# Cryptochrome
**Cryptochromes** are flavoprotein photoreceptors that absorb UV-A and blue light and regulate growth, development, and circadian rhythms in organisms ranging from plants to animals. They belong to the cryptochrome/photolyase family (CPF), a class of structurally related blue-light-absorbing flavoproteins found in all kingdoms of life, and are closely related to photolyases, the ancient flavoenzymes that use light energy to repair UV-induced DNA damage[1](https://www.sciencedirect.com/science/article/abs/pii/S0176161714001692). A definition that fits most cryptochromes is that they are photolyase-like proteins with impaired double-stranded [DNA repair](https://www.edgepedia.org/dna-repair) function, some of which have evolved novel roles in signaling[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC10940379/).
| Key fact | Detail |
|---|---|
| Cofactors | Flavin adenine dinucleotide (FAD) plus a pterin (folate) as chromophore/cofactors; cryptochromes show 25–50% sequence identity to photolyase[3](https://www.annualreviews.org/content/journals/10.1146/annurev.biochem.69.1.31) • [4](https://www.sciencedirect.com/science/article/pii/S0021925820772303) |
| Name coined | Jonathan Gressel proposed "cryptochrome" in 1979 for the then-unidentified pigments responsible for plant UV-A/blue photoresponses[5](https://academic.oup.com/plphys/article/195/1/213/7618129) |
| First molecular identification | The HY4 gene of *Arabidopsis thaliana*, cloned in 1993 from a T-DNA-tagged allele of the hy4 mutant in Anthony Cashmore's laboratory[5](https://academic.oup.com/plphys/article/195/1/213/7618129) |
| Mammalian genes | Humans and mice have two cryptochrome genes, CRY1 and CRY2, differentially expressed in the retina relative to opsin-based photoreceptors[3](https://www.annualreviews.org/content/journals/10.1146/annurev.biochem.69.1.31) |
| Mammalian function | Light-independent transcriptional repressors within the core circadian clock mechanism[6](https://onlinelibrary.wiley.com/doi/full/10.1111/php.12669) |
| Magnetoreception | Prime candidates for magnetic sensing in migratory animals via the radical pair mechanism, but the hypothesis remains unproven[7](https://journals.physiology.org/doi/full/10.1152/physrev.00032.2025) |
## Discovery and naming
The name predates the molecule. Jonathan Gressel, inspired by the successes in identifying the phytochromes, proposed the term "cryptochrome" in 1979 for the yet-to-be molecularly identified pigments having both absorption and action spectra in the UV-A/blue light region and actually responsible for the plant photoresponse; the name referred to their importance in cryptogamic plants and their cryptic nature[5](https://academic.oup.com/plphys/article/195/1/213/7618129). Earlier biochemical approaches, including the light-induced absorbance change (LIAC) assay in membrane fractions of fungi, algae, and higher plants, correctly predicted that the blue-light receptors would be flavoproteins containing FAD or FMN, but failed to identify the molecules themselves because of their low abundance relative to photosynthetic pigments[5](https://academic.oup.com/plphys/article/195/1/213/7618129).
**Molecular genetics broke the deadlock.** In 1980, [Maarten Koornneef](https://www.edgepedia.org/maarten-koornneef) isolated the *Arabidopsis* hy4 mutant, which shows a long-hypocotyl phenotype in blue light[5](https://academic.oup.com/plphys/article/195/1/213/7618129). Kenneth Feldmann generated 8,000 *Arabidopsis* T-DNA insertion-mutant lines in 1991, and in 1993 Anthony Cashmore's laboratory screened this population, identified a T-DNA-tagged allele of hy4, and cloned the HY4 gene, which encodes a protein with high similarity to DNA photolyase[5](https://academic.oup.com/plphys/article/195/1/213/7618129) • [4](https://www.sciencedirect.com/science/article/pii/S0021925820772303). Cryptochromes were subsequently identified in many other plant species, bacteria, fungi, animals, and humans[8](https://uodiyala.edu.iq/uploads/PDF%20ELIBRARY%20UODIYALA/EL34/Light%20and%20Plant%20Development.pdf).
## Structure and photochemistry
Cryptochromes share two structural elements with photolyases: the FAD cofactor and a chain of three conserved tryptophans (TrpA, TrpB, and TrpC, named for their position proximal, medial, and terminal with respect to the FAD) connecting the flavin cofactor with the protein surface[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC3915827/). Beyond the N-terminal photolyase homology region (PHR), most cryptochromes carry a more variable C-terminal extension (CCE) that confers functional specificity[10](https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2024.1436322/pdf) • [4](https://www.sciencedirect.com/science/article/pii/S0021925820772303).
Photoreduction of FAD is the core photochemical event. For *Arabidopsis* cryptochromes 1 and 2, signaling arises through a photocycle in which the flavin is photoreduced to FAD•− and a conserved aspartate (AtCRY1: D396; AtCRY2: D393) then donates a proton, stabilizing a FADH• state that transduces the signal.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC8284579/) The resting state of the FAD cofactor, the intermediates in photoreactions, and the necessity of particular tryptophans for activity have all been points of contradiction across studies[12](https://www.sciencedirect.com/science/article/abs/pii/S1389556714000586). Some mutations within the Trp triad prevent photoreduction of FAD in vitro but do not affect biological activity in vivo, implying alternative electron-transfer pathways in the cellular environment[13](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biophys-032116-094545).
## Functions in plants
Plant cryptochromes, first identified in *Arabidopsis*, play key roles in growth and development[14](https://doi.org/10.1146/annurev-arplant-042110-103759). Documented plant responses involving cryptochromes include light-dependent transcription, circadian rhythm, inhibition of hypocotyl elongation, promotion of floral initiation, mediation of gravitropic response, and responses to biotic and abiotic stress[15](https://www.sciencedirect.com/science/article/pii/S0168945224004084). Plant Cry1 and Cry2 are closely related to the type I CPD photolyases within their N-terminal photolyase-like domain[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC10940379/).
## Circadian roles in animals
Cryptochromes serve as circadian photoreceptors in *Drosophila* and *Arabidopsis*, raising the possibility that they are universal circadian photoreceptors[3](https://www.annualreviews.org/content/journals/10.1146/annurev.biochem.69.1.31). In the fly, Cryptochrome is expressed in circadian pacemaking cells of the brain and directly senses light; the eyes are not required for photic resetting of the *Drosophila* clock, so photic resetting in the fly brain is cell autonomous[16](https://doi.org/10.1146/annurev-vision-111815-114558).
The animal kingdom contains two functional types. *Drosophila*-like CRYs act as non-visual photopigments linking circadian rhythms to the light/dark cycle, whereas vertebrate-like CRYs do not appear to sense light directly but control rhythm generation as transcriptional repressors; some animals have both types, others only one[17](https://pmc.ncbi.nlm.nih.gov/articles/PMC5397253/).
**In mammals, cryptochromes are central clock components rather than light sensors.** Humans and mice have CRY1 and CRY2, and CRY1 is highly expressed with circadian periodicity in the suprachiasmatic nucleus (SCN), the mammalian circadian pacemaker[3](https://www.annualreviews.org/content/journals/10.1146/annurev.biochem.69.1.31). Mutant mice lacking Cry1 or Cry2 have impaired light induction of the clock gene mPer1 and abnormally short or long intrinsic periods, respectively; the double mutant has normal vision but lacks molecular and behavioral rhythmicity in constant darkness[3](https://www.annualreviews.org/content/journals/10.1146/annurev.biochem.69.1.31). Cryptochrome function also extends beyond the clock, participating in regulation of the DNA damage response, cancer progression, and glucocorticoid signaling[17](https://pmc.ncbi.nlm.nih.gov/articles/PMC5397253/).
## Magnetoreception
Cryptochrome flavoproteins are the leading molecular candidates for the radical pair mechanism (RPM) of magnetoreception, a spin-dependent process initiated by photoinduced electron transfer, in which blue-light activation of FAD creates radical pairs whose spin dynamics are sensitive to the geomagnetic field[18](https://arxiv.org/abs/2508.21350). The canonical FAD-tryptophan radical pair exhibits pronounced anisotropic hyperfine couplings, enabling sensitivity to geomagnetic-strength fields[18](https://arxiv.org/abs/2508.21350). Evidence for cryptochrome-mediated physiological responses to applied magnetic fields has been summarized in plant, *Drosophila*, and mammalian cell systems, although areas of controversy exist in all of these cases[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC10940379/).
The hypothesis carries substantial experimental support but remains unproven. A key puzzle is how weak radiofrequency fields around 1 nT, roughly 50,000 times weaker than [Earth's magnetic field](https://www.edgepedia.org/earths-magnetic-field), can disrupt magnetic compass responses, far below levels predicted by theoretical models; modeling studies propose a third radical acting as a scavenger to enhance sensitivity[7](https://journals.physiology.org/doi/full/10.1152/physrev.00032.2025). There is also no direct evidence that Earth-strength fields produce the radical-pair recombination effects reported for millitesla-range fields[7](https://journals.physiology.org/doi/full/10.1152/physrev.00032.2025). Research on *D. melanogaster* mutants has suggested that full-length cryptochrome is not required for magnetosensitivity, supporting an interpretation in which cryptochrome plays a structural or scaffolding role while FAD itself may be the sensory molecule[7](https://journals.physiology.org/doi/full/10.1152/physrev.00032.2025). Even the cellular localization is unresolved: migratory birds were disoriented by a homogeneous 5-nT radiofrequency field but oriented normally when the same field was applied solely to their eyes with miniaturized coils[7](https://journals.physiology.org/doi/full/10.1152/physrev.00032.2025).
## References
1. [Dealing with light: The widespread and multitasking cryptochrome/photolyase family in photosynthetic organisms, Journal of Plant Physiology](https://www.sciencedirect.com/science/article/abs/pii/S0176161714001692)
2. ['Seeing' the electromagnetic spectrum: spotlight on the cryptochrome photocycle, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10940379/)
3. [Aziz Sancar (2000). Cryptochrome: The Second Photoactive Pigment in the Eye and Its Role in Circadian Photoreception. Annual Review of Biochemistry 69:31-67](https://www.annualreviews.org/content/journals/10.1146/annurev.biochem.69.1.31)
4. [Regulation of the Mammalian Circadian Clock by Cryptochrome, Journal of Biological Chemistry](https://www.sciencedirect.com/science/article/pii/S0021925820772303)
5. [Light signaling in plants—a selective history, Plant Physiology 195(1):213-231 (2024)](https://academic.oup.com/plphys/article/195/1/213/7618129)
6. [The Photolyase/Cryptochrome Family of Proteins as DNA Repair Enzymes and Transcriptional Repressors, Photochemistry and Photobiology (2017)](https://onlinelibrary.wiley.com/doi/full/10.1111/php.12669)
7. [Magnetosensation: the unsolved mystery, Physiological Reviews](https://journals.physiology.org/doi/full/10.1152/physrev.00032.2025)
8. [Cryptochromes, in Light and Plant Development](https://uodiyala.edu.iq/uploads/PDF%20ELIBRARY%20UODIYALA/EL34/Light%20and%20Plant%20Development.pdf)
9. [Magnetic field effects in flavoproteins and related systems, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3915827/)
10. [Review of cryptochrome architecture and signal transduction, Frontiers in Chemistry (2024)](https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2024.1436322/pdf)
11. [Cryptochromes: Photochemical and structural insight into magnetoreception, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8284579/)
12. [Photoactivation of the cryptochrome/photolyase superfamily, ScienceDirect](https://www.sciencedirect.com/science/article/abs/pii/S1389556714000586)
13. [The Radical-Pair Mechanism of Magnetoreception, Annual Review of Biophysics](https://www.annualreviews.org/doi/pdf/10.1146/annurev-biophys-032116-094545)
14. [The Cryptochromes: Blue Light Photoreceptors in Plants and Animals, Annual Review of Plant Biology 62:335-364 (2011)](https://doi.org/10.1146/annurev-arplant-042110-103759)
15. [Dissecting sequence-structure-function-diversity in plant cryptochromes, ScienceDirect](https://www.sciencedirect.com/science/article/pii/S0168945224004084)
16. [Ocular Photoreception for Circadian Rhythm Entrainment in Mammals, Annual Review of Vision Science](https://doi.org/10.1146/annurev-vision-111815-114558)
17. [Animal Cryptochromes: Divergent Roles in Light Perception, Circadian Timekeeping and Beyond, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5397253/)
18. [The Quantum Compass Mechanism in Cryptochromes, arXiv](https://arxiv.org/abs/2508.21350)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolism and metabolic pathways › Elemental and cofactor metabolism › Coenzyme-dependent enzyme groups › Flavin-dependent enzymes*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license