ESR1 and breast cancer: Evaluation of the 394 ER-positive primary breast cancers demonstrated that Ox-E/ER index values correlated negatively with PR mRNA levels (rp = -0.2; P = 0.00011) and positively with tumor grade (rp = 0.2; P = 9.741 × e-5), and were significantly higher in ER-positive/PR-negative versus ER-positive/PR-positive breast cancers (t-test, P = 0.0008).