In the presence of both ER and PR agonist ligands, immunoprecipitation of ER shows a specific increase in PR-B interaction in the ER+, PR+ breast cancer cell line T47D.13 However, in the presence of activated ER, unliganded PR promotes expression of a subset of ER target genes and enhances proliferation,11 highlighting the importance of ligand activation of PR. This evidence concerns the gene RB1 and breast carcinoma.