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Two faulty ATM copies: understanding ataxia-telangiectasia | CION Cancer Clinics
Biallelic ATM means a child has inherited a faulty ATM gene from both parents. This causes ataxia-telangiectasia, a rare condition that affects balance, immunity and the body's ability to repair DNA, and raises the risk of childhood blood cancers. It is very different from carrying one faulty copy. This page explains the condition, how it is inherited, and why it matters where relatives marry. 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 biallelic ATM mean?
- What does ATM mean for each person in the family?
- How can two healthy parents have a child with A-T?
- The words you will meet, in plain language
- How does one faulty ATM copy differ from two?
- Four things families tell us, and what is actually true
- What this page cannot tell you
- Common questions about biallelic ATM and A-T
The short answer
What does biallelic ATM mean?
Biallelic means both copies. Everyone has two copies of the ATM gene, one from each parent. When both copies are faulty, the body has no working ATM at all, and the result is a condition called ataxia-telangiectasia, often shortened to A-T.
How A-T shows itself
Ataxia means unsteady movement. Most children with A-T begin to wobble soon after they start walking, and the unsteadiness slowly worsens through childhood. Telangiectasia means small, widened blood vessels, often seen as red threads on the whites of the eyes. Many children also have frequent chest and sinus infections, because their immune system is weaker.
Why cancer comes into it
ATM helps cells notice broken DNA and repair it. Without any working copy, damage builds up faster. Children and young adults with A-T have a much higher risk of leukaemia and lymphoma, which are cancers of the blood and lymph glands, than other children. They are also very sensitive to radiation.
A-T is rare. Being told you carry one faulty ATM copy does not mean you or your child have A-T.One gene, four positions
What does ATM mean for each person in the family?
The same gene means different things depending on how many faulty copies someone has.
The child with A-T
Two faulty copies. The child lives with the condition itself, needs care from a team that knows A-T, and must be protected from unnecessary radiation.
Often looks like
- Unsteady walking from the toddler years
- Red threads on the whites of the eyes
- Frequent chest and sinus infections
Each parent
One faulty copy each. Parents are carriers. They do not have A-T and will not develop it. Carrier mothers have a moderately raised breast cancer risk of their own, which is worth a separate conversation.
Brothers and sisters
Each may have A-T, carry one copy, or carry none. A sibling who is walking steadily beyond the toddler years is very unlikely to have A-T, but may still be a carrier.
Wider relatives
Aunts, uncles and cousins on either side may carry one faulty copy. This matters most where families marry within the family or community, because two carriers are more likely to meet.
Not sure whether this applies to you?
Ask an oncologistHow it passes down
How can two healthy parents have a child with A-T?
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Each parent carries one faulty copy
Neither has any symptoms, because one working copy is enough. Most carriers never know until a child is diagnosed or a gene panel is done.
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Each pregnancy draws one copy from each parent
The child receives one of the mother's two copies and one of the father's, by chance, every time.
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The chances are the same in every pregnancy
One in four children will have A-T. One in two will be carriers like their parents. One in four will inherit two working copies.
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Chance has no memory
Having one child with A-T does not make the next pregnancy safer or riskier. Each pregnancy starts again at the same odds.
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Options exist for the next pregnancy
Once both parents' exact faults are known, testing in early pregnancy, or testing embryos during IVF, can be discussed with a counsellor before trying again.
On the report
The words you will meet, in plain language
- Biallelic
- Both copies of the gene are faulty, one inherited from each parent.
- Homozygous
- The two faulty copies are exactly the same change. This is more common when the parents are related.
- Compound heterozygous
- The two faulty copies are different changes, one from each parent.
- Carrier
- Someone with one faulty copy and one working copy. Carriers do not have A-T.
- Alpha-fetoprotein
- A blood marker that is usually raised in A-T. It is one of the clues doctors check when A-T is suspected.
- Autosomal recessive
- The pattern where a child needs two faulty copies, one from each parent, to be affected.
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Side by side
How does one faulty ATM copy differ from two?
One faulty copy (carrier)
Two faulty copies (A-T)
Healthy, and usually unaware
Unsteady movement from early childhood
A moderately raised adult cancer risk
A high risk of blood cancers from childhood
Radiotherapy is not ruled out by the result
Radiation is kept to the minimum possible
Adult screening set by the family history
Lifelong care from a specialist team
Commonly believed
Four things families tell us, and what is actually true
"We are cousins, so our child will have A-T."
Being related raises the chance that both of you carry the same rare fault. It does not make it certain. If a fault runs in the family, both partners can be tested before planning a pregnancy, and the answer is often reassuring.
"A-T is caused by something the mother did in pregnancy."
A-T is inherited from both parents equally, and nothing either parent did caused it. Families sometimes blame the mother. The genetics simply do not support that.
"Physiotherapy will make the unsteadiness go away."
Physiotherapy, speech and occupational therapy help children keep strength and independence for longer. A gene fault cannot be corrected or reversed, and the condition does change over time. Honest planning helps families use the good years well.
"Our other children are fine, so they are not carriers."
A child who walks steadily is very unlikely to have A-T, but carriers show no signs at all. Carrier testing is usually left until they are adults and can choose it for themselves.
Being straight with you
What this page cannot tell you
It cannot diagnose A-T. The condition is confirmed by a paediatric neurologist or clinical geneticist, using the child's symptoms, blood tests and a genetic report. If your child is unsteady, that assessment comes first, not a search online. What your specific variant means is a question for the counsellor who ordered the test.
It cannot tell you how your child's condition will progress
A-T varies. Some families have a milder form, called variant A-T, where some ATM still works and symptoms start later. The exact changes on the report help predict which form a child has, but only the team following your child can judge how things are going.
Who this does not apply to
If you are an adult told that you carry one faulty ATM copy, you do not have A-T and will not develop it. Your questions are about adult cancer screening, which our ATM carrier pages cover. Most people who search this term are carriers, not families living with A-T.
If a marriage within the family is being planned and ATM has come up, a counsellor can test both partners and explain the result before any decision is made.Questions we are asked
Common questions about biallelic ATM and A-T
Is A-T the same as being an ATM carrier?
No. A carrier has one faulty copy and is healthy. A-T needs two faulty copies, one from each parent. Carriers have a moderately raised risk of some adult cancers, mainly breast cancer, but they never develop the unsteadiness, eye changes or infections of A-T.
At what age does A-T usually show?
Most children show unsteady walking soon after they start to walk. The red threads in the eyes often appear later in childhood. Milder forms can show much later, even in adult life. Diagnosis is often delayed because early unsteadiness is mistaken for clumsiness.
Can A-T be treated?
No treatment corrects the gene. Care focuses on keeping the child as strong, well and independent as possible, with physiotherapy, help with speech and swallowing, good nutrition, protection from infections and, when needed, antibody replacement. Some medicines are being studied in trials.
Why must X-rays and CT scans be limited?
Children with A-T cannot repair DNA damage properly, and radiation causes exactly that kind of damage. Scans are used only when truly needed, and ultrasound or MRI are preferred where they give the same answer. Radiotherapy for cancer is avoided or given at much lower doses.
If my child with A-T develops cancer, can they have chemotherapy?
Often yes, but doses and drugs are adjusted, because children with A-T are more sensitive to some treatments. This needs a paediatric oncology team that has managed A-T before. Tell every new doctor about the diagnosis before any treatment or scan.
Can we test in the next pregnancy?
Yes, once both parents' exact ATM changes are known. Testing can be done in early pregnancy, or embryos can be tested during IVF. Each choice has its own timing, cost and emotional weight, and a counsellor can walk you through them before you decide.
Do the parents need their own cancer checks?
Yes. Both parents are carriers. Carrier mothers have a moderately raised breast cancer risk, and breast screening often starts earlier than usual. Carrier fathers should ask about pancreatic and prostate risk. A doctor who knows the family history will set each plan.
Where can a family in Telangana get help?
A-T is looked after by paediatric neurologists and geneticists, with oncology involved when needed. Call the CION helpline and describe the situation. Someone will help you find counselling in Telugu, arrange testing for the parents and point you to the right specialist team.
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)
Medical Oncologist
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Sources
- GeneReviews (NCBI) — Ataxia-Telangiectasia
- MedlinePlus Genetics — Ataxia-telangiectasia
- MedlinePlus Genetics — ATM gene
- National Institute of Neurological Disorders and Stroke — Ataxia Telangiectasia
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
The ATM gene: what it does and why it mattersWhat ATM does when it works normallyATM carriers and radiation sensitivityWhy radiation matters so much more in A-T than for carriersTesting the family for ATMCarrier testing for parents, partners and relativesATM mutation: which cancers and what riskThe adult cancer risks that apply to carrier parents
Talk to us
Has ATM come up for your child or your family?
A genetic counsellor can explain the result, test both parents and talk through options for a future pregnancy, in Telugu if you prefer. One helpline serves every CION centre.