ESR1 and breast cancer: Luminal BC, the most common BC subtype (over 70% of all BCs), expresses estrogen receptor-α (ESR1; ERα) and is successfully treated with ERα-targeted therapies, which include receptor antagonists, such as tamoxifen or fulvestrant, and aromatase inhibitors that interfere in 17β-estradiol ligand production, such as anastrazole and letrozole [80].