The PER1 gene encodes the period circadian protein homolog 1 protein in humans. Function The PER1 protein is important to the maintenance of circadian rhythms in cells, and may also play a role in the development of cancer. This gene is a member of the period family of genes. It is expressed with a daily oscillating circadian rhythm, or an oscillation that cycles with a period of approximately 24 hours. PER1 is most notably expressed in the region of the brain called the suprachiasmatic nucleus (SCN), which is the primary circadian pacemaker in the mammalian brain. PER1 is also expressed throughout mammalian peripheral tissues. Genes in this family encode components of the circadian rhythms of locomotor activity, metabolism, and behavior. Circadian expression of PER1 in the suprachiasmatic nucleus will free-run in constant darkness, meaning that the 24-hour period of the cycle will persist without the aid of external light cues. Subsequently, a shift in the light/dark cycle evokes a proportional shift of gene expression in the suprachiasmatic nucleus. The time of gene expression is sensitive to light, as light during a mammal's subjective night results in a sudden increase in per expression and thus a shift in phase in the suprachiasmatic nucleus. Alternative splicing has been observed in this gene; however, these variants have not been fully described. There is some disagreement between experts over the occurrence of polymorphisms with functional significance. Many scientists state that there are no known polymorphisms of the human PER1 gene with significance at a population level that results in measurable behavioral or physiological changes. Still, some believe that even silent mutations can cause significant behavioral phenotypes, and result in major phase changes. Functional conservation of the PER gene is shown in a study by Shigeyoshi et al. 2002. In this study, mouse mPer1 and mPer2 genes were driven by Drosophila timeless promoter in Drosophila melanogaster. They found that both mPer constructs could restore rhythm to arrhythmic flies (per01 flies). Thus mPer1 and mPer2 can function as clock components in flies and may have implications concerning the homology of per genes. Role in chronobiology The PER1 gene, also called rigui, is a characteristic circadian oscillator. PER1 is rhythmically transcribed in the SCN, keeping a period of approximately 24 hours. This rhythm is sustained in constant darkness, and can also be entrained to changing light cycles. PER1 is involved in generating circadian rhythms in the SCN,