MELBOURNE, Aug. 14 -- Researchers in Australia have discovered why breast cancer treatment stops working for some patients, uncovering an unexpected role of a key cancer-fighting protein.

The study, published in Nature, found that the retinoblastoma (Rb) protein, long viewed as a natural brake on cancer growth, can unexpectedly activate genes that respond to the hormone estrogen. Some of these genes can help cancer cells withstand treatment and retain the ability to regrow, according to a statement released Thursday by Australia's Peter MacCallum Cancer Center.

The research team found that Rb does more than switch off genes that drive cell division. It can also "switch on biological programs that partially work against its own protectiv...