sentences:  Two years later, Albrecht et al.found genetic evidence to support this result when they discovered that mPer1 mutants are not able to advance the clock in response to a late-night light pulse (ZT22) and that mPer2 mutants are not able to delay the clock in response to an early night light pulse (ZT14).Thus, mPer1 and mPer2 are necessary for the daily resetting of the circadian clock to normal environmental light cues.