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Redox (oxidation-reduction) reactions regulate signal transduction. Oxidants such as superoxide, hydrogen peroxide, hydroxyl radicals, and lipid hydroperoxides (i.e., reactive oxygen species) are now realized as signaling molecules under subtoxic conditions. Nitric oxide is also an example of a redox mediator. Reactive oxygen species induce various biological processes such as gene expression by stimulating signal transduction components such as Ca²⁺-signaling and protein phosphorylation. Various oxidants increase cytosolic Ca²⁺; however, the exact origin of Ca²⁺ is controversial. Ca²⁺ may be released from the endoplasmic reticulum, extracellular space, or mitochondria in response to oxidant-influence on Ca²⁺ pumps, channels, and transporters. Alternatively, oxidants may release Ca²⁺ from Ca²⁺ binding proteins. Various oxidants stimulate tyrosine as well as serine/threonine phosphorylation, and …