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.