In addition to their critical role in the production of ATP through the electron transport chain (Figure 1), these organelles function in intracellular Ca2+ homeostasis, synthesis of steroids, heme and iron-sulfur clusters, and programmed cell death (PCD) [6,7,8]. Mitochondria are also sites of formation of reactive oxygen species (ROS), including superoxide anion (O2•-) [9] and the highly reactive hydroxyl radical (•OH) or its intermediates [10], and reactive nitrogen species such as nitric oxide (•NO) [6]. Mitochondria generate endogenous ROS as by-products of oxidative phosphorylation (Figure 1) [8]. Oxygen- and proton pump-driven ATP production by the electron transport chain (Figure 1, lower left) is one function. The respiratory chain proteins (complex I-IV) establish an electrochemical gradient across the inner mitochondrial membrane (IMM) by extruding protons out of the matrix into the intermembrane space, thereby creating an energy gradient that drives the production of ATP by complex V (Figure 1, lower left). Superoxide (O2•-) is produced as a by-product in the process of electron transport. Electrons in the electron carriers, such as the unpaired electron of ubisemiquinone bound to coenzyme Q binding sites of complexes I, II, and III, can be donated directly to O2 to generate O2•- [8]. O2•-  does not easily pass through biological membranes and thus must be inactivated in compartments where it is generated [9]. The mitochondrial matrix enzyme manganese superoxide dismutase (MnSOD or SOD2) or copper/zinc SOD (Cu/ZnSOD or SOD1) in the mitochondrial intermembrane space and cytosol convert O2•- to hydrogen peroxide (H2O2) in the reaction O2•- + O2•- + 2H+→ H2O2 + O2  (Figure 1) [9]. H2O2 is more stable than O2•- and can diffuse from mitochondria into the cytosol and nucleus. H2O2 is detoxified by glutathione peroxidase in mitochondria and the cytosol and by catalase in peroxisomes.