CLOCK and neoplasm: In a study of sporadic and familial primary breast cancer, Winter et al. [78] found that the expression of Per1 and Per2 in tumor tissues was significantly lower than that in normal breast tissues, especially the expression of Per1 in familial primary breast cancer, which was significantly lower than that in sporadic breast cancer specimens, indicating that the disorder of clock gene expression may be an important factor in the pathogenesis of familial breast cancer [77].