Blue light-dependent regulator that is the input of the circadian feedback loop. Has no photolyase activity for cyclobutane pyrimidine dimers or 6-4 photoproducts. Regulation of expression by light suggests a role in photoreception for locomotor activity rhythms. Functions, together with per, as a transcriptional repressor required for the oscillation of peripheral circadian clocks and for the correct specification of clock cells. Genes directly activated by the transcription factors Clock (Clk) and cycle (cyc) are repressed by cry. Necessary for light-dependent magnetosensitivity, an intact circadian system is not required for the magnetoreception mechanism to operate. Required for both the naive and trained responses to magnetic field, consistent with the notion that cry is in the input pathway of magnetic sensing. Binds 1 FAD per subunit. The bound form of FAD in the inactive state of cry is oxidized FAD, not reduced. After activation by blue light the FAD is in an anionic radical state, which would be paramagnetic. Green light, which reduces levels of radical intermediate, has an antagonistic effect on function. Interacts with tim and per; promoted by light conditions (PubMed:10417378, PubMed:11448767). Interaction with tim irreversibly commits tim to proteasomal degradation (PubMed:10417378). Interacts with l(1)G0136/CG8198 (PubMed:26569474). Nuclear translocation initiates after the perception of a light signal. Accumulates in the perinuclear region about one hour before translocation into the nucleus. Translocation occurs through interaction with other Clock proteins such as tim and per. Expressed at higher levels in the head than in body and it is more expressed in antennae than in legs, wings and mouth appendages. Prominent expression is seen in cells of the lateral brain, which are close to or coincident with the clock neurons. Abundance oscillates in a circadian manner. Expression is regulated by light and circadian rhythms. Under circadian regulation, expression is influenced by the clock pacemaker genes period, timeless, Clock and cycle. FAD-binding region regulates cry stability, cry-tim interaction, and circadian photosensitivity. Photolyase/cryptochrome alpha/beta domain is sufficient for light detection and phototransduction. Flies exhibit poor synchronization to light-dark cycles and show no response to brief light pulses. Mutant abolishes rhythmic tim and per expression in photoreceptor and glial cells, but not within certain pacemaker neurons of adult brain. Unstable upon light exposure. Light induces the degradation of cry, likely due to conformational change in the photoreceptor leading to targeting to the proteasome. Appears to bind 5,10-methenyltetrahydrofolate at substoichiometric levels. Belongs to the DNA photolyase class-1 family.