Before and even after pursuing genetic testing, you may have questions about what the test results may mean for you and your family. A genetic counselor can explain your test results and answer your questions.

Here are answers to some of the most frequently asked questions about genetic testing.

What are genes and genetic mutations?

We each have millions of cells in our bodies. Inside each of those cells is our DNA, which is organized into genes. Our genes provide instructions for proteins to be formed and carry out different functions in our bodies, such as how to grow and develop. Some genes protect our bodies from developing cancer.

The DNA in our genes is made up of a code of four chemical bases. They are referred to by the letters A, T, C, and G. Each gene has a specific order of these letters. When there is a genetic mutation, the order of the letters is altered. This can prevent a gene from working properly. In the case of cancer genes, if a genetic mutation is present, that person may have a higher chance of developing certain types of cancer throughout their life.

How much of cancer is hereditary?

In general, only about 5 to 10 percent of all cancers are hereditary. In families with hereditary cancer, a genetic mutation has been identified in family members that contributed to the development of certain cancers in the family. The likelihood of a cancer being hereditary can vary by tumor type. For example, about 15 to 20 percent of ovarian cancer is hereditary. Other cancer types, such as lung and skin cancers, have a lower likelihood of being hereditary.

About 15 to 20 percent of cancers are familial. This means a certain cancer type seems to be more common in the family but may be connected to shared environments among family members, not a genetic mutation.

Most cancers and tumors are sporadic. This means they occur due to chance, environment, or other factors that are not well understood.

What are indications for possible hereditary syndromes?

You may want to consider genetic testing in many scenarios. Some indications that may suggest a possible hereditary syndrome in an individual or family include:

• receiving a cancer diagnosis at a young age (usually age 50 for breast or colon cancer)
• receiving a diagnosis of rare cancer at any age
• having multiple people in a family diagnosed with the same or related cancers
• having multiple primary cancer diagnoses in the same person
• having cancer in both sides of a paired organ (for example, both breasts or both kidneys)
• being of certain ethnic backgrounds
• when individuals or families have had some type of genetic mutation identified through genetic testing by another doctor, hospital, or company

How do people develop hereditary cancer?

Our bodies naturally have tumor-suppressor genes, which are supposed to help protect us from developing cancer. Everyone has two copies of these types of genes in each cell: You inherit one copy from your mother and one copy from your father. Think of it like brakes on a car: The front brakes represent one copy, and the back brakes represent the second copy.

Throughout our lives, a random genetic mutation may occur on one copy of our tumor-suppressor gene in one cell of the body. Perhaps this random mutation occurs due to age, chance, or an environmental exposure. Essentially, the back brakes of the car stop working. Later in that person’s life, another random genetic mutation may occur on the second copy of that tumor-suppressor gene in that same cell line. This is as if the front brakes of the car also stop working. When both sets of brakes stop working, sporadic (nonhereditary) cancer can develop. In other words, all brakes on the car have failed.

In the development of hereditary cancer, a person is born with one copy of a tumor suppressor gene that already has a genetic mutation in every cell of the body. It is as if the car was built without any back brakes. If a random genetic mutation occurs in the front brakes, a cancer can develop. Being born with one copy of the tumor-suppressor gene already not working makes it easier for that person to lose the second copy. This is why we tend to see hereditary cancers develop earlier in life and develop more often.

How are cancer genes inherited?

Genetic conditions can be passed through a family in several ways. We have two copies of every gene because we inherit one copy from our mother and one copy from our father. In turn, one copy of each gene is passed on to a child.

For some mutations, called autosomal dominant, inheriting one copy of a mutated gene is enough to increase the risk of developing cancer. When a person has an autosomal dominant mutation, there is a 50% chance that the mutation was inherited from their mother and a 50% chance that the mutation was inherited from their father.

If a parent has an autosomal dominant mutation, there is a 50% chance for each of their children to inherit the mutation. Examples of autosomal dominant cancer genes are the five genes associated with Lynch syndrome, which increases the risk of developing several types of cancer at a young age, especially colorectal cancer.

For other mutations, called autosomal recessive, the risk usually exists if the person inherits the same mutation from both parents. An example of an autosomal recessive cancer syndrome is xeroderma pigmentosum, which results in an extreme sensitivity to sunlight that leads to skin cancer.

People who have only one copy of a recessive genetic mutation are referred to as carriers. In many autosomal recessive conditions, being a carrier of one genetic mutation does not have any effect on a person’s health. However, in some autosomal recessive cancer conditions, carriers may have an increased risk of developing certain types of cancer. If an autosomal recessive condition runs in your family, a genetic counselor can discuss with you what risks, if any, we know about for carriers.

If both parents are carriers for a particular autosomal recessive condition, there is a 25% chance for each of their children to inherit both genetic mutations (one mutation from each parent) and have the associated condition. Alternatively, there is a 75% chance that each child will not have the condition.

Rarely, a new genetic alteration can occur during embryonic development. In this situation, the mutation was not inherited from the mother or father, but it can be passed on to future children.

What is multigene panel genetic testing?

Multigene panel genetic testing can evaluate multiple genes at one time. The goal is to identify genetic mutations that may increase one’s risk for certain cancers.

The advantage of this type of testing is that we can assess multiple genes associated with a particular type of cancer all at the same time. This maximizes the chances of finding an explanation for an individual’s or family’s history of cancer.

The disadvantages of multigene panel testing include:

• finding an unexpected genetic mutation that indicates a risk for certain cancers that may not have been previously seen in the family
• an increased chance of finding a genetic variant of uncertain significance

An unexpected result occurs when an identified genetic mutation indicates that there is a risk for certain cancers that have not been previously seen in the family. When a variant of uncertain significance is identified, a definitive answer regarding the risk for cancer is not available. Additional research may be needed.

What is the difference between germline and somatic genetic testing?

Every cell of our body contains approximately 20,000 genes. All cancers are caused by genetic changes of some kind. Any test that examines genes is a genetic test. Within the realm of cancer genetic testing, there are two main types of testing: germline testing and somatic testing.

Germline testing refers to testing for inherited changes in genes. This testing can be performed on a variety of tissue types but is most often done by analyzing blood. Most of our cells, including blood cells, contain DNA that comes from our mother’s egg and father’s sperm. The egg and sperm are called germ cells. Therefore, germline genetic testing looks at the DNA that we inherited from our parents’ germ cells.

Since germline mutations are present from birth, a person born with, say, a BRCA1 mutation will have the mutation in their cells throughout their life. These mutations can be passed on to a child through their own egg or sperm.

Somatic genetic testing involves the detection of genetic mutations that were not present from birth. Their causes include random cell errors, cell aging, and environmental exposures that can cause mutations in genes. For example, UV light can cause DNA damage, resulting in genetic mutations in the skin cells. An accumulation of mutations in critical genes can cause a cell to grow abnormally and ultimately result in a tumor.

Somatic genetic testing of tumors identifies which genes are mutated in a specific cancer. These mutations are typically specific only to the tumor itself and are not found in other cells from different areas of the body. Given this, somatic mutations found in a cancer cell are not passed on to a person’s children. Understanding which genes are mutated in a person’s tumor may help doctors with treatment planning.

What is the difference between using blood and saliva samples for genetic testing?

Genetic testing can be done using a blood sample or a saliva (spit) sample. The decision whether to give a blood or saliva sample for testing usually depends on personal preference and convenience. 

If you choose to pursue genetic testing through a blood sample, we will schedule you for a blood draw at an MSK location that is convenient for you. If you choose to pursue genetic testing through saliva, the MSK laboratory will ship a saliva kit to your home with instructions on how to collect the saliva sample. You can then send it back to the laboratory using a prepaid shipping label.

Once the blood or saliva sample is received by the MSK laboratory, DNA is extracted from the sample for genetic testing.  Whether you choose blood and saliva testing, the results are usually available within a few weeks of when the sample is submitted to the laboratory. 

Genetic testing through blood and saliva are equally accurate.  Rarely, the saliva sample does not contain enough DNA. If that happens, you may need to provide an additional saliva sample or a blood sample. 

What does it mean to have a positive genetic test result?

A positive genetic test result means that a mutation was identified in a gene. People who carry a genetic mutation in a cancer gene may have a hereditary predisposition to cancer and a higher than average risk of developing cancer at some point in their lifetime. Cancer risks vary according to the affected gene and sometimes the type of mutation identified within that gene. Having a mutation does not mean a person will develop cancer. It just means that there is an increased risk.

A positive result may:

• provide an explanation for an individual’s cancer diagnosis
• help guide treatment decisions for a person affected with cancer
• inform family members about their future risk of developing cancer and allow them to take steps to reduce their risk or to detect cancer at an earlier stage
• prompt family members to pursue genetic testing to determine if they inherited the same genetic mutation and, thus, the associated cancer risks
• provide information to assist with reproductive decision-making

Genetic counselors can help identify who in a family is at risk for inheriting a mutation. We will discuss with you at what age testing would be appropriate for those relatives.

What does it mean to have a negative genetic test result?

A negative test result means that no genetic mutations were found. There are different reasons why a test result can be negative:

• The cancer in the family could have happened by chance.
• There may be a genetic mutation in other family members that you did not inherit.
• There may be a genetic mutation in the family that could not be found with current testing methods or has not been discovered yet.

Based on your personal and family history, a genetic counselor will provide an individualized risk assessment. We will help you best understand what your negative test result means. We will recommend the cancer screenings that may be best for you.

What does it mean to have a genetic variant of uncertain significance identified?

This means that a change was identified in a gene, but it’s not yet known whether this change is associated with an increased risk of cancer. The scientific community is still learning which changes in our genes are harmless and which are associated with a risk for disease, including cancer. If a variant of uncertain significance is identified, we will look at a person’s individual and family health history. With that information, we can make recommendations for medical management.

What is in vitro fertilization and preimplantation genetic testing?

If a genetic mutation is identified in your family, you may want to learn about family planning options to either start a family or have more children. Embryos (fertilized eggs) can be tested for the family’s genetic mutation. This process is called preimplantation genetic testing (PGT).

PGT requires eggs to be fertilized in a laboratory. This process is called in vitro fertilization (IVF). Genetic testing can be performed on a few cells removed from the embryo. The embryos that do not have the family’s genetic mutation are transferred into a women’s uterus to achieve pregnancy. IVF-PGT may be an option for families who wish to have children without the hereditary cancer syndrome.

What is DNA banking?

You and your family members may wish to consider the option of commercial DNA banking. DNA banking is long-term storage of your genetic material. DNA can be obtained from blood, saliva, or other tissue from a patient’s body. DNA banking provides a way for patients and family members to store genetic material to be used if additional genetic testing becomes available in the future. Your genetic counselor can help identify laboratories that offer DNA banking.