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Abundant evidence leaves no doubt that reactive oxygen species (ROS) are not only inevitable by-products of oxygen metabolism but also play a role in cellular signaling. ROS are produced by a family of NADPH oxidases for signaling purposes and mediate or augment the effects of insulin, growth factors, cytokines and G-protein-coupled receptors. Disturbances of ROS signaling leading to overproduction of these intermediates inflict oxidative damage of cell components in the course of various diseases. Restoration of proper ROS signaling, especially inhibition of cellular sources of ROS, may thus provide new ways of therapy.