What is genetic testing?

Genetic testing looks for specific inherited changes (mutations) in a persons chromosomes, genes, or proteins. Genetic mutations can have harmful, beneficial, neutral (no effect), or uncertain effects on health. Mutations that are harmful may increase a persons chance, or risk, of developing a disease such as cancer. Overall, inherited mutations are thought to play a role in about 5 to 10 percent of all cancers.

Cancer can sometimes appear to run in families even if it is not caused by an inherited mutation. For example, a shared environment or lifestyle, such as tobacco use, can cause similar cancers to develop among family members. However, certain patternssuch as the types of cancer that develop, other non-cancer conditions that are seen, and the ages at which cancer typically developsmay suggest the presence of a hereditary cancer syndrome.

The genetic mutations that cause many of the known hereditary cancer syndromes have been identified, and genetic testing can confirm whether a condition is, indeed, the result of an inherited syndrome. Genetic testing is also done to determine whether family members without obvious illness have inherited the same mutation as a family member who is known to carry a cancer-associated mutation.

Inherited genetic mutations can increase a persons risk of developing cancer through a variety of mechanisms, depending on the function of the gene. Mutations in genes that control cell growth and the repair of damaged DNA are particularly likely to be associated with increased cancer risk.

Genetic testing of tumor samples can also be performed, but this Fact Sheet does not cover such testing.

Does someone who inherits a cancer-predisposing mutation always get cancer?

No. Even if a cancer-predisposing mutation is present in a family, it does not necessarily mean that everyone who inherits the mutation will develop cancer. Several factors influence the outcome in a given person with the mutation.

One factor is the pattern of inheritance of the cancer syndrome. To understand how hereditary cancer syndromes may be inherited, it is helpful to keep in mind that every person has two copies of most genes, with one copy inherited from each parent. Most mutations involved in hereditary cancer syndromes are inherited in one of two main patterns: autosomal dominant and autosomal recessive.

With autosomal dominant inheritance, a single altered copy of the gene is enough to increase a persons chances of developing cancer. In this case, the parent from whom the mutation was inherited may also show the effects of the gene mutation. The parent may also be referred to as a carrier.

Continued here:
Genetic Testing Fact Sheet - National Cancer Institute

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