It is true that only a small percentage of all cancers, including breast cancer, is inherited, that is, the likelihood of getting them is transmitted at conception from parent to son or daughter, either through the egg or through the sperm that fertilized it. In such cases, the gene defect appears in every body cell, including blood cells. A mutation that allows uncontrolled cell growth can occur at any time during life, induced either directly or indirectly, by exposure to a toxic substance like tobacco smoke or to a chronic irritant like alcohol or stomach acid. Such mutations can also occur spontaneously, when a mistake is made during cell division that is not detected by the cell's built-in surveillance system. A cancer-causing mutation in a breast or ovarian cell acquired during a woman's lifetime cannot be passed on to her children.
Oncogenes dictate the production of proteins that stimulate normal cell growth.
If a mutation occurs in an oncogene, cells can proliferate at a greatly accelerated rate. Tumor-suppressor genes, as their name implies, place brakes on cell division. If a mutation inactivates a tumor-suppressor gene, cell multiplication can continue unchecked. These same mutations have also been linked to a three to four-fold risk of developing ovarian cancer. In 1971, Dr. Alfred Knudson of Fox Chase Cancer Center in Philadelphia, proposed a scheme by which an inherited mutation might result in a predisposition to cancer.