Cryptochromes are a class of light-sensitive proteins found in many organisms. In animals, they’re an integral part of the circadian clock: the collection of biochemical oscillations that align physiology with the day–night cycle.1 A network of positive and negative feedback loops in gene expression and protein production couple those oscillations to downstream processes. For example, they link external light and temperature conditions to levels of hormones that stimulate hunger and sleepiness.
The involvement of cryptochromes in circadian clocks is well established; they’re part of a negative feedback loop that suppresses transcription. But much about them remains poorly understood—for example, their structures are not well characterized, and the light-reactive proteins participate in cycles that don’t have light as an input.
Cryptochromes are also the only biomolecules other than chlorophylls that are known to host so-called radical pairs. When a photon hits a cryptochrome, it can excite an electron that hops...
