ESR1 and breast cancer: Loss of ER and/or PR in recurrent breast cancer is considered a poor prognostic sign, and has been shown to correlate with increased risk of death (hazard ratio (HR) 3.62; 95% confidence interval (CI) 1.65–7.94) and (HR 2.34; 95% CI 1.01–5.47) compared with patients with stable ER or PR positive tumors [8, 9].