Circadian rhythms are behavioral or physiological rhythms with periods of approximately 24 hours. They have been identified in many species from bacteria to humans. In mammals, the circadian regulator is the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. Multiple genes involved in the SCN circadian rhythm have been isolated. The Drosophila period (dper) gene is the most completely characterized mammalian clock gene. In mouse, multiple per genes have been identified and constitute a mammalian period gene family (mper1, mper2, and mper3). These proteins contain a dimerization PAS domain and a cytoplasmic localization domain. SCN expression of mper1 and mper2 overlaps, but is not synchronous. Although not light-inducible, mper2 transcripts are detected approximately two hours following those of mper1. This suggests that the mper1 gene product, a putative transcription factor, is produced in response to light and promotes the expression of mper2. Although mper1 and mper2 are similar, they are not redundant. Thus, these different circadian regulators may allow integration of a wider range of input signals and interact with different downstream effectors.