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Many migrating animals use Earth’s magnetic field as a guide, but how they sense magnetic stimuli at the molecular level has been unclear. Cells containing iron-oxide crystals called magnetite that are sensitive to magnetic fields may be involved. To look for genes linked to magnetoreception, Robert Fitak at Duke University in Durham, North Carolina, and his colleagues exposed rainbow trout (Oncorhynchus mykiss) to a magnetic pulse and studied how gene expression in their brains changed compared with that in an unexposed control group. Fish subject to the magnetic stimulus showed increased expression of ferritin genes, which encode proteins that store iron.