ESR1 and breast cancer: BC is a multifactorial disease including four molecular subtypes, i.e., luminal A (ER+, PR+, and human epidermal growth factor receptor-negative, Her−), luminal B (ER+, PR+, and Her+), Her2 (ER−, PR−, and Her+), and basal like (ER−, PR−, and Her−, also called triple-negative BC, TNBC) [3,4,5].