English: In the intact animal, the phase of circadian mPer2 cycles is dictated by systemic Zeitgeber cues such as temperature or chemical cues influencing HSF activity (see text). Since mPER2 is also an integral part of the clockwork circuitry, this protein might confer the phase of systemic Zeitgebers to the local oscillator. If the oscillator is inactivated (e.g., by the repression of Bmal1), mPER2 is still expressed in a circadian manner in the intact animal. Under free-running conditions (i.e., in liver explants cultured in vitro), rhythmic mPer2 expression persists, but with the phase and period imposed by the local oscillators. However, when this oscillator is arrested, the expression of mPer2 and probably that of all clock and clock-controlled genes becomes arrhythmic.