ESR1 and breast carcinoma: The four-type classification of the different types of breast cancer into luminal A-like (ER + and/or PR+, Ki67 low and HER2−), luminal B-like (ER+ and/or PR+, Ki67 high and/or HER2+), HER2 positive (ER+/−, PR+/− and HER2+) and triple-negative (ER−, PR− and HER2−).