PER1 (period circadian clock 1) is an important core clock protein and members of the circadian family of proteins, involved in the regulation of normal cell proliferations and physiological rhythms of human beings. It is expressed in a circadian pattern in the suprachiasmatic nucleus, the primary circadian pacemaker in the mammalian brain and its function is to regulate physiological rhythms, cell cycle and DNA damage response of normal living subjects. Aberrant expressions and altered rhythms of PER1 are highly linked to the carcinogenesis and development of malignant tumors, such as prostate cancer, colon cancer, leukocythemia and breast cancer.