ESR1 and breast cancer: After adjusting for age at diagnosis, race, nuclear grade, clinical stage, lymph vascular invasion, adjuvant radiation, and pCR, patients with ER expressions < 10% showed a significantly higher risk for breast cancer death or recurrence compared to those with ER expressions between 10% and 20% (HR = 2.181, 95% CI = 1.317–3.612, p = 0.002 for TTR).