ESR1 and breast cancer: In this younger age group, the strength of the association between ER- and/or PR-receptor positive breast cancer and BMI (HR, 0.75; 95% CI, 0.70 – 0.81) was much stronger than that for ER- and PR-receptor negative breast cancer (HR, 0.85; 95% CI, 0.76 – 0.95).