Circadian Locomoter Output Cycles Protein Kaput (CLOCK)

Circadian rhythmicity of biologic processes is a fundamental property of all eukaryotic and some prokaryotic organisms. These rhythms are driven by an internal time-keeping system. Changes in the external environment, particularly in the light-dark cycle, entrain this biologic clock.Under constant environmental conditions devoid of time cues, rhythms driven by the biologic clock show a period near, but usually not equal to, 24 hours.

The CLOCK gene encodes a basic helix-loop-helix (bHLH)-PAS transcription factor that is essential for circadian rhythm.The gene encodes an 855-amino acid polypeptide that is a novel member of the bHLH-PAS family of transcription factors and has features predicting DNA binding, protein dimerization, and activation domains.