A discussion of the roles of oestrogen and progestin in human mammary carcinogenesis.

Oestrogens and progestins are important for both the genesis of human breast cancer and growth of those tumours once formed. Their role at different stages of the neoplastic process are reviewed and discussed within the context of a change in sensitivity of epithelial cells during either initiation or promotion stages. Evidence favours, but does not conclusively prove, the view that progestins are the predominant mitogen for normal breast epithelium whilst oestrogen assumes that function in neoplastic epithelium. Alterations in oestrogen receptor levels could provide the key for such a change. There are insufficient data on physiological progestin concentrations to judge their effect on established cancer. Models for steroidal effects on cell proliferation and oestrogen and progestin receptor regulation that are based on endometrial data are not appropriate for breast.