Putative transcription factor involved in the generation of biological rhythms. Activates cycling transcription of Period (PER) and Timeless (TIM) by binding to the E-box (5'-CACGTG-3') present in their promoters. Efficient DNA binding requires dimerization with another bHLH protein. Forms a heterodimer with Clock in order to activate PER and TIM transcription. Expressed in head and ovary. Cyc mutants don't display PER and TIM cycling, and are completely arrhythmic.