Reactive oxygen species (ROS), such as superoxide anion (O₂^-), hydrogen peroxide (H₂O₂), and hydroxyl radical (HO•), consist of radical and non-radical oxygen species formed by the partial reduction of oxygen. Cellular ROS are generated endogenously during mitochondrial oxidative metabolism as well as in cellular response to xenobiotics, cytokines, and bacterial invasion.

ROS also activates MAPK pathways by the direct inhibition of MAPK phosphatases. Through PTEN, the PI3K pathway is subject to reversible redox regulation by ROS generated by growth factor stimulation. The activation of autophagy may be a cellular defense mechanism in response to ROS.