mPer1 and mPer2 are essential for normal resetting of the circadian clock

Mammalian Per1 and Per2 genes are involved in the mechanism of the circadia n clock and are inducible by light. A light pulse can evoke a change in the onset of wheel-running activity in mice by shifting the onset of activity to earlier times (phase advance) or later times (phase delays) thereby adva ncing or delaying the clock (clock resetting). To assess the role of mouse Per (mPer) genes in circadian clock resetting, mice carrying mutant mPer1 o r mPer2 genes were tested for responses to a light pulse at ZT 14 and ZT 22 , respectively. The authors found that mPer1 mutants did not advance and mP er2 mutants did not delay the clock. They conclude that the mammalian Per g enes are not only light-responsive components of the circadian oscillator b ut also are involved in resetting of the circadian clock.