Breast cancers can be grouped into four basic subtypes dependingon the expression of the estrogen receptor (ER), progesterone receptor(PR), and human epidermal growth factor receptor 2 (HER2): luminalA (ER+/PR+/HER2-), luminal B (ER+/PR+/HER2+), HER2 positive (ER-/PR-/HER2+),and basal-like or triple negative (ER-/PR-/HER2-).17 Despite luminal A being the most prevalent (68% of cases),2 it is poorly researched relative to other subtypes.One reason might be due to the estrogen dependency of luminal A cancers,18,19 rendering it difficult to be grown in animal models. Here, ESR1 is linked to breast cancer.