We observed 10 046 incident breast cancer cases (NHS,n = 6621; NHS II, n = 3425) over20.8 years median follow-up (3 644 137 person-years);85.5% of these cases had data available for tumor ER status (ER+,n = 7005; ER−, n = 1583). This evidence concerns the gene ESR1 and breast cancer.