In women with BRCA-1, the naturally occurring female hormone progesterone speeds the proliferation of mammary cells. “If we block the progesterone pathway using an antiprogesterone, it could prevent breast cancer,” says Eva Lee, lead author of the study. That’s exactly what mifepristone did for the experiment’s mice, all of which had the BRCA-1 gene. At age 1, none of those treated with mifepristone had developed tumors. But all the untreated mice had tumors by the time they were 8 months old.

From what I have heard on NPR, all 14 of the treated mice remained cancer-free, while all 27 mice who received either placebo or nothing, developed breast cancer. Of course, such clear-cut, all-or-none data are both exciting and suspicious and I am sure someone will try to replicate this very soon.
I do not know exact details of the protocol, but it appears that the mice were given RU-486 throughout life. This is, of course, impossible in humans – you cannot give this to girls/women from birth till death every day! We are looking primarily for treatment, not prevention (certainly not a pharmaceutical for prevention – behavioral methods will be welcome if discovered).
Still, knowing that breast cancer cells have progesterone receptors, that progesterone promotes breast cancer and that RU-486 prevents it will place a sharp focus of future research on this mechanism, discovering its details and potentially developing a new drug that can be used in the treatment of breast cancer.