ESR1 and breast cancer: Molecular expression profiling, in combination with the expression status of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) [1-3], has led to the classification of breast cancers into four subtypes [4, 5]: luminal A (ER+, PR+ and HER2-), luminal B (ER+, PR+ and HER2+), HER2 positive (ER-, PR- and HER2+), and basal-like (ER-, PR- and HER2-, or triple negative) [3].