We demonstrated that in mice, vaccination with a plasmid gene vaccine and an adenovirus gene vaccine, each containing the gene for HER2/neu, prevented growth of a HER2/neu-expressing breast cancer cell line injected into the mammary fat pad or i.v. The gene vaccines were not effective individually in therapeutic vaccine models in which vaccination took place after tumor cells were injected. The gene discussed is ERBB2; the disease is breast carcinoma.