Understanding Genetic Testing for Cancer

Genes, mutations, and cancer risk

Genetics is the field of science that looks at how traits (such as eye color) are passed down from parents to their children through genes.

Genes are pieces of DNA (deoxyribonucleic acid) inside each of our cells that tell the cell how to make the proteins the body needs to function. DNA is the genetic “blueprint” in each cell. Genes affect inherited traits passed on from a parent to a child, such as hair color, eye color, and height. They also affect whether a person is likely to develop certain diseases, such as cancer.

Changes in genes, called mutations, play an important role in the development of cancer. Mutations can cause a cell to make (or not make) proteins that affect how the cell grows and divides into new cells. Certain mutations can cause cells to grow out of control, which can lead to cancer. Still, only about 5% to 10% of all cancers are thought to be related to an inherited gene mutation.

    Usually several gene changes or mutations are needed before a cell becomes cancer.

Most cancers start because of acquired gene mutations that happen during a person’s lifetime. Sometimes these gene changes have an outside cause, such as exposure to sunlight or tobacco. But gene mutations can also be random events that sometimes happen inside a cell, without a clear cause.

Acquired mutations only affect the cells that grow from the mutated cell. They do not affect all the cells in the person’s body. This means all the cancer cells will have the mutations, but normal cells in the body will not. This is very different from inherited mutations, which are in every cell in the body – even the cells without cancer.  

What is genetic testing?

Genetic testing is the process of using medical tests to look for mutations in a person’s genes. More than 1,000 genetic tests are used today, and more are being developed.

Genetic testing can be used in many ways.

Genetic testing for diseases that can be inherited

Predictive genetic testing is the type of testing most often used to check for cancer risk. It’s used to look for gene mutations that might put a person at risk of getting a certain kind of cancer. Most people do not need this type of testing. It’s usually done when family history suggests there’s a cancer that may be inherited. An example is testing for changes in the BRCA1 and BRCA2 genes (known breast cancer genes) in a woman whose mother and sister had breast cancer.

Genetic testing can also be used for other reasons, such as:

  • Carrier testing can be used to help couples learn if they carry a gene mutation for a disorder they might pass on to a child, such as cystic fibrosis, sickle-cell anemia, or Tay-Sachs disease.
  • Prenatal screening can be used to diagnose some conditions in babies before they are even born, such as Down syndrome.
  • Newborn screening is the most widespread form of genetic testing. Newborns are screened for a number of inherited conditions, such as phenylketonuria (PKU), cystic fibrosis, sickle-cell disease, and others.

Each of these types of genetic testing look for gene changes that are passed from one generation to the next. Except for the newborn screening tests, they’re used mainly for people with certain types of diseases that seem to run in their families.

Genetic testing for cancer risk

Cancer-related genetic tests are most commonly done as predictive genetic tests. They may be used:

  • To see if a person has a certain gene mutation known to increase the risk for a certain cancer (or cancers)
  • To confirm a suspected gene mutation in a person or family

Testing cancer cells for genetic changes

Sometimes after a person has been diagnosed with cancer, the doctor will do tests to look for gene changes in the cancer cells. These tests can give information on a person’s outlook (prognosis) and can sometimes help tell whether certain types of treatment might be useful.

These types of tests look for gene changes only in the cancer cells that are taken from the patient. These tests are not the same as the tests used to find out about inherited cancer risk.

For more about this kind of testing and its use in cancer treatment, see our information on specific types of cancer.

Who should have genetic testing?

If you are concerned about a pattern of disease in your family, cancer you’ve had in the past, or other cancer risk factors, you may want to talk to a health care provider about genetic testing. Only you can decide to have genetic testing to learn about your cancer risk.

Genetic testing may be recommended for people who have had a certain diseases or certain patterns of disease in their family. If you have any of the following, you might consider genetic testing:

  • Several first-degree relatives (mother, father, sisters, brothers, children) with cancer
  • Many relatives on one side of the family who have had the same type of cancer
  • Cancers in your family that are known to be linked to a single gene mutation or a family member whose had more than one of these linked types of cancer (breast, ovarian, and pancreatic cancers are examples of this).
  • More than 1 type of cancer in a family member
  • Family members who had cancer at a younger age than normal for that type of cancer
  • Close relatives with cancers that are linked to rare hereditary cancer syndromes
  • A family member with certain rare cancers, such as breast cancer in a male or retinoblastoma
  • Ethnicity (for example, Jewish ancestry is linked to ovarian and breast cancers)
  • A physical finding that’s linked to an inherited cancer (such as having many colon polyps)
  • A known genetic mutation in one or more family members who have already had genetic testing

You need to know your family history and what kinds of tests are available. For some types of cancer, no known mutations have been linked to an increased risk. Other cancer types may have known mutations, but there’s no way to test for them yet.

Family Cancer Syndromes gives you more information on the types of cancers that may be linked to inherited genes.

Next steps

It’s important to find out how useful testing may be for you before you do it. Talk to your health care provider and plan to meet with a genetic counselor before the actual test. This will help you know what to expect. The counselor can tell you about the pros and cons of the test, what the results might mean, and what your options are. 

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Matloff ET, Bonadies DC. Genetic counseling. In: DeVita VT, Hellman S, Rosenberg SA, eds. Cancer: Principles & Practice of Oncology. 9th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2011:631-638.

National Cancer Institute. Genetic Testing for Hereditary Cancer Syndromes. April 11, 2013. Accessed at www.cancer.gov/about-cancer/causes-prevention/genetics/genetic-testing-fact-sheet on March 11, 2016.

Last Medical Review: May 23, 2016 Last Revised: May 23, 2016

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