The three primary types of reaction oxygen species include: the superoxide anion (02-), hydrogen peroxide (H202), and the hydroxyl radical (HO). The most common ROS is O2, which is converted by mitochondrial SOD into H2O2. This in turn transmutes into oxidants such as OH radicals. This process primarily occurs through a Fenton reaction, which oxidizes a large quantity of crucial biomolecules. The superoxide anions have an unpaired electron, making them highly reactive and capable of inducing oxidative reactions. H2O2 is a fairly stable ROS formed when superoxide radicals react with each other or when enzymes such as superoxide dismutase convert superoxide radicals into hydrogen peroxide. H2O2 can penetrate cell membranes and act as signaling molecules to produce more reactive species. HO radicals are highly reactive and considered to be one of the most damaging ROS. They are formed through the reaction of hydrogen peroxide with metal ions (e.g. copper or iron), in the Haber-Weiss or Fenton reactions. These radicals can cause widespread and nonspecific damage to molecules including DNA, lipids and proteins.