ESR1 and breast cancer: In receptor negative breast tumours, the mechanism would be reversed: the absence of ER would lead to a reduced PgR expression43, 44, and allow NF-kB complex activation and miR-9-5p upregulation that in turn might contribute to the maintenance of ER negative status by negatively targeting ESR1 mRNA and promote the development of typical features associated with clinical aggressiveness of breast cancer.