What is a hereditary cancer syndrome?

A hereditary cancer syndrome is a condition in which a change in a single gene, inherited from a parent and present from birth, raises a person’s lifetime risk of developing certain cancers. Well-known examples include Lynch syndrome and hereditary breast and ovarian cancer syndrome, caused by changes in the BRCA1 and BRCA2 genes.

The most useful thing to know at the outset is that carrying one of these gene changes is not a diagnosis of cancer, and it is not a prediction that cancer will happen. It is information about risk, and it is information that can be acted on. People who know they carry one of these changes are screened earlier, more often, and more effectively than anyone else, and in some cases cancers can be prevented entirely.

This article is written for anyone approaching this subject for the first time: people considering genetic testing, people who have just received a result, and people whose relative has been diagnosed with a hereditary syndrome. It explains the ideas that apply across all of these conditions. Articles on individual syndromes are linked at the end.

How much cancer is inherited?

Less than most people expect. Roughly 5% to 10% of all cancers are caused by an inherited gene change. The other 90% or more are not.

This is worth sitting with, because a cancer diagnosis in a family often produces immediate fear that everyone is at risk. Usually they are not. Cancer is common: something close to two in five people will develop cancer at some point. In a large family, several relatives having cancer may simply reflect how common the disease is, particularly if they were diagnosed at older ages and with different types of cancer.

At the same time, 5% to 10% is not a small number, and the people in that group benefit enormously from being identified. The purpose of genetic testing is to find them.

Sporadic, familial, and hereditary cancer

Cancer is usually sorted into three groups, and knowing which one applies changes what happens next.

Sporadic cancer — The large majority, around 70% to 80%. These cancers arise from genetic damage that accumulates in a single cell during a person’s lifetime, from aging, environmental exposures, and simple copying errors. Nothing was inherited, and relatives are not at increased risk.
Familial cancer — Around 15% to 20%. More cancer occurs in the family than expected, but no single gene change explains it. The cause is usually a combination of many common genetic variants that each nudge risk slightly, together with shared environment and habits. Relatives may have a modestly increased risk, and enhanced screening is often offered even though no gene change is found.
Hereditary cancer — Around 5% to 10%. A single inherited gene change substantially raises risk, and it can be identified by a test. This is what a hereditary cancer syndrome means.

The boundary between the second and third groups is not fixed. Families once labeled familial are sometimes reclassified as hereditary when a new gene is discovered or a test is repeated with better technology.

What makes a cancer syndrome hereditary?

Two ideas explain most of what follows.

The first is where the gene change is. A somatic change arises inside a tumor during a person’s life. It exists only in the cancer cells; it cannot be passed to children, and it says nothing about relatives. A germline change is present in the DNA a person was born with, exists in every cell of the body including egg and sperm cells, and can be passed on. Hereditary cancer syndromes are germline conditions.

This distinction causes constant confusion, because modern cancer care tests both. When a tumor is sent for molecular testing, the report may name genes that are also hereditary cancer genes. That does not mean the change is inherited. Confirming that requires a separate test on blood or saliva. If a tumor report has named such a gene and nobody has raised germline testing with you, it is reasonable to ask.

The second idea is that we all carry two copies of every gene. Most hereditary cancer genes are tumor suppressor genes, whose normal job is to repair damage or restrain cell growth. A person with a hereditary syndrome is born with one working copy and one that does not work. One working copy is enough for normal life, which is why carriers are healthy. But every cell in their body is one step closer to trouble than everyone else’s. If the remaining copy is damaged in any single cell over a lifetime, that cell loses the protection entirely.

This explains the pattern these syndromes produce. Cancers appear earlier than usual, because the process has a head start. They appear in more than one organ, because every cell in the body carries the change. And they appear in more than one place in the same organ, or in both of a paired organ, more often than expected.

Signs that cancer in a family may be inherited

Certain patterns raise the possibility of a hereditary syndrome and are the reason a doctor may suggest genetic testing:

Cancer diagnosed at an unusually young age — Often under 50, and younger still for some cancers.
The same or related cancers in several relatives on the same side of the family — Which side matters, because a gene change is inherited from one parent.
More than one separate cancer in the same person — Two different cancers, or the same cancer in both breasts, both kidneys, or both eyes.
A rare cancer — Some cancers are uncommon enough that their appearance alone prompts testing, such as male breast cancer, medullary thyroid carcinoma, or cancer of the adrenal gland in a child.
A specific tumor type — Certain subtypes point to specific syndromes. This is where the pathology report matters, because it records the type of cancer in the detail these criteria require.
Non-cancer features — Some syndromes produce characteristic skin marks, benign growths, or other findings that are easy to overlook.
Ancestry — Certain populations carry specific gene changes more commonly, so ancestry can be relevant even when the family history seems unremarkable.

Gathering an accurate family history is genuinely useful preparation. What matters is who had cancer, how they are related to you, which side of the family they are on, the exact type of cancer, and the age at diagnosis. Obtaining a relative’s pathology report is often the single most valuable thing a family can do, because the specific subtype recorded there frequently determines whether testing criteria are met.

How lifetime risk is described, and why it is given as a range

People are usually told their risk as a percentage. Understanding what that number is, and is not, matters more than any other part of this subject.

Risk is always a range, not a single figure. Published estimates for the same syndrome often differ substantially, and there is a specific reason. Early studies of these conditions looked at families identified precisely because so many members had cancer. Those families were selected for having a lot of cancer, so the risks calculated from them were higher than the risk faced by an average carrier. As testing became cheaper and was offered more widely, carriers began to be found in families with unremarkable histories, and the estimates fell, sometimes by half or more.

Your own family history shifts the number. For most syndromes, a carrier with several close relatives affected at young ages faces a considerably higher risk than a carrier discovered incidentally in a family with little cancer. The same gene change can carry meaningfully different implications in two different families.

Risk is not the same for every gene, even within one syndrome. Lynch syndrome is caused by changes in any of five genes, and the risks differ so much between them that they are now managed as related but distinct conditions.

A high lifetime risk is not a certainty. This is the point most often lost. A 70% lifetime risk means that about three in ten carriers will never develop that cancer. The reverse is also true, and both halves deserve stating: the risk is high enough to justify serious preventive measures, and low enough that many carriers never develop the disease. These figures describe groups of people, not individuals, and a genetics service can give an estimate that reflects your genes, your sex, and your own family.

Finally, a risk figure describes what happens without screening or preventive treatment. It is a description of the natural course of the condition, not a forecast of your future, and the whole point of knowing is to change it.

How genetic testing is done

Testing for a hereditary syndrome is done on a blood sample or, less often, a saliva sample. It examines the DNA a person was born with rather than the DNA of a tumor, so it can be done whether or not a person has ever had cancer.

The laboratory uses next-generation sequencing to read the genes letter by letter, together with a separate method that detects large deletions and duplications, where whole sections of a gene are missing or repeated. Both are needed, because a meaningful share of changes in some genes are large rearrangements that sequencing alone will not find. Someone tested many years ago may reasonably ask whether their test included this step and whether retesting is worthwhile.

Testing is almost always done as a panel covering many genes at once rather than one gene at a time. This finds more, but it also means a result may name a gene nobody was expecting, including genes whose implications are less well understood. This is one reason testing is arranged alongside genetic counseling rather than ordered as a standalone laboratory test.

How results are reported

Laboratories classify each change they find according to how confident they are that it affects the gene:

Pathogenic or likely pathogenic variant — A change known, or strongly expected, to stop the gene working. This is a positive result and is the basis for everything described below.
Variant of uncertain significance — A change was found, but there is not yet enough evidence to say whether it matters. This is not a positive result. It is managed as though the result were negative, with screening guided by family history, and it is never a basis for preventive surgery. Most such variants are eventually reclassified as harmless as evidence accumulates. It is worth asking the genetics service how you will be told if the classification changes.
Benign or likely benign variant — A harmless difference. Everyone carries many of these, and they are usually not reported.
No variant identified — No change was found in the genes tested.

What the result means

A positive result confirms the syndrome. It means an increased lifetime risk of specific cancers, a plan for screening and possibly for reducing risk, and implications for blood relatives.

A negative result means two quite different things depending on the situation, and confusing them is consequential:

True negative — A specific gene change is already known in your family, and you tested negative for that exact change. You did not inherit it, your risk returns to roughly that of the general population, and your children cannot inherit it from you.
Uninformative negative — No gene change has been identified in anyone in your family. This rules out the genes tested, but it does not rule out an inherited cause, since not every hereditary syndrome has a known gene. Screening continues based on family history.

This second point is misunderstood often enough to be worth stating directly: a negative result in a family with a striking cancer history does not explain the family history away, and it does not mean enhanced screening can stop.

What can be done with the information

Knowing about a hereditary syndrome is useful because it opens options that are not available to people at average risk. Which of these apply depends entirely on the syndrome.

Earlier and more frequent screening — Starting years or decades before routine screening would begin, and repeating more often. For some cancers this finds disease early; for others, such as colon cancer, screening removes precancerous growths and prevents the cancer altogether.
Different screening tests — Some syndromes call for tests not used in the general population, such as breast MRI or whole-body MRI.
Medication to lower risk — Available for some syndromes, such as aspirin in Lynch syndrome.
Risk-reducing surgery — Removing an at-risk organ before cancer develops. This is the most effective option available and also the most consequential, since it means an operation on a healthy person with permanent effects. It is discussed as one option among several, and where guidelines support screening as an alternative, choosing screening is a legitimate decision rather than a refusal of care.
Different cancer treatment — Some syndromes change how a cancer is treated if one develops, including eligibility for particular drugs and decisions about how much tissue to remove at surgery.
Information for relatives — Often the most valuable outcome, since it allows family members to find out whether the same applies to them.

These are options the care team discusses rather than instructions. Decisions about timing, and particularly about surgery, are personal and are usually revisited more than once over the years.

What a result means for your family

Most hereditary cancer syndromes are inherited in an autosomal dominant pattern, which means a single altered copy is enough to cause the condition. Each child, sibling, and parent of a carrier has a 50% chance of carrying the same change. A few syndromes are recessive, requiring a non-working copy from both parents, and the explanation for those families is different.

Three points are worth emphasizing because they are commonly misunderstood:

These conditions pass through fathers exactly as they do through mothers. A syndrome causing breast and ovarian cancer is inherited from a father as readily as from a mother. Assuming otherwise leads whole branches of families to conclude wrongly that they are not at risk.
Skipping a generation does not mean skipping the gene. A carrier may live a long life without developing cancer and still pass the change to their children.
Testing a relative is easier than the first test in the family. Once a specific change is identified, relatives are tested for that exact change, a process called cascade testing. It is simpler, cheaper, and gives a clear yes or no.

Testing is usually offered to adult relatives. The right age for children depends entirely on the syndrome: for most, testing is deferred until adulthood because nothing about management changes before then, but for some, screening genuinely begins in childhood and testing is offered early. Genetics services help with this and with the difficult practical work of telling relatives, including providing a letter that can be passed on.

Insurance, employment, and privacy

Concern about how a genetic result might be used is common and is one of the main reasons people hesitate to be tested. It deserves a direct answer rather than reassurance.

Legal protections exist, and they differ by country. In Canada, federal legislation prohibits requiring a person to undergo genetic testing or to disclose results as a condition of obtaining goods, services, or a contract, including insurance. In the United States, federal law prohibits health insurers and employers from using genetic information, but those protections do not extend to life, disability, or long-term care insurance. Other countries vary considerably, some through legislation and some through voluntary agreements with insurers.

Because the rules differ and change over time, this is a question worth raising directly with a genetics service before testing rather than afterward. They will know the current position where you live and can explain what is and is not protected. It is also worth knowing that results from clinical genetic testing are part of your medical record and are subject to the same privacy protections as the rest of it, and that direct-to-consumer testing companies operate under different rules than clinical laboratories.

Questions to ask your doctor

Based on my personal and family history, do you think genetic testing is appropriate for me?
Which genes will be tested, and will the test detect large deletions as well as sequence changes?
Should I be seen by a genetic counselor before testing?
Would it be more useful to test an affected relative first?
What information about my relatives would be most helpful for me to gather?
If a gene change is found, what would my estimated risks be, given my own family history?
What screening or preventive options would be available to me?
If the result is a variant of uncertain significance, how will that be managed, and how will I be told if it is reclassified?
If no gene change is found, does my family history still mean I need extra screening?
Which of my relatives should be offered testing, and how is that arranged?
At what age should my children be tested?
How is my result protected, and could it affect my insurance where I live?
Is support available to help me work through these decisions?