ESR1 and breast cancer: In HER2-positive bone metastatic breast cancers, the BCSS was best in the ER-positive/PR-positive subtype (Figure 3A), ER-positive/PR-negative (HR = 1.24, 95% CI: 1.02–1.51), and ER-negative/PR-positive (HR = 1.96, 95% CI: 0.94–4.10) breast cancers which manifested poorer BCSS than the ER-positive/PR-positive ones, while the BCSS difference between ER-positive/PR-negative, ER-negative/PR-positive, and ER-negative/PR-negative breast cancers was not statistically significant (Figure 3C).