Current clinical classification of breast cancer relies largely on the use of immunohistochemistry (IHC) markers of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki‐67 to classify breast cancer into four subtypes, that is, Luminal A (ER+ and/or PR+, HER2−, low Ki‐67), Luminal B (ER+, HER2±, high Ki‐67), HER2‐enriched (ER−, PR−, HER2+), and triple‐negative breast cancer (TNBC). This evidence concerns the gene ESR1 and breast cancer.