Cryptochromes are highly conserved molecules (evolutionary very old) derived from photolyase, a bacterial enzyme activated by light and participating in DNA damage repair. In eukaryotes the chryptochromes lost their enzymatic activity. Cryptochromes possess two chromophores: pterin and flavin (a chemical relative of pterin). Pterin absorbs a photon, which causes it to emit energy; the latter is absorbed by flavin, which probably mediates the phosphorylation of a certain domain in cryptochrome. This triggers a signal transduction chain that affects gene regulation in the cell nucleus.
Studies in animals and plants suggest that Cryptochromes play a pivotal role in the generation and maintenance of circadian rhythms . In corals they are part of the mechanism that triggers coordinated spawning for a few nights after a full moon in the spring. Cytochromes are also involved in magnetic orientation of birds during migration and essential for the ability of fruit flies to sense magnetic fields.