In this context, the analysis of the BC’s molecular profile based on the expression of ERα, PR, and HER2, as first established in 2000 by Perou et al. [71], is essential for patient management, and breast tumors are currently grouped into four main subtypes: (i) luminal A, (ii) luminal B, (iii) HER2-enriched (HER2E), and (iv) triple-negative (TN) [72]. This evidence concerns the gene ESR1 and breast cancer.