Introduction
Although estrogen has long been thought of as the major ovarian mitogen involved in proliferation of normal human mammary epithelium and in neoplastic progression, there is some evidence that progesterone may play a role. In both mouse and rat mammary gland, it is apparent that progesterone is specifically involved in inducing epithelial proliferation and this has been supported by progesterone receptor knockout (PRKO) mice where development of the gland is retarded. The luminal compartment of mouse mammary epithelium has been shown to be the target of the progesterone-induced proliferative signal and is also the site of mammary epithelial tumorigenesis.