Any of a class of proteins that are sensitive to blue and ultraviolet light and are found in a wide range of organisms, including plants, insects, birds, and mammals. Cryptochromes are bound to FAD, which acts as a chromophore, and are involved in a variety of light-dependent signalling pathways. They are structurally similar to DNA lyases, enzymes that help to repair light-induced DNA damage in cells. In plants, cryptochromes regulate light-induced aspects of growth and development, such as hypocotyl elongation and onset of flowering. In animals, cryptochromes occur in the retina of the eye and act to entrain circadian rhythms with the light-dark cycle. The activity of cryptochromes is enhanced by magnetic fields, and it has been suggested that cryptochromes might form part of an internal magnetic compass for migrating birds and other animals.