Superoxide dismutase (SOD) is responsible for the elimination of cytotoxic active oxygen by catalyzing the dismutation of the superoxide radical to oxygen and hydrogen peroxide. There are three SOD isoenzymes in mammalian cells. They are: extracellular SOD (EC SOD), copper and zinc-containing SOD (CuZn SOD) and manganese-containing SOD (Mn SOD, SOD-2). The CuZn form contains Cu and Zn ions and exists as a 32kD dimer in the cytosol. Mn SOD is an 80kD tetramer that contains Mn ion and resides in the mitochondrial matrix. Mn SOD is a tumor necrosis factor (TNF)-inducible enzyme that protects cells from TNF-mediated apoptosis via superoxide anion detoxification and the subsequent regulation of apoptosis through cytochrome c release and the modulation of the redox state of the mitochondria. Mn SOD has also been shown to be a tumor suppressor in human breast cancer. Overexpression of this enzyme protects neurons from NMDA-and nitric oxide-induced neurotoxicity.