Transcriptional control of the progesterone receptor gene by estrogen is a complex mechanism. It involves estrogen receptora which uses several enzymes that locally modify histone tails as cofactors. Using MCF-7 cells as a model, we found that Jumonji AT-rich interactive domain 1A (JARID1A; KDM5A⁄RBP2), an enzyme that removes the activating H3K4 di- and trimethylation marks, was involved in the fine-tuning of pro-gesterone receptor gene expression.