A chemical used to abort pregnancy could fight breast and ovarian cancer, say scientists.
Some women are genetically prone to breast cancer
The compound mifepristone or RU-486 works by blocking the sex hormone progesterone, which feeds the growth of certain cancers.
The Science study on mice suggests the chemical would be most effective in women genetically prone to the cancers.
By the age of 70, more than half of women with a faulty BRCA1 gene will develop breast or ovarian cancer.
Mifepristone is designed to abort pregnancy in its first three months by blocking the action of progesterone and ending the life of the foetus.
In smaller doses, it is also used as an emergency contraceptive - stopping a foetus developing in the first days after intercourse.
Professor Eva Lee, from the University of California at Irvine, and colleagues studied genetically engineered mice lacking the BRCA1 gene.
They found that mammary glands in the mice accumulated large numbers of progesterone receptors - molecules that allow progesterone to exert its effects.
When progesterone attached itself to the receptors, it appeared to encourage abnormal cell growth.
By eight months of age, all the mice had developed tumours.
But similar mice treated with mifepristone had not developed tumours, even after a year.
The scientists do not believe mifepristone would be the best treatment to give to women to prevent cancer because the drug acts on other receptors that could produce unwanted side effects.
But they think that harnessing its specific progesterone-blocking action could work.
Dr Kat Arney from Cancer Research UK, agreed, saying: "This is an interesting discovery, which could one day help prevent breast cancer.
"We already know that sex hormone levels can have a big impact on breast cancer risk, and can fuel the growth of some breast cancers.
"These new results tell us a lot about the way that flaws in the BRCA1 gene may lead to breast cancer, by making faulty breast cells more responsive to progesterone.
"The next step is to prove that this finding holds true in human breast cancers."
Only a minority of breast cancers are due to BRCA1 mutations.