Fig. 7. Likely photocycle of crypotochrome in birds. Light can be absorbed either by the fully oxidized or semiquinone form of flavin, the active chromophore in cryptochrome. Magnetic field effects can, in principle, occur on the reduction from activated flavin (FAD^*) to the semiquinone, or on the reoxidation from fully reduced FADH^– to fully oxidized FAD.