A biochemical paradox that has been observed for many years is now becoming understood: O₂, essential for the aerobic life forms, can be inappropriately metabolized, becoming toxic to an organism. Mammals derive most of their cellular adenosine triphosphate (ATP) from the controlled four-electron reduction of O₂, to form H₂O by the mitochondrial electron-transport system. Approximately 98% of all O₂ consumed by cells enters the mitochondria, where it is reduced by a terminal oxidase, such as cytochrome oxidase.