PAQR7 and breast carcinoma: A natural steroid, 17α-hydroxyprogesterone, has also been used to identify mPR-specific actions in human breast cancer cells, although high concentrations (1 μM) of this agonist are required to activate the mPRs [40], which is expected due to its relatively low RBA for mPRα (~1% that of progesterone) [3].