In breast cancer, some of the cells in your breast begin growing abnormally. These cells divide more rapidly than healthy cells do and may spread (metastasize) through your breast, to your lymph nodes or to other parts of your body. The most common type of breast cancer begins in the milk-producing ducts, but cancer may also begin in the lobules or in other breast tissue.
In most cases, it isn't clear what causes normal breast cells to become cancerous. Doctors do know that only 5 percent to 10 percent of breast cancers are inherited. Families that do have genetic defects in one of two genes, breast cancer gene 1 (BRCA1) or breast cancer gene 2 (BRCA2), have a much greater risk of developing both breast and ovarian cancer. Other inherited mutations - including the ataxia-telangiectasia mutation gene, the cell-cycle checkpoint kinase 2 (CHEK-2) gene and the p53 tumor suppressor gene - also make it more likely that you'll develop breast cancer. If one of these genes is present in your family, you have a 50 percent chance of having the gene.
Yet most genetic mutations related to breast cancer aren't inherited. These acquired mutations may result from radiation exposure - women treated with chest radiation therapy for lymphoma in childhood or during adolescence when breasts are developing have a significantly higher incidence of breast cancer than do women not exposed to radiation. Mutations may also develop as a result of exposure to cancer-causing chemicals, such as the polycyclic aromatic hydrocarbons found in tobacco and charred red meats.