Does Cancer Run in Families? Understanding Hereditary Cancer and Genetic Testing

Start with the size of the thing

About 5% to 10% of all cancers are thought to come from a harmful gene change inherited from a parent. That is NCI's figure, and it is the number most headlines leave out.

So roughly nine cancers in ten are not inherited. They come from changes a cell picks up during life, from tobacco, alcohol, infection, radiation, weight, age, or plain bad luck in cell division. Our page on what causes cancer covers those.

That does not make family history useless. It means family history is a flag, not a verdict.

Cancer can cluster in a family with no gene behind it

NCI makes this point directly. Relatives share more than DNA. They share houses, diets, habits and jobs. If three people in a family smoked, three lung cancers in that family may say more about tobacco than about genes.

Chance matters too. Cancer is common. In a large family with older members, several cancers of different types is close to what you would expect anyway.

The patterns that do suggest an inherited change

NCI lists the features that make a hereditary cancer syndrome worth checking. A hereditary cancer syndrome is a gene change, passed down in a family, that raises cancer risk. The flags are:

• A relative already known to carry a harmful cancer-risk gene change.
• Cancer diagnosed young, for example colon cancer before age 50.
• Several different cancers in the same person.
• The same cancer in several close relatives, meaning parents, siblings or children.
• Breast and ovarian cancer in the same family, or colon and endometrial cancer in the same family.
• Cancer in both organs of a pair, such as both breasts or both kidneys.
• A rare cancer, such as breast cancer in a man.

Guidelines also recommend testing for anyone diagnosed with triple-negative breast cancer, ovarian cancer, pancreatic cancer, colorectal cancer before age 50, metastatic prostate cancer, or male breast cancer. For those people the result can change treatment, not just future screening.

One practical note from NCI: when possible, testing should start with a relative who has had cancer. That person's result tells the rest of the family what to look for. Our page on family history and cancer risk works through how to collect that history.

What the test is, and what it is not

Testing for inherited risk reads genes you were born with. It is done on blood, saliva, cheek cells or skin cells, and results usually come back in about two to three weeks.

The standard approach is a panel test, which reads many genes at once. A narrow panel targets one cancer family. A broad panel covers genes tied to many common cancers.

This is a different test from tumor testing. Tumor testing, also called somatic or biomarker testing, reads changes the cancer picked up during life, and it is used to pick treatment. NCI is clear that tumor testing does not replace inherited testing. Our page on tumor testing versus inherited genetic testing sets the two side by side.

Four answers, not two

A result is rarely a simple yes or no.

Positive. The lab found a pathogenic or likely pathogenic variant, meaning a change known to raise risk. It may explain a cancer already diagnosed, guide treatment, and trigger earlier or more frequent screening, risk-reducing medication or surgery.

True negative. A specific variant is known in the family, and this person did not inherit it. Their risk is probably close to the general population's. It is not zero.

Uninformative negative. The family history is strong, but no known variant was found in anyone. There may be a variant that current tests cannot see.

Variant of uncertain significance. A change was found, and there is not enough data to say whether it matters. NCI notes most of these are eventually reclassified as harmless, so a VUS is usually not used to make decisions. It should be revisited later.

Because of that spread, NCI recommends genetic counseling around testing. Our page on genetic counseling describes what those appointments cover.

What a BRCA result means in real numbers

BRCA1 and BRCA2 are genes for repairing DNA. Numbers make the risk concrete.

More than 60% of women who inherit a harmful BRCA1 or BRCA2 change will develop breast cancer in their lifetime, against about 13% of women in general. For ovarian cancer, it is about 39% to 58% with a BRCA1 change and about 13% to 29% with BRCA2, against about 1.1% in general.

Men are affected too. About 1.8% to 7.1% of men with a harmful BRCA2 change develop breast cancer by age 70, against about 0.1% of men in general.

Notice what these are: raised odds, not certainty. A large share of carriers never develop the cancer in question.

Why an at-home kit is the wrong tool for this

NCI is blunt here. As of January 2024, the only FDA-approved direct-to-consumer test for inherited cancer risk looks at three BRCA variants that are common in people of Ashkenazi Jewish descent.

There are hundreds of BRCA variants that can raise risk. About 80% of cancer-causing BRCA variants are missed by that approach. A reassuring result from such a kit is not reassurance for most people.

When to ask about testing

Bring it up with a doctor or genetic counselor if any of the flags above fit your family, in particular:

• Any close relative with a known harmful cancer-risk gene change.
• Colon or endometrial cancer diagnosed in a relative before age 50.
• Breast cancer before age 50, or breast and ovarian cancer in the same family line.
• Two or more close relatives with the same cancer.
• Male breast cancer, or ovarian, pancreatic or metastatic prostate cancer in a close relative.

What this does not mean

A positive result is a risk figure, not a diagnosis and not a schedule.

A negative result does not remove ordinary risk. Routine screening still applies.

And a result belongs to a family, not just a person. Siblings and children of a carrier each have a chance of carrying the same change, which is why NCI treats the conversation with relatives as part of the process rather than an afterthought.

Sources

• NCI, Genetic Testing for Inherited Cancer Risk — https://www.cancer.gov/about-cancer/causes-prevention/genetics/genetic-testing-fact-sheet
• NCI, BRCA Gene Changes: Cancer Risk and Genetic Testing — https://www.cancer.gov/about-cancer/causes-prevention/genetics/brca-fact-sheet
• NCI, Cancer Genetics — https://www.cancer.gov/about-cancer/causes-prevention/genetics

How this article was prepared

An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.

The National Cancer Information Foundation publishes Cancer Explained. This page is for learning. It is not medical advice and does not suggest a test or treatment.

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