ESR1 and breast cancer: Gene expression profiling greatly facilitates the diagnosis and treatment of BC patients by identifying four molecular subtypes (1): luminal A, presence of the estrogen receptor (ER) and/or the progesterone receptor (PR), absence of human epidermal growth factor receptor 2 (HER2); (2) luminal B, ER positive, PR negative, and/or HER2 positive; (3) HER2-enriched; (4) triple-negative breast cancer (TNBC), ER negative, PR negative, and HER2 negative (4).