ESR1 and breast cancer: Moreover, patients with ER-positive/PR-negative bone metastatic breast cancer had significantly better BCSS than ER-negative/PR-positive (HR = 1.64, 95% CI: 1.12–2.38) and ER-negative/PR-negative (HR = 1.33, 95% CI: 1.19–1.47) breast cancers (both log-rank p < 0.001, Figure 2C).