In this study, they identified 456 genes (out of 8,102 candidate genes) that could classify breast tumours into six subtypes (including ER-positive, luminal subtype A; ER-positive, luminal subtype B; ER-positive, luminal subtype C; HER2-positive, ER-negative subtype; basal-like, ER-negative, progesterone receptor- (PGR) negative, HER2-negative; normal breast-like) and subsequently validated these subtypes in an independent cohort of 49 patients with locally advanced breast cancer. Here, ESR1 is linked to breast cancer.