ESR1 and breast cancer: There was no effect modification by age, family history, or tamoxifen therapy; elevated RRs of ER-/PR- CBC following ER-/PR- first breast cancer were also observed for women <45 years of age at first diagnosis (RR = 5.9, 95% CI 2.9–12.2), those without family history of breast cancer (RR = 5.2, 95% CI 2.8–9.7) and those who were not treated with tamoxifen (RR = 6.5, 95% CI 3.2–12.9).