Int. J. Environ. Res. Public Health 2010, 7 946 are light-activated DNA-repair enzymes [48]. This suggests adoption of a transcription regulatory function (cryptochrome) by a redox repair enzyme (photolyase) during evolution. Presumably, CRY activity does not necessarily require light exposure. Figure 4. Scheme of the magnetic field effect on the radical pairs between FADH and tryptophan in cryptochrome active site. The unpaired electron spins (S1 and S2) precess about a local magnetic field produced by the addition of the external magnetic field B with contributions I1 and I2 from the nuclear spins on the two radicals. The spin precession continuously alters the relative spin orientation, causing the singlet (anti-parallel) to triplet (parallel) interconversion, which underlies the magnetic field effect. Electron back-transfer from a tryptophan to FADH quenches cryptochrome's active state. However, this back-transfer can only take place when the electron spins are in the singlet state, and this spin-dependence allows the external magnetic field, B, to affect cryptochrome activation. This image was made with VMD and is owned by the Theoretical and Computational Biophysics Group, an NIH Resource for Macromolecular Modeling and Bioinformatics, at the Beckman Institute, University of Illinois at Urbana-Champaign. (From [51], reproduced by the kind permission of the authors and the publisher (Elsevier)). With regard to the localization of CRY in selected organs, it is important to note the presence of CRY proteins in the supraoptic nucleus of the hypothalamus, which is the central regulator of periodicity of daily (circadian) rhythms. This means that cells of the supraoptic nucleus can emit and spread signals regulating functional activity of organs, tissues and whole organism. CRY proteins were also found in endocrine glands, fibroblasts and other cell types. 4.2. Cryptochrome-Mediated Pathways and Biological Effects The above data clearly show the possibility that magnetic (including geomagnetic) fields can alter the functional activity of CRY proteins. To appreciate the possible effects of such alterations, we need to understand CRY-dependent signaling pathways.