ESR1 and breast cancer: Luminal breast cancers are characterized by ER positivity and 70% of all breast cancer cases are of the luminal A subtype.14 In these tumors, obesity is associated with reduced overall survival (OS) and relapse-free survival (RFS).15 Premenopausal obese women have 30% lower risk of developing luminal A breast cancer,16 whereas postmenopausal women with a higher body mass index (BMI) have a higher risk of developing luminal B tumors.