ESR1 and breast cancer: Among the full BBD Cohort, a frequency-matched case–control sample set of BBD biopsies was selected based on outcome in follow-up at 16 years: incident ER + BC (BBD-ER+), incident ER− BC (BBD-ER−) or cancer-free (BBD-control), matched on age at biopsy and year of biopsy or censoring.