Breast cancer is the most common cancer detected in women, accounting for nearly one out of every three cancers diagnosed in the United States.Most cancer patients do not die from the primary tumor but die due to metastasis.Therefore, the study of metastasis is of most importance both to the clinician and patient.In the past, animal models have been used in breast cancer research and mammary gland biology.Our group has also established several animal models to address the function of a novel tumor suppressor gene maspin in breast tumor progression.Maspin was initially isolated from normal mammary epithelial cells.Its expression was down regulated in breast tumors.To test the protective role ofmaspin overexpression in mammary tumor progression, we crossed maspin overexpression transgenic mice (WAP-maspin) with a strain of oncogenic WAP-SV40 T antigen mice.The bitransgenic mice had reduced tumor growth rate and metastasis.