ESR1 and breast cancer: As estrogen receptor α (ERα), prostaglandin E2 (PGE2) produced by cyclooxygenase-2 (COX-2) and individual E-series of prostaglandin (EP) receptors are known to play critical roles in breast cancer cell pathogenesis, progression, malignancy and metastasis24, 25, we assessed the expression of each protein in tumor tissue.