Scientists are hailing the results of a new study that shows a drug already used to treat advanced cancer has proven extremely effective against an aggressive form of early breast cancer.

In an article published Thursday in The New England Journal of Medicine, researchers found the drug Herceptin can prevent 50 percent of all breast tumors from recurring in patients with early stages of the disease.

The drug, which targets only diseased cells, only works for the estimated 20-percent of breast cancer cases in which tumors produce too much of a protein known as HER-2. 

Breast cancer is the most common cause of cancer among women in the United States.  More than 200,000 American women will be diagnosed with breast cancer, and about 40,000 women will die of the disease in the United States in 2005.

Officials say Herceptin will be marketed internationally by the Swiss drugmaker Roche.