A single gene appears to play a crucial role in deadly breast cancers, increasing the chances the cancer will spread and making it resistant to chemotherapy, US researchers said on Monday.

They found people with aggressive breast cancers have abnormal genetic alterations in a gene called MTDH, and drugs that block the gene could keep local tumors from metastasizing or spreading, increasing a woman's chances for survival.

"Not only has a new metastasis gene been identified, but this also is one of a few such genes for which the exact mode of action has been elucidated," said Dr Michael Reiss of The Cancer Institute of New Jersey in New Brunswick, whose study appears in the journal Cancer Cell.