Despite great advances in breast cancer research, many of the genes responsible for the formation and spread of this disease remain unidentified. We have generated a mutant strain of mice with in-born defects that resemble some of the pre-cancerous characteristics of human breast cancer cells. We intend to study how growth control is lost in our mice and use this information to better understand how growth control is lost in human breast cancer patients. Ultimately we expect this approach will reveal new gene mutations that cause breast cancer. Understanding the underlying causes of breast cancer (through gene mutations) will directly lead to new diagnostic tests for patients with these mutations and better treatments.