The Period (Per) genes are key circadian rhythm regulators in mammals. Expression of the mouse Per (mPer) genes have diurnal pattern in the suprachiamstic nuclei and in peripheral tissues. Genetic ablation mPER1 and mPER2 function results in a complete loss of circadian rhythm control based on wheel running activity in mice. In addition, these animals also display apparent premature aging and significant increase in neoplastic and hyperplastic phenotypes.