ESR1 and breast cancer: Notably, PR antagonists are available (17), and a recent clinical trial (MIPRA; NCT02651844) provided evidence that the progesterone antagonist, mifepristone (RU486), may provide benefits in ER+ breast cancer expressing a high PRA/PRB isoform ratio; an approximately 50% decrease in Ki67 staining was observed in all surgical specimens from patients treated with mifepristone compared with baseline (P = 0.0003) (18).