Most organisms show circadian 24-h rhythmicity in their behavior and phsysiology. In mammals,biological clock is located in the suprachiasmatic nucleus (SCN), generates circadian rhythms in behaviourand physiology. These biological rhythms are adjusted daily to the environmental light/dark cycle via theretinohypothalamic tract (RHT). Three mammalian priod genes (per1, per2, and per3) that resemble theclock-regulating gene of Dorosophia melangaster, period (per ), have been cloned. Circadian clocks arealso located in peripheral tissues of mammals that are synchronized by the SCN. A molecular description ofthe mammalian circadian system has revealed that circadian oscillations may be a fundamental property ofmany cells in the body.Recently, it has been shown that the phosphorylation of PER proteins by casein kinase Iepsilon can modulatethe nuclear localization of mPER1 and mPER3 but not of mPER2.This antibody is useful tool to clarify molecular functions that regulate biological clock.