ESR1 and breast carcinoma: It has been concluded that the frequency of ER positivity in MBC is higher than that in female breast cancer (FBC), with more than 90% of tumors being positive for ER, and more than 80% positive for PR.[7,11,17] Therefore, the presence of both ER and PR plays a crucial role in determining whether the primary disease behind the axillary metastatic lymph node is breast cancer.