ESR1 and breast cancer: The mean age at baseline was 53.0 years, mean BMI 23.7 kg/m2, 49.3% of the women were postmenopausal, 34.1% current smokers, 27.0% highly physically active, 22.8% heavy alcohol drinkers, 27.1% ever HT users, 58.9 ever OC users, and 14.1% were nulliparous and the mean age at 1st childbirth in parous women was 25.9 years (Table 1 compares baseline characteristics of non BC cases to all 1193 BC cases in the cohort, and Additional file 1: Table S1 compares baseline characteristics of non BC cases to 611 BC cases with complete date on ER and PR status).