ESR1 and cancer: In univariate analysis, those ER+ BRCA1 cancers retaining wt BRCA1 more often had a low mitotic rate (100% vs 27%; P < 0.001), were less often of pure ductal histology (50% vs 85%; P = 0.05), and less often grade 3 (0% vs 68%; P < 0.001).