Cryptochrome pigment photocycle: Light absorption reduces FAD^{ox} to the semiquinone FADH* and to the fully reduced FADH–. The cycle is closed by the re-oxidation of FADH– to FADH* by oxidising agents generated by oxidative stress. The magnetic field (MF, here) affects the speed of photoactivation from FAD^{ox} and the reoxidation step of FADH–. Adapted from Thorsten Ritz, Margaret Ahmad, Henrik Mouritsen, Roswitha Wiltschko, and Wolfgang Wiltschko. Photoreceptor-based magnetoreception: optimal design of receptor molecules, cells, and neuronal processing. Journal of the Royal Society, Interface / the Royal Society, 7 Suppl 2:S135–S146, Apr 2010