ESR1 and breast cancer: Similarly, there was no association between progesterone and breast cancer risk in women age < 45 at blood draw (doubling ER+/PR+, OR: 1.1, 95% CI: 0.9 to 1.3) and a suggestively inverse association in women ≥ 45 (doubling ER+/PR+, OR: 0.8, 95% CI: 0.7 to 1.0).