My research group uses animals to study mammalian development and the processes that transform normal cells into cancer cells.Breast cancer is the most common cancer. It is the second leading cause of cancer mortality in women, with approximately one in nine being affected over their lifetime. One of our projects is to study functions of the breast tumor suppressor gene BRCA1 in mammary gland development and tumor formation. After studying mice models carrying various mutations in the BRCA1 gene, our data indicated that BRCA1 is essential for genetic stability. Loss of BRCA1 increases the mutation rates of all genes, including tumor suppressors and oncogenes, which results in tumor formation.