Synonyms:FASPS, hPER2, KIAA0347, PER2, period homolog 2 Background:PER2, also named as KIAA0347, is a component of the circadian clock mechanism which is essential for generating circadian rhythms. PER2 is a negative element in the circadian transcriptional loop. PER2 influences clock function by interacting with other circadian regulatory proteins and transporting them to the nucleus. It negatively regulates CLOCK|NPAS2-BMAL1|BMAL2-induced transactivation. Defects in PER2 are a cause of familial advanced sleep-phase syndrome (FASPS) which is characterized by very early sleep onset and offset.