A team of scientists have discovered some unexpected benefits with the abortion pill RU486 (Mifepristone).
They say the pill, which is used to end unwanted early pregnancies, can prevent certain breast cancers.
The scientists at the University of California, Irvine, say the chemical compound for the abortion pill prevents the growth of mammary tumours caused by the mutant gene responsible for the majority of breast and ovarian cancers.
A mutated version of the gene BRCA-1 significantly raises the possibility of breast and ovarian cancers, so it is widely studied by cancer geneticists.
As a rule by age 70, more than 50 percent of women with the mutated BRCA-1 gene develop breast or ovarian cancer.
The compound, Mifepristone, prevents breast tumours developing by inhibiting the hormone progesterone in breast tissue cells and offers the possibility of a new prevention method to women who have a genetic predisposition to breast and ovarian cancers.
At present such women with a family history of the disease, face the prospect of having their breasts or ovaries surgically removed to reduce the risk of developing cancer.
Lead author Eva Lee, a professor of developmental and cell biology and biological chemistry at UCI, says they found that progesterone encourages the proliferation of mammary cells that carry a breast cancer gene and Mifepristone can block that response.
She says they hope the discovery will lead to new options for women with a high risk for developing breast cancer.
Lee and her colleagues studied mice that carried the mutated BRCA-1 gene and found that mice treated with Mifepristone did not develop mammary tumours by the time they reached one year of age while the untreated mice developed tumours by eight months of age.
Progesterone is secreted by the ovaries and is essential to the maintenance of a pregnancy; Mifepristone is designed to abort pregnancy in the first trimester by blocking the progesterone, thereby ending the viability of the fetus.
In smaller doses, it is used as an emergency contraceptive.
The UCI researchers found that progesterone encourages the development of cancer when the mutated BRCA-1 is present because it speeds up the division of cells. Mifepristone was found to block a binding process that is necessary for progesterone to cause the cell division.
It is estimated that five per cent of the 41,000 cases diagnosed in the UK each year are hereditary, mostly caused by flaws in a gene called BRCA1.
In excess of 13,000 British women and 43,300 American women a year die of breast cancer; those without the rogue gene have an 11 per cent chance of developing it, but women with it have an 85 per cent risk.
A family history of breast cancer also raises the risk of ovarian cancer to 40 per cent.
Genetic testing will confirm the presence of the BRCA-1 genes and annual mammograms can spot the early stages of the disease but are not as reliable in younger women, where they only pick up 40 per cent of cancers.
Experts have welcomed the research, but say a treatment is at least 15 years away.
The research was supported by the National Cancer Institute, the Breast Cancer Research Foundation and the Department of Defense.
The study is published in the current issue of Science.