PurposeTo investigate the role of the PER3 circadian rhythm gene, located within the commonly deleted region of chromosome 1p36, in human breast cancer development.Patients and MethodsThe frequency of genetic alterations at 1p36 and PER3 gene copy number status were analyzed in 180 lymph node–negative breast cancers from patients who had received treatment with chemotherapy and/or tamoxifen. The expression levels of PER3 were also analyzed using published microarray profiles from > 400 breast cancer samples. Finally, the effect of loss of Per3 on tumor susceptibility was tested using two mouse models of breast cancer.ResultsDeletion of PER3 is directly related to tumor recurrence in patients with estrogen receptor (ER) – positive breast cancers treated with tamoxifen. Low expression of PER3 mRNA is associated with poor prognosis, particularly in a subset of tumors that are ER positive, and either luminal A or ERBB2-positive tumors. Mice deficient in Per3 showed increased susceptibility to breast cancer induced by carcinogen treatment or by overexpression of Erbb2.ConclusionDisruption of PER3 function may serve as an indicator of probability of tumor recurrence in patients with ER-positive tumors. Further investigations of this pathway may reveal links between deregulation of sleep homeostasis and breast tumorigenesis.