Superoxide dismutase (SOD) exists in 3 isoforms in mammals: the cytoplasmic Cu/Zn-SOD (SOD1), the mitochondrial Mn-SOD (SOD2), and the extracellular Cu/ZnSOD (SOD3). All 3 isoforms require catalytic metal for their activation. Copper/zinc superoxide is a cytoplasmic enzyme and is the most common form of SOD found in the cytosol of cells. Zinc and copper exist in its active site, and are involved in the catalytic process. Cu/Zn-SOD is a dimer composed of 2 subunits. This isoform is encoded by the SOD1 gene and has an active role in eradicating superoxide radicals. Mn-SOD is found within the mitochondria and contains manganese ions in its active site. It is also a tetramer composed of 4 subunits and encoded by the SOD2 gene. This isoform is responsible for protecting the mitochondria from oxidative damage. Extracellular SOD is found within the extracellular space, including spaces between cells and in body fluids. Extracellular SOD uses copper and zinc ions in its active site, and is a tetramer composed of 4 subunits. This isoform is encoded by the SOD3 gene and has an active role in protecting tissues from oxidative damage.