In the mammallian circadian clock, for example, transcription factors CLOCK and BMAL1 are the positive regulators.
Heterodimer CLOCK-BMAL1 activates E-box elements present in the Per1 promoter, as well activating the E box promoters of other components of the molecular clock such as PER2, CRY1, and CRY2.
CLOCK (circadian locomotor output cycles kaput) was first cloned in mouse and BMAL1 (brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like 1) is the primary homolog of Drosophila CYC.