PER2 and cancer: We found that when kept in 24 hour alternating light-dark conditions (24hr LD cycles), mice deficient in Bmal1 (Bmal1+/−), Cry1 and Cry2 (Cry1−/−;Cry2−/−), Per1 and Per2 (Per1−/−;Per2m/m) or Per2 alone (Per2−/−) were all cancer-prone.