PGR and breast cancer: As estrogen is known to induce progesterone receptor (PgR) expression in mammary epithelial cells,138–140 exogenous and endogenous estrogens may also contribute to breast cancer risk by augmenting progesterone signaling.141 Endogenous testosterone, whose levels are significantly reduced post-oophorectomy,15 is unlikely to account for the progestin-mediated increased breast cancer risk.