Clinically, breast cancers are firstly categorized, according to the expression of three receptors routinely assessed by immunohistochemistry assay in the following subtypes: estrogen and progesterone receptor positive (ER+, PR+), human epidermal growth factor receptor positive (HER2+) and triple-negative (ER–, PR–, HER2–) malignancies (15, 16). Here, ESR1 is linked to breast cancer.