ESR1 and breast cancer: A prior analysis in a subset of the population included in the current analysis [13], in women predominantly premenopausal at diagnosis (n = 197 cases; n = 192 premenopausal at diagnosis), found a significant positive association between follicular estradiol with breast cancer overall (4th vs. 1st quartile OR: 2.1, 95% CI: 1.1 to 4.1, Ptrend = 0.08), that appeared somewhat stronger for invasive and ER+/PR+ disease.