Cancer arises from the uncontrolled growth of cells. Cancer is caused by harmful changes (mutations) in the genetic messages (genes) which control the growth and division of cells which prevent them from being able to do their jobs effectively. We receive one complete copy of our genes from our mother and another from our father. It is the accumulation of multiple mutations over many years that disrupts the growth control of the cell and allows a normal cell to grow without control, and eventually become a cancer.

Most cases of cancer occur in the absence of a significant family history, and are not inherited. In these families the mutations causing the cancer occur only in the tumor itself and are all acquired after birth. Although the cause is seldom known, these acquired mutations may be the result of environmental or hormonal exposures, or mistakes which can occasionally occur when a cell divides. Acquired gene mutations cannot be passed from one generation to the next, so this type of cancer is considered "sporadic" (a chance event) and not hereditary. Just by chance, some families have several members affected with sporadic cancers.

The interaction of multiple minor genes and environmental influences may also increase the risk of developing cancer. Although little is known in this area, it is possible, for example, that individuals with a moderate family history of cancer may be more susceptible to cancer-causing agents in the environment. This type of moderately increased cancer risk can be called a "familial" risk. At this time, we do not have genetic testing available for familial cancers and instead rely on family history interpretation.

About 5 to 10 percent of cancers are thought to be hereditary. In these cases, an individual inherits a copy of a growth control gene with a mutation from one parent, and a working copy of the same gene from the other parent. The gene with the mutation is also called a "cancer susceptibility gene." Since this cancer susceptibility gene is inherited, it is found in every cell of the body, but the working copy of the gene keeps each cell working properly. However, if the working copy of the gene in a cell becomes damaged by a mutation, that cell can lose its growth control and become cancerous. Thus, individuals who inherit a cancer susceptibility gene have a much greater chance for developing certain cancers in their lifetime. However, not everyone with an inherited cancer susceptibility gene will develop cancer.

Damaged cancer susceptibility genes can be inherited, and passed on, by men just as easily as women. If a parent carries a cancer susceptibility gene, each of their children has a 50 percent chance of inheriting the gene, and thus the susceptibility to cancer. Each child also has a 50 percent chance of inheriting the working copy of the gene, in which case their cancer risk would be no higher than that of the general population. It should be noted that some cancer susceptibility genes only have links to cancers that may not be seen in both sexes, such as ovarian and prostate cancers. Parents of the unaffected sex can still carry and pass on these genes.

Hereditary cancers generally are not significantly different from non-hereditary cancers. It is the way the cancers occur in the family that indicates whether they may be hereditary. Signs suggesting hereditary cancer include:

• two or more relatives with the same type of cancer, on the same side of the family,
• several generations affected,
• early ages of cancer diagnosis,
• individuals with more than one primary cancer,
• the occurrence in one family of cancers which are known to be genetically related (such as breast and ovarian cancer, or colon and uterine cancer),
• the presence of physical signs which are known to be associated with hereditary cancer (such as moles and melanoma, or polyps and colon cancer), and
• certain rare types of cancer.