Approximately 75% of breast tumors express ERα, and patients with breast cancer with ERα-positive tumors generally receive endocrine therapy, such as receptor antagonists (antiestrogens), including tamoxifen or fulvestrant, or aromatase inhibitors such as anastrozole or letrozole that inhibit ligand (17β-estradiol) production [31–33]. The gene discussed is CYP19A1; the disease is breast neoplasm.