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CION Cancer Clinics
The NBN gene: what it does and why it matters | CION Cancer Clinics
NBN is a DNA repair gene. It helps the cell notice when both strands of its DNA have snapped, so the break can be mended. Most adults with an NBN result carry one faulty copy, and the effect on cancer risk is usually modest. Two faulty copies cause a rare childhood condition. This page explains what the gene does and what your report is telling you. At CION Cancer Clinics, our oncologists explain what a gene result means for you and your family, and plan the checks that follow.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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On this page
- What does the NBN gene actually do?
- What does one faulty copy mean, and what do two mean?
- What happens inside the cell when DNA snaps?
- What do the words on an NBN report mean?
- What does an NBN result change, and what does it not?
- What can this page not tell you?
- What do families get wrong about NBN?
- Common questions about the NBN gene
The short answer
What does the NBN gene actually do?
NBN is a DNA repair gene. It makes a protein called nibrin, which helps the cell notice when both strands of its DNA have snapped and raise the alarm so the break can be mended. One faulty copy of NBN usually causes little or no trouble. Two faulty copies cause a rare childhood condition called Nijmegen breakage syndrome.
Part of a three-member repair team
Nibrin does not work alone. It joins two other proteins to form a small team that sits at the site of a broken strand. This team holds the broken ends together and calls in another protein, made by the ATM gene, which tells the cell to pause and repair before it divides again.
Why a snapped strand is serious
Most DNA damage affects one letter on one strand, and the other strand acts as a template for the fix. When both strands break, there is no template in that spot. If the break is mended badly, pieces of instruction can be lost or joined in the wrong place. Those errors are one of the ways a cell can drift towards cancer.
NBN also helps the immune system build its defences, which is why the childhood condition affects immunity too.Carrier or condition
What does one faulty copy mean, and what do two mean?
The gene name on a report tells you less than the number of faulty copies. Check that first.
One faulty copy
You are a carrier. Your working copy still does the job in most cells. Some studies have linked one particular NBN fault to a moderately raised breast cancer risk. For most other NBN faults, the evidence is thin and still being gathered.
Two faulty copies
This causes Nijmegen breakage syndrome, which shows up in early childhood. It is rare everywhere and very rare in India.
Usually looks like
- A small head size from birth
- Slow growth through childhood
- Frequent chest and ear infections
- A high risk of lymphoma in childhood
Where the name comes from
The syndrome was first described by doctors in Nijmegen, a city in the Netherlands. Nibrin, the protein, was later named after the syndrome.
Why it turns up on panels
NBN is included on many breast and general cancer gene panels. So most adults who learn they carry an NBN fault find out that way, often while being tested for something else.
Not sure whether this applies to you?
Ask an oncologistFrom break to repair
What happens inside the cell when DNA snaps?
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Both strands of DNA break
This can follow radiation, some chemicals, or simple errors when a cell copies itself. It happens in healthy bodies every day and is usually repaired without trouble.
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The nibrin team arrives at the break
Nibrin and its two partners gather at the broken ends within moments. They hold the ends close and mark the spot for repair.
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The alarm is passed to ATM
The team switches on the ATM protein. ATM tells the cell to stop dividing for a while, so there is time to mend the break properly.
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The break is mended or the cell stands down
Most breaks are repaired cleanly. If the damage is too great, the cell is told to switch itself off, which protects the rest of the body.
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When NBN is missing, breaks slip through
With both copies faulty, the alarm is weak. Cells keep dividing with broken DNA. That is why people with the syndrome are so sensitive to radiation and so prone to cancer.
On your report
What do the words on an NBN report mean?
- Pathogenic variant
- A spelling change known to stop the gene working. This is what people mean by a gene fault.
- Heterozygous or monoallelic
- One faulty copy and one working copy. This is the carrier state, and it is what most adults with an NBN result have.
- Biallelic
- Both copies are faulty. In NBN, this causes Nijmegen breakage syndrome.
- Founder variant
- One specific fault handed down from a shared ancestor, so it is more common in one community. The best-known NBN founder fault comes from Eastern Europe.
- Double-strand break
- A break across both strands of the DNA ladder. It is the kind of damage the NBN team is built to detect.
- Variant of uncertain significance
- A spelling change the laboratory cannot yet call harmful or harmless. It should not change your care.
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Side by side
What does an NBN result change, and what does it not?
What it can change
What it usually does not
Relatives can be tested for your exact fault
Your treatment plan for a cancer you already have
Breast screening may be reviewed against family history
Routine X-rays for a carrier with one faulty copy
Your partner may be offered testing before a pregnancy
Your children's health, if your partner has no fault
Your oncologist knows before planning radiotherapy
The need for preventive surgery
Being straight with you
What can this page not tell you?
It cannot tell you what your own result means. What your specific variant means is a question for the counsellor who ordered the test. The evidence differs a great deal between one NBN fault and another, and only someone who has seen your report can say which group yours falls into.
It cannot give you a firm risk figure
Most of what is known about NBN carriers comes from studies of one Eastern European fault. Studies of other faults are small. Indian data are very limited. So the honest answer for many carriers is that the risk is probably modest and not yet well measured.
Who this does not apply to
Most people do not need NBN testing, and there is no reason to seek it out on its own. If your oncologist mentioned NBN from a tumour test, that is a separate question, covered under targeted therapy. A tumour result does not tell you whether the fault is inherited.
If your parents are related by blood, tell your counsellor. It matters for the chance of a child inheriting two faults.Commonly believed
What do families get wrong about NBN?
"An NBN fault is as serious as BRCA."
For most NBN faults, the evidence points to a much smaller effect, if any. Carriers are usually managed on their family history. Treating it like BRCA can lead to worry and decisions nobody needed.
"As a carrier, I must avoid all X-rays and scans."
There is no firm evidence that carriers of one faulty copy need to avoid routine imaging. Children with two faulty copies are a different case and need great care with radiation.
"My child will have the syndrome because I carry NBN."
A child needs a faulty copy from both parents to have the syndrome. If your partner does not carry an NBN fault, your children can at most be carriers like you.
"Nobody in our family had cancer, so the result must be wrong."
Carriers often have quiet family histories, because the effect of one faulty copy is small. The result can be correct and still change very little for you.
Questions we are asked
Common questions about the NBN gene
Is NBN a cancer gene?
It is a DNA repair gene. Working normally, it protects you from cancer by helping mend broken DNA. It becomes linked to cancer only when it stops working. The serious risk is in children with two faulty copies. For adults with one faulty copy, any effect on risk is usually modest.
Which cancers are linked to one NBN fault?
Breast cancer is the one studied most, and the clearest link is with a single Eastern European fault. Prostate cancer has also been suggested. For many other NBN faults, the evidence is too thin to say. Your counsellor will tell you which group your result belongs to.
How is NBN related to the ATM gene?
They work in the same repair pathway. The NBN team spots a broken strand and switches on ATM, which then pauses the cell for repair. Faults in either gene can cause childhood conditions with similar features, including sensitivity to radiation and immune problems.
Can I pass an NBN fault to my children?
Yes. Each child has a one in two chance of inheriting your faulty copy and becoming a carrier. A child would have the syndrome only if they also inherited a faulty copy from the other parent. Testing your partner tells you whether that is possible.
Does marrying within the family matter?
It can. Related partners are more likely to carry the same rare fault from a shared ancestor, which raises the chance of a child inheriting two. Marriages between relatives are common in many Telangana families. It is worth mentioning plainly to your counsellor.
Can I have radiotherapy if I carry NBN?
Tell your oncologist about the result before radiotherapy is planned. For adults with one faulty copy there is no clear evidence that radiotherapy is unsafe, but the team will want to know. The concern is much greater for children with two faulty copies.
Can the gene fault be fixed?
No. An inherited fault is present in every cell and cannot be corrected or reversed with current medicine. What can change is the screening you are offered and the information your relatives receive. For most carriers, that is where the value of the result lies.
Where do I start if my report mentions NBN?
Check whether it says one faulty copy or two, and whether it is called pathogenic or uncertain. Take the report to a genetic counsellor or your oncologist with a list of relatives who had cancer. Call the CION helpline if you are unsure who to see.
Your Specialists
Meet CION's oncologists. Bring your family history or genetic report to them.
Our medical oncologists see people with a strong family history of cancer, arrange genetic counselling and testing where it fits, and plan the checks that follow.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
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Sources
- MedlinePlus Genetics — NBN gene
- GeneReviews (NCBI) — Nijmegen Breakage Syndrome
- MedlinePlus Genetics — Nijmegen breakage syndrome
- National Cancer Institute — Genetics of Breast and Gynecologic Cancers (PDQ) – Health Professional Version
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
NBN mutation: which cancers and what riskWhat is known, and not known, about risk for carriersBiallelic NBN and Nijmegen breakage syndromeThe childhood condition caused by two faulty copiesNBN positive: what happens nextThe practical steps after a resultThe ATM gene: what it doesThe partner gene that NBN switches on
Talk to us
Not sure what an NBN result means for you?
Bring your report and a list of relatives who had cancer. A counsellor will explain what the result changes and what it does not. One helpline serves every CION centre.