ESR1 and breast carcinoma: Three major breast cancer subtypes are easily distinguished by immunohistochemical assessment (IHC): triple-negative (ER, PR and HER2-negative), HER2-positive (HER2-positive (ER and PR may be positive or negative)) and ER-positive/HER2-negative (ER-positive, HER2-negative (PR may be positive or negative)) [8, 9].