Creatures as varied as mammals, fishes, insects, reptiles, and birds have an intriguing 'sixth' sense that allows them to navigate in the Earth's magnetic field. Despite decades of study, the physical basis of this sense remains elusive. A likely mechanism is furnished by magnetic field sensitive reactions occurring in the retina of animal eyes. A decade ago it was suggested (see our magnetoreception page) that the photoreceptor cryptochrome, arising in the retina, endows birds with magnetoreceptive abilities. The hypothesis gained support during the last years, after it had been shown that the protein exhibits properties required for an animal magnetoreceptor to operate properly. (see prior highlights on A Compass in the Eye, July 2010; on Where's North, Ask Superoxide, July 2009; and on Animal Magnetic Sense Shared by Plant, April 2007). However, the biophysical mechanism of cryptochrome activation and signaling is still poorly understood. A recent study proposed a theoretical analysis method for identifying cryptochrome's signaling reactions involving comparison of measured and calculated reaction kinetics. Application of the method suggest a light-driven reaction cycle which combines electronic excitation with electron and proton transfer reactions in the protein. More details on cryptochrome functioning as a light-driven magnetic compass can be found on our cryptochrome webpage .