Any change that is not normal in the genes in a cell. The change can result in a damaged gene, missing gene, or a gene that ends up in the wrong place. It may be caused by mistakes when a cell divides. It may be caused by something in the environment. The change (called a mutation) can be harmful, good, or have no effect. If this happens in a mother's egg or a father's sperm, it can be passed on to the parent's child. Some of these changes can lead to cancer or other diseases.
Read about genetics and breast cancer risk.
Audio contributed by: David Hyde Pierce