ESR1 and breast cancer: In addition, our results indicated a positive correlation between plasma prolactin levels and risk of breast cancer among the ER+/PR+ patients (RR (95% CI) as 1.26 (1.04, 1.53)), although no association was uncovered in the ER-/PR- patients (RR (95% CI) as 1.0 (0.70, 1.43)).