The Georgetown University Medical Center researchers said their findings might provide a way to identify patients using tamoxifen -- the most commonly used breast cancer prevention and treatment drug -- who have become resistant and no longer benefit from the drug. That would allow doctors to try other therapy options sooner.

Professor Robert Clarke and his team found breast cancer cells resistant to tamoxifen display few "alpha" estrogen receptors, but many more "gamma" estrogen-related receptors, which tamoxifen seems to activate.

"Until now, this (gamma) receptor has not been viewed to be of much importance in any type of breast cancer," said Assistant Research Professor Rebecca Riggins. "All that was known (was) there were more of these receptors in breast cancer than in normal breast tissue; we hadn't gone much further than that."

She said the discovery could help explain why invasive lobular carcinoma -- the sub-type of breast cancer in which the findings were made -- might not respond as well to tamoxifen as other subtypes.

The research appears in the journal Cancer Research.