An experimental breast cancer vaccine may eventually help treat or prevent up to 80% of breast cancers, according to preliminary tests.Researchers say the vaccine is based on the protein mammaglobin-A, which has been found in 80% of breast cancer tumors but is largely absent in normal, healthy breast cells.In a new study, researchers found that vaccination against the protein reversed tumor growth in mice bred to have breast cancer. The vaccine is thought to work by stimulating the immune system to launch an attack against breast cancer tumor cells.Researchers say the results show for the first time that vaccination with the mammaglobin-A protein may provide a new alternative for breast cancer treatment and prevention. The results appear in the Sept. 15 issue of the Journal of the National Cancer Institute.