RB1 and breast cancer: In conclusion, while there is abundant clinical data showing that PR agonists are beneficial in postmenopausal patients with advanced ER+ breast cancer,56–59 their use as an adjuvant therapy is not established and trials are in progress.28 Several recent pre-clinical studies provide compelling evidence that the key to effective progestogen therapy is the ability of activated PR (specifically PR-B) to reprogram the genomic activity of activated ER.