According to the IHC-based categorization of breast cancer subtypes [66], the majority of MBCs are luminal A subtype (ER+ and/or PR+, HER-2−, and Ki-67 low), followed by luminal B subtype (ER+ and/or PR+, HER-2+, and/or Ki-67 high). This evidence concerns the gene ESR1 and breast carcinoma.