This book describes cellular level sensors that act as switches, turning on gene expression and other metabolic processes necessary for cell survival and differentiation. These responses can also initiate programmed cell death or activate latent human immunodeficiency virus or animal leukemia viruses. These redox sensors are nonspecific in sensitivity but specific in response. Unlike ligand/antiligand-type specific sensors, they respond to ionizing and ultraviolet radiations, pH gradients, heat, light, electric and magnetic fields, redox chemicals, mechanical stress, and other nonspecific stressors. The sensors are type-b cytochromes, including NADPH oxidases, NO synthases, and nitrogen oxide reductases. The intense radiation of early pre-biotic earth may have been the evolutionary driving force for the development of their common ancestor.