ESR1 and breast carcinoma: Receptor status categorizes breast cancer into 3 distinct biological types based on over-expression of estrogen receptors (ER), progesterone receptor (PR), and epidermal growth factor receptor (HER2), namely, hormone receptor breast cancer (positive ER and/or PR over-expression and negative for HER2 over-expression), HER2 positive (HER2 over-expression regardless of ER or PR expression status) and triple negative (negative for ER, PR and HER2).