Mifepristone, also known as RU-486, stops the development of breast cancers caused by mutations in the BRCA1 gene by blocking the female hormone progesterone, scientists in the United States have discovered.

The findings indicate that the drug, which already has a good safety profile, could become an alternative way for thousands of women with BRCA1 mutations to protect themselves against breast cancer without surgery.

As the mutation confers an 80 per cent lifetime risk of the disease, as well as a 40 per cent risk of ovarian cancer, many women who inherit it choose to have their breasts and ovaries removed as a preventive measure.

Mutations in BRCA1 and BRCA2, a similar gene, together account for about 5 per cent of the 44,000 cases of breast cancer diagnosed annually in Britain.

If the results of animal studies are replicated in patients, women who know they have a high genetic risk of breast cancer could potentially take mifepristone or related drugs to stop tumours.

Much more work, however, is still required before such a preventive drug becomes available. Mifepristone itself is unlikely to be suitable because it blocks the action of progesterone. This property is used to provoke early abortions, as the female hormone is essential to maintaining a pregnancy.

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