Superoxide dismutase (SOD) is an antioxidant enzyme involved in the defense system against reactive oxygen species (ROS). SOD catalyzes the dismutation reaction of superoxide radical anion (O2-) to hydrogen peroxide, which is then catalyzed to innocuous O2 and H2O by glutathione peroxidase and catalase. Several classes of SOD have been identified. : Cu/Zn SOD (SOD1) is localized in cytosol, Mn SOD (SOD2) in mitochondria, and ec SOD (SOD3) in extracellular space. SOD2, the primary antioxidant enzyme that scavenges superoxide radicals in mitochondria, is essential for the survival of aerobic life. It exists as a homotetramer with an individual subunit molecular weight of about 23 kDa. SOD2 has been shown to play a major role in promoting cellular differentiation and tumorigenesis and in protecting against hyperoxia-induced pulmonary toxicity. Recent studies have reported that Mn SOD activity is related to cancer, aging, progeria, asthma, and transplant rejection. The expression of SOD2 in many cancers shows elevated levels of AP2 transcription factor. SOD2 expression is regulated not only at the level of transcription, but also at the level of translation by an RNA-binding protein. Lack of SOD2 expression results in dilated ventricular cardiomyopathy, neonatal lethality, and neurodegeneration. Overexpression of SOD2 has been shown to protect against oxidative stress-induced cell death and tissue injury.