PER1 and cancer: This gene is a member of the period gene family and is expressed in a circadian pattern in the SCN. The genes of this family encode components of the circadian rhythms of motor activity, metabolism, and behavior. PER2 is activated by CLOCK/ARNTL heterodimers, but then represses this activation in a feedback loop using PER/CRY heterodimers to interact with CLOCK/ARNTL. PER2 polymorphisms may increase the risk of certain types of cancer and are associated with sleep disorders [22,23,28].