A substantial body of data published during the past 30 years makes a strong case for the existence of cellular radioprotective mechanisms that can be up-regulated in response to exposure to small doses of ionizing radiation. Either these "induced" mechanisms can protect against a subsequent exposure to radiation that may be substantially larger than the initial "priming" or "conditioning" dose, or they may influence the shape of the survival response to single doses so that small radiation exposures are more effective per unit dose than larger exposures above a threshold where the induced radioprotection is triggered. Evidence for these effects comes from studies in vitro with protozoa, algae, higher plant cells, insect cells, mammalian and human cells, and studies on animal models in vivo. Work at the molecular level is now confirming that changes in levels of some cytoplasmic and nuclear proteins, and the increased expression of some genes, may occur within a few hours or even minutes of irradiation. This would be sufficiently quick to explain the phenomenon of induced radioresistance although the precise mechanism, whether by repair, cell cycle control or some other process, remains yet undefined.

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