Approximately 10% of breast cancer is considered "hereditary." The majority of breast cancers, however, are called "sporadic," meaning we don't know why they happened. Hereditary cancer refers to cancer that is caused by a gene mutation that is present at birth and in all cells of the body. This gene change makes individuals more likely to develop cancer in their lifetime but doesn't mean they will definitely develop the disease. Certain cancers, including those of the breast, ovary and colon, are more likely than others to be hereditary.