Previous preclinical and clinical studies have demonstrated that Y537S mutation status is associated with a higher degree of resistance to endocrine therapies used for treating ER+ breast cancer, such as aromatase inhibitors, tamoxifen, and fulvestrant, relative to other ESR1 LBD mutations (4, 5, 8, 10–12, 33). The gene discussed is CYP19A1; the disease is breast cancer.