Men's prostate cancer therapies might help women with hard-to-treat breast cancer, a study has shown.
Prostate cancer drugs which block the tumour-fuelling effects of testosterone may shrink certain breast tumours, research suggests.
Although the work is at an early stage, scientists believe it could transform treatment of deadly forms of breast cancer.
Most breast tumours are stimulated by the female hormone oestrogen but more than a fifth are not. These cancers are hard to treat because they do not respond to anti-oestrogen therapies such as the drug tamoxifen.
The new research suggests that a subset of "oestrogen negative" breast cancers could be treated with prostate drugs which work in a similar way to tamoxifen.
These breast cancers appear to respond to a testosterone pathway that can switch on genes normally activated by oestrogen.
Laboratory tests showed it might be possible to combat them by blocking this pathway, using prostate hormone therapy.
Lead researcher Dr Ian Mills, from Cancer Research UK's Cambridge Research Institute, said: "This important discovery suggests that patients with a type of oestrogen-receptor-negative breast cancer may potentially benefit from therapies given to prostate cancer patients, which could transform treatment for this patient group in the future."
The research is published today in the EMBO Journal.