All types of cancer are caused by mutations in the genetic material (DNA) of cells and breast cancer is no exception. Mutations are a natural part of the cell division process, and cells can and do regularly accumulate mutations without getting cancer. But if the mutations occur in the genes that normally control cell growth and division, they can lead to cancer. It usually takes several mutations within a single cell to cause that cell to become cancerous, but when those mutations occur, all of the cells that are descendents of the original will be cancerous too. Because these cells lack the normal safeguards that regulate cell division, the abnormal cells will soon number in the millions and become a tumor. When the tumor invades surrounding tissue, it gains official status as cancer.

If this were the end of the story, cancer would not be the public health problem that it is today. Unfortunately, when cells have lost control of how fast they divide, they are likely to accumulate even more mutations and will eventually acquire the ability to break free of the tumor and enter the blood or lymph vessels. Metastasis, as this is called, disseminates cancer cells throughout the body and makes it much more difficult to treat.

If we know that mutations in the DNA of cells cause breast cancer, then what causes the mutations? In most cases, it is not clear what causes breast cells to mutate and become cancerous. Breast cancer may eventually prove to have a number of causes. In 5 to 10 percent of cases, a predisposition to develop breast cancer is inherited from your parents. Two genes have been found that can greatly increase this risk, called BRCA1 and BRCA2. These genes are normally involved in repairing damaged DNA and so if they are not working, mutations become much more common. Other genes may also increase the chances of developing breast cancer. These include a gene called the cell-cycle checkpoint kinase 2 (CHEK-2) gene and another called the p53 tumor suppressor gene.

So is cancer a genetic disease? It is, but that just means that it is caused by mutations in the DNA, not necessarily by inherited mutations. Researchers currently believe that most cancers are acquired, meaning they are caused by spontaneous mutations that develop in your body cells after you are born. We do not know exactly why these mutations occur but there are some environmental causes of mutations that may be to blame. Exposure to radiation is one major cause of mutations, especially if it occurs during adolescence, when the breasts are first developing. Exposure to several types of chemicals such as polycyclic aromatic hydrocarbons (PAHs) can also cause mutations that may lead to breast cancer.

Another factor that contributes to the frequency of breast cancer is how breast tissue responds to hormones. The glands and ducts of the breast undergo almost constant changes during the reproductive life of a female. This is caused primarily by the hormones of the menstrual cycle (especially estrogen). Breast tissue must prepare itself each month to undergo changes if a pregnancy occurs. The small changes that occur each month allow the breast to become capable of producing milk, but if no pregnancy occurs, the breast returns to normal and prepares to start the cycle again. The hormones estrogen and progesterone cause some of these changes in breast tissue by increasing the rate of cell division. Over time, this increased rate of cell division that happens several times each year increases the risk that some cells may acquire mutations and start dividing uncontrollably, leading to cancer.