ESR1 and breast cancer: The subject of this analysis were four proteins routinely determined in breast cancer patients: estrogen receptor (ER), progesterone receptor (PgR), human epidermal growth factor receptor 2 (HER2) and Ki67, and six investigational proteins selected by their presumed association with increased risk of BM: cyclooxygenase 2 (COX2) [10], cytokeratins 5/6 (CK5/6) [5], chemokine receptor (CXCR4) [11], parathyroid hormone related protein receptor type 1 (PTHrPR1) [12, 13], osteopontin (OPN) [14, 15] and calcium sensing receptor (CaSR) [16].