In a case–case analysis (data not shown), ever use of OCs (OR = 1.09, 95% CI = 0.92 to 1.29) and OC use within the previous 5 years (OR = 1.08, 95% CI = 0.79 to 1.46) were not associated with ER– breast cancer, relative to ER+ breast cancer. Here, ESR1 is linked to breast cancer.