Circadian regulator that acts as a transcription factor and generates a rhythmic output with a period of about 24 hours. Oscillates in antiphase to the cycling observed for period (PER) and timeless (TIM). According to PubMed:9742131, reaches peak abundance within several hours of the dark-light transition at ZT0 (zeitgeber 0), whereas PubMed:9616122 describes bimodal oscillating expression with maximum at ZT5 and ZT23. Clock-cycle heterodimers activate cycling transcription of PER and TIM by binding to the E-box (5'-CACGTG-3') present in their promoters. Once induced, Period and Timeless block Clock's ability to transactivate their promoters. Efficient DNA binding requires dimerization with another bHLH protein. Forms a heterodimer with Cycle. Widely expressed. Found in head, body, and appendage fractions. Contains three polyglutamine repeats which could correspond to the transactivation domain. The length of the repeats is polymorphic. In the arrhythmic mutant JRK, deletion of this region leads to the loss of circadian rhythmicity and altered light response. The variability in length of the polyglutamine stretch is due to polymorphism of this region. Variant B encodes two conceptual proteins, the first consists only of the bHLH domain, the other consists of the PAS-1 and all C-terminal domains. Variant B is expressed weakly at all the times of the day, and it cycles in phase with the full-length form.