Int. J. Environ. Res. Public Health 2010, 7 945 Thus, the radical pair mechanism is a plausible way in which weak magnetic field variations can affect chemical reactivity, allowing radical pairs containing substances that can function as chemical/biological magnetic sensors. 4. Signaling Pathways Capable of Implementing Bio-Regulatory (Including Genome-Regulatory) Functions of Magnetic Fields To prove the existence of the effects of magnetic fields on genome regulation, it is necessary to identify immediate receptors of the magnetic regulatory signals and their role in the genetic machinery. We propose a theory that magnetic fields induce definite genetic effects due to the existence of magnetic field-sensitive transcription factor repressors capable of regulating biological activity of organisms through epigenetic mechanisms. These substances are proteins of the Cryptochrome/ Photolyase family. Valuable information regarding magneto-sensitivity of Cryptochromes (CRY) and their biological responses to weak magnetic fields was obtained with the plant model, Arabidopsis thaliana [46]. If plants—for which magnetic responses have no apparent function—are sensitive to external magnetic fields, it is conceivable that other CRY-containing species are also sensitive [47]. 4.1. Cryptochromes: Ancient Regulatory Proteins Sensitive to Electromagnetic Radiation and Magnetic Fields Cryptochromes are blue-light photoreceptor flavoproteins (50–70 kDa) found in plants, bacteria, insects, and animals [48]. They contain two non-covalently bound chromophores, a redoxactive flavin adenine dinucleotide (FAD), and a light-harvesting cofactor. Identification of a new cryptochrome class (cryptochrome DASH) in bacteria and plants suggests that cryptochromes evolved before the divergence of eukaryotes and prokaryotes [49]. It was proposed in 2000 that CRY could host magnetically sensitive radical pairs [32]. The process of cryptochrome signaling presumably involves conformational changes in the protein that promote interaction with its downstream signaling partners [48,50]. The most recent scheme concerning the magnetic field effects on CRY activity comes from [51], the concept is shown and described in Figure 4. It should be noted that the FADH-access cavity of the helical domain is predicted to be a functionally active site of the CRY molecule. What, however, is known about localization and function of CRY proteins in the eukaryotic cell and in the whole organism? Cryptochromes are ubiquitously expressed in the organs and tissues of all organisms, and have a predominantly nuclear localization [48]. What is especially intriguing is that CRY protein function is related to cell genome expression regulation; they belong to the transcriptional repressors group [52,53]. Among known functions of CRY are the regulation of growth and development (in plants) and entrainment of circadian clocks [40]. They act as integral parts of the central circadian oscillator in animal brains and play an important role in mechanisms of circadian variations in expression of genes [54,55]. The circadian control system increases fitness and allows organisms to adapt to their physical and ecological environment. Therefore, animals with mutated a CRY gene have severe molecular and behavioral problems [56]. In plants, CRY proteins act as receptors controlling photomorphogenesis in response to blue or ultraviolet (UV-A) light. CRY are probably the evolutionary descendants of DNA photolyases, which