reducing agent used in gated delivery system is glutathione (GSH) because it has been determined that GSH is the most abundant reducing agent in the body. GSH also has significantly different concentrations between the intracellular and extracellular environments making it easier to target either environment without getting triggered by the other. Furthermore, GSH is found in higher concentration within tumor cells. This provides another way to have sustained and local release of drug at tumor sites. There are generally 2 different mechanisms for this type of gated system. One method is to cleave disulfide bonds. Another method is to cleave bonds through the use of reactive oxygen species (ROS). Bonds that are able to be cleaved by ROS are generally thioketals, ketals, and diselenides.