From LA Times: 

A sustained dose of RU-486, the so-called abortion pill, prevented breast cancer tumors in mice with a genetic mutation that made them highly susceptible to the disease, researchers at UC Irvine report today.

The mutation in a gene known as BRCA1 leads to uncontrolled cell growth in the mammary glands. About one in 800 women inherits a version of BRCA1 that is damaged in some way, and the National Cancer Institute estimates that up to 80% of such women will develop breast cancer.

Cancer researcher Eva Lee and colleagues figured that RU-486 could be used to block the hormone progesterone, whose unchecked production is believed to be a primary cause of hereditary breast cancer.

RU-486 binds to receptors that normally link with progesterone. It is used as an abortion pill because progesterone is needed in the uterus to maintain a pregnancy.  more

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It is amazing what you can find out when you actually study a drug!!!  Interesting study.  Unfortunately, it does need more study.