PER2 and breast cancer: According to the molecular type of breast cancer, the statistically significant lowest expression of PER1 was observed in the triple negative breast cancer patients, while in the case of HER2-enriched, the lowest expression was observed for the negative loop of the central clock genes such as CRY2, PER2, PER3. We found a strong statistical association between the core circadian genes and hormone receptor status as well as tumor grading.