RB1 and breast cancer: To probe changes in PR target gene expression in the presence or absence of commonly used PR ligands (R5020, RU486), including diverse antiprogestins (aglepristone, onapristone) currently in development for clinical use, we utilized the well-characterized model system of T47D breast cancer cells, stably expressing either unmodified wild-type (WT) PR-B or a transcriptionally hyperactive form of deSUMOylated K388R PR-B (KR; this receptor faithfully mimics phosphorylated PR-B with regard to target gene selection) [19, 31].