Breast cancers that express the hormone receptors (HRs) Estrogen Receptor-α (ER) and Progesterone Receptor (PR), but not Human Epidermal Growth Factor Receptor-2 (HER2) [HR+/HER2−] are the most common subtype (68%), followed by HR+/HER2+ and HR−/HER2− (10% each) and HR−/HER2+ (4%) [NCI]. Here, ESR1 is linked to breast cancer.