PGR and breast cancer: We demonstrated the validity of such an PgR based “estrogen-challenge test” through proof-of-principle preclinical studies in syngeneic murine mammary cancer models derived from STAT1 knockout mice [170]; by serial Western blot analyses and FFNP-PET imaging, PgR levels decreased with estradiol deprivation and increased after a 1-day estradiol stimulation in the ER-positive and responsive SSM3 tumors, whereas there were no apparent changes in the ER-positive nonresponsive SSM2 tumor system [84,85].