The main objective of the study was to present expression pattern of ten core clock genes (CLOCK, NPAS2, BMAL, CRY1, CRY2, PER1, PER2, PER3, TIMELESS, and CSNK1E) in breast cancer tissues in comparison to the paired non-tumor adjacent tissue according to the histological and clinical features, including. This evidence concerns the gene PER3 and breast cancer.