According to behavioral experiments, it is an ��inclination compass�� based on the axial course of the field lines rather than on their polarity, and it is light-dependent, requiring light in the shortwavelength part of the spectrum. How birds perceive magnetic directions remained largely unknown. Several hypotheses were forwarded; the one presently favored is the Radical Pair Model by Ritz and colleagues : it proposes that photon absorption in BAY-678 specialized receptor molecules leads to an electron transfer and the formation of radical pairs. These occur in two states, singlet and triplet, which are in a chemical balance that depends on the alignment of the receptor molecules in the magnetic field. To obtain information on magnetic directions, the singlet or triplet yield in the various spatial directions would have to be compared. Hence, for a magnetic compass based on radical pair processes, three crucial requirements must be fulfilled: light has to reach the receptor molecules to induce the formation of radical pairs, the arrangement of the receptor cells has to cover all spatial directions to allow the comparison of the respective singlet or triplet yields, and within any one receptor cell, all receptor molecules have to be aligned in the same direction to act as a functional unit. The eye with its spherical shape meets the first KT109 conditions and was therefore suggested as site for magnetoreception. This has been supported by experimental evidence: magnetoreception is indeed mediated by the eye, and, by using radio frequency fields as diagnostic tools, the underlying mechanism could be identified as radical pair process. Ritz and colleagues already discussed cryptochromes as suitable candidates for the receptor molecules. Cryptochromes are blue light-sensitive flavoproteins that can form radical pairs ; they are related to the photolyases which catalyze DNA repair in plants via electron transfer. Cryptochromes were first identified in plants, but then also found in animals, where they are e.g. involved in the circadian clock. Cryptochrome-controlled processes were found to be affected by magnetic fields, indicating that this molecule has the potential to mediate magnetic information.