We observed the reduced accessibility of EREs in two independent cohorts (42 and 75 samples in the JFCR–BRCA and TCGA–BRCA cohorts, respectively), suggesting that the current classification system for ER-positive tumours based on gene expression, particularly the expression of ER, PgR and Ki67, is not sufficient for understanding the nature of BC. This evidence concerns the gene PGR and breast cancer.