Anti-progestins (drugs that block progesterone) are a relatively new category of anti-hormonal agents. Drugs in this class (e.g. mifepristone, RU 486) may be useful in the treatment of breast cancer. However, they are under early clinical investigation and are not approved for treatment of breast cancer. So far, the anti-progestins studied have had limited success in breast cancer clinical trials. Since Tamoxifen has been proven effective and approved for treatment of estrogen receptor positive breast cancer, you hear about this anti-estrogen drug more often than the anti-progestins.