This study, conducted over a two year period, shows that the single addition of MMP-3, a gene that encodes an enzyme that degrades extracellular components such as fibronectin, laminin, collagens III, IV, IX, and X, and cartilage proteoglycans, is sufficient to induce moderate to severe mammary hyperplasia, lymphocytic infiltrates, ductal carcinoma in situ, and mammary carcinomas. The gene discussed is LAMB2; the disease is breast carcinoma.