A derivative of the synthetic progestin norethindrone with antiprogesterone activity. Mifepristone competitively binds to the progesterone receptor, resulting in inhibition of the effects of endogenous or exogenous progesterone. This agent also exhibits antiglucocorticoid and weak antiandrogenic activities. or A drug used to end early pregnancies. It is also being studied in the treatment of some types of cancer and other conditions. Mifepristone blocks the action of progesterone, a hormone that helps some cancers grow. It is a type of antiprogesterone. Also called Mifeprex and RU 486.