Int. J. Environ. Res. Public Health 2010, 7 949 the sensitivity of normal cells and tissues to genotoxic stress induced by anticancer therapy correlate with CLOCK/BMAL1/CRY1 transcriptional activity. We propose that magnetic fields can implement this mechanism of active transcriptional repression via the CRY activity modulation, thereby influencing transcription of numerous genes. Such a concept is appropriate in explaining some previous findings, for example, the marked decrease in expression of mRNA for ALF1 and histone H3.3A in hippocampal neurons cultured under sustained exposure to static magnetic fields [66]. Significance of ascertaining the pertinent mechanisms was emphasized because “it appears that static magnetism may modulate cellular integrity and functionality through expression of a variety of responsive genes required for gene transcription and translation, proliferation, differentiation, maturation, survival, and so on” [66]. To summarize, CRY are unique bio-compounds, which combines sensory and bioregulatory functions. They act as mediators between living beings and their physical environment, providing a mechanism for both the reception of electromagnetic signals and the triggering (or entraining) of biological responses. 4.3. Ca2+-Operated Pathways and Ion Resonance Effects Ca2+-mediated signaling represents another feasible target for regulatory influences of magnetic fields on biological processes. Much experimental data and corresponding theoretical considerations have accumulated on this issue (reviewed in [67]). We will briefly mention some results corroborating the main idea that external magnetic fields can modify some biological parameters and signaling pathways, including those of immunological and epidemiological significance. Calcium is the most universal signal used by living organisms to convey information for many different cellular processes. There are several well-known and recently identified proteins that sense and decode the calcium signal, and which are key elements in the nucleus in the regulation of the activity of various transcriptional networks (reviewed in [68,69]). Ca2+ has a central role in transcriptional responses [70]. Several Ca2+-dependent transcriptional pathways acting on cytokine genes have been described [71]. The d scheme presented in Figure 7) summarizes some effects of electromagnetic field on the Ca2+-dependent transcriptional network. In spite of the many hypotheses and the theoretical considerations and experimental data that have accumulated in this field, the exact mechanisms of Ca2+-mediated biological effects of weak magnetic fields is not completely understood. Among feasible initial events of magnetic field-induced alterations in Ca2+-operated pathways there is modulation of the cell membrane potential and consequent changes in voltage-activated ion channel function. For example, Nuccitelli et al. [72] noted alterations in plasma membrane potential in tumor cells under the influence of magnetic fields of different types (static or pulsating) and intensities. The authors suggested that plasma membrane hyperpolarization may be part of the signal transduction chain determining the anti-apoptotic effect of magnetic fields. A four-fold increase in transmembrane calcium influx induced by ELF EMF was observed [73]. Specific frequencies of EMF have a differential effect on calcium ion activity. For example, it was demonstrated that radio frequency EM radiation exposure causes changes in calcium ion activity when the EM radiation is amplitude modulated, and only within a certain frequency of amplitude modulation: effects occurred for modulations of 6, 9, 11, 16 and 20 Hz, but no effects occurred for