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that play important roles in plant development and entrainment of the circadian clock in several 5]. In addition to the primary FAD chromophore, there is evidence (except for cry-DASH that repairs single-stranded DNA [7]). Unlike photolyases, where flavins are generally reduced [8], cryp-flavin occurs, generating the neutral radical form in plants (Ara-animals (droso-ivated sig eoxidation ed flavin ( state). The degree of activation of crys therefore depends equilibrium reached under illumination conditions betwe forward (photoreduction) reaction and the reverse (oxidation) reaction of flavin, and both forward and reverse reactions are essential for photoreceptor function. Although numerous studies exist on the mechanisms involved in the forward (light activation) reactions of cryptochromes and the related photolyases [2,14,16], the mechanism of the flavin oxidation (dark inactivation) reaction either in vivo or in vitro is virtually unexamined. To the best of ou