Although there was no statistically significant heterogeneity, for HR- tumors, the increased CBC risk was greatest in the first 10 years following the first breast cancer diagnosis, with no association seen among women who were diagnosed with CBC more than 10 years after their first diagnosis (<5 years ER-/PR- vs. ER+/PR+ RR = 1.5, 95% CI 1.1–2.1; ≥10 years ER-/PR- vs. ER+/PR+ RR = 1.1, 95% CI 0.7–1.6) (Table 3). This evidence concerns the gene ESR1 and breast cancer.