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Constitutional mismatch repair deficiency in children | CION Cancer Clinics
Constitutional mismatch repair deficiency, or CMMRD, is a rare condition where a child inherits a faulty DNA repair gene from both parents. It causes cancers in childhood, most often in the brain, blood and bowel. This page explains how it differs from Lynch syndrome, the signs doctors look for, what testing and surveillance involve, and what it means for the rest of the family. At CION Cancer Clinics, our oncologists plan screening and care for families with an inherited cancer syndrome, explained in plain words.
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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 is constitutional mismatch repair deficiency?
- Which signs make a doctor think of CMMRD?
- What happens once CMMRD is suspected
- The words you will meet, in plain language
- How CMMRD compares with Lynch syndrome
- What this page cannot tell you
- Four things families tell us, and what is actually true
- Common questions about CMMRD in children
The short answer
What is constitutional mismatch repair deficiency?
Constitutional mismatch repair deficiency, usually shortened to CMMRD, is a rare inherited condition in which a child is born with two faulty copies of a DNA repair gene instead of one. It causes cancers in childhood, most often in the brain, the blood and the bowel. It is related to Lynch syndrome but is a much more serious condition.
How it differs from Lynch syndrome
In Lynch syndrome, a person inherits one faulty copy and one working copy of a repair gene. The working copy protects them through childhood, and cancer risk mainly rises in adult life. In CMMRD, the child inherits a faulty copy from each parent. There is no working copy anywhere in the body, so the cell's proofreading system fails from birth.
Why the parents are often surprised
Each parent usually carries one faulty copy, which means each has Lynch syndrome. Many never knew, because the genes most often involved in CMMRD carry lower adult risks and the family may have little cancer history. The child's diagnosis is often how the parents learn about their own.
A CMMRD diagnosis in a child is also a Lynch diagnosis for both parents.What doctors look for
Which signs make a doctor think of CMMRD?
No single sign proves it. Doctors think of it when several of these appear together in one child or one family.
Cancers in childhood
Children with CMMRD can develop more than one cancer, sometimes years apart. A second cancer in a child who has already been treated is itself a strong reason to ask about CMMRD.
The most common
- Fast-growing brain tumours
- Lymphoma or leukaemia
- Bowel polyps or bowel cancer in the teenage years
Marks on the skin
Flat, light-brown patches called café-au-lait spots are common. They can look like the spots seen in neurofibromatosis, and some children are first labelled with that condition instead. Some children also have paler patches of skin. The spots cause no harm and need no treatment.
Clues in the family
Parents who are related by blood, a brother or sister with a childhood cancer, or Lynch-type cancers on both sides of the family.
A tumour result showing an extremely high number of mutations can also point towards it.Who this does not apply to
A child with one Lynch parent and no childhood cancer. That child can inherit at most one faulty copy, which is Lynch syndrome, and testing usually waits until adulthood. Most children with a single brain tumour or leukaemia do not have CMMRD either.
Not sure whether this applies to you?
Ask an oncologistFrom suspicion to a plan
What happens once CMMRD is suspected
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The tumour gives the first clue
Tests on a child's tumour may show the repair proteins are missing, or an unusually large number of mutations. Tumour testing itself is covered in our targeted therapy section.
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A blood test looks for two faulty copies
The child's blood is tested for faults in the repair genes. One of these genes has a close look-alike in the DNA, so the laboratory must be experienced. Specialist tests on blood cells help when the gene result is unclear.
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Both parents are tested
This confirms that each parent carries one of the child's two faults. Each parent then needs their own adult Lynch surveillance plan.
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Brothers and sisters are tested early
Unlike Lynch syndrome, CMMRD screening begins in early childhood, so siblings are tested young. The result decides whether they need the full programme.
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Surveillance begins and continues
International programmes combine regular brain MRI, whole-body MRI, blood tests and bowel and stomach examinations. The paediatric oncology team sets the schedule.
On your child's report
The words you will meet, in plain language
- Constitutional
- Present in every cell of the body from birth, rather than only inside a tumour.
- Biallelic
- Both copies of a gene are faulty, one inherited from each parent. This is what makes CMMRD different from Lynch syndrome.
- Mismatch repair
- The cell's proofreading system for copying mistakes. The genes involved are MLH1, MSH2, MSH6 and PMS2.
- Hypermutated tumour
- A tumour carrying far more mutations than usual, because nothing is correcting the mistakes as cells divide.
- Café-au-lait spots
- Flat, coffee-coloured patches on the skin. Harmless in themselves, but a useful clue when there are several.
- Consanguinity
- Parents who are related by blood, such as cousins. It raises the chance that both carry the same faulty gene.
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Side by side
How CMMRD compares with Lynch syndrome
Lynch syndrome
CMMRD
One faulty copy, from one parent
Two faulty copies, one from each parent
Cancer risk rises mainly in adult life
Cancers often begin in childhood
Children are usually tested as adults
Brothers and sisters are tested as children
Screening starts in adulthood
Screening starts at diagnosis, often in early childhood
Being straight with you
What this page cannot tell you
It cannot tell you whether your child has CMMRD. That needs a paediatric oncologist, a clinical geneticist and a laboratory used to testing these genes. What your child's specific variant means is a question for the counsellor who ordered the test.
What the evidence does and does not show
CMMRD is rare, so everything known comes from small international registries. Early results suggest children in a surveillance programme have cancers found earlier. Some tumours in children with CMMRD have responded to immunotherapy, but studies so far are small and this is not right for every child.
On related parents
Marriage within the family is common in some communities in Telangana and across South India. It is not a cause for blame. It simply raises the chance that both parents carry the same fault. If your family is planning another child, a counsellor can explain your options in Telugu, including testing during pregnancy.
If you are unsure whether your child's history fits, call the helpline. Someone will tell you honestly whether a referral is worth making.Commonly believed
Four things families tell us, and what is actually true
"Nobody in our family had cancer, so it cannot be inherited."
The genes most often behind CMMRD carry low adult risks, so parents and grandparents may never have been ill. A quiet family history is common in CMMRD.
"The spots mean it is neurofibromatosis, so we already have the answer."
Café-au-lait spots appear in both conditions. If a child with spots develops a brain, blood or bowel cancer, doctors should ask whether CMMRD has been checked.
"Our other children look healthy, so testing can wait."
In CMMRD, cancers can start before any sign appears. Testing brothers and sisters early lets those who carry both faults start surveillance straight away.
"This happened because of something we did in pregnancy."
It did not. CMMRD is decided at conception by the genes each parent passes on. No food, medicine or event in pregnancy causes it.
Questions we are asked
Common questions about CMMRD in children
If one child has CMMRD, what are the chances for the next?
When both parents carry one faulty copy, each pregnancy has a one in four chance of CMMRD. There is a one in two chance the child carries one faulty copy, meaning Lynch syndrome, and a one in four chance of neither. A counsellor will explain what this means for your family.
Is CMMRD the same as childhood Lynch syndrome?
No. A child who inherits one faulty copy has Lynch syndrome and is usually well through childhood. CMMRD means two faulty copies, which is why cancers begin so much earlier and why the surveillance is so much more intensive.
What does surveillance involve for a child?
Usually regular brain MRI, whole-body MRI, blood tests and bowel and stomach examinations under sedation as the child grows. Young children may need sedation for the MRI too. The team will set a schedule and adjust it with age.
Do both parents need their own check-ups?
Yes. Each parent carries Lynch syndrome and needs adult surveillance, usually starting with colonoscopy. The mother may also need a plan for womb and ovarian risk. Their own brothers and sisters can then be offered testing.
Can the diagnosis change my child's cancer treatment?
It can. Tumours in CMMRD carry many mutations, and some have responded to immunotherapy. The oncology team will consider the diagnosis when planning treatment and will explain the options and their limits.
Can we avoid passing it to a future child?
Options include testing during pregnancy and IVF with testing of embryos before transfer. Both raise personal, practical and cost questions. A counsellor can walk you through each one before you decide.
Where in India can the genes be tested?
Several laboratories test the repair genes, but PMS2 is technically difficult and needs an experienced lab. Ask your counsellor which laboratory they trust for this gene before sending a sample.
Who should we see first?
If your child is already under a paediatric oncologist, start there and ask about a genetics referral. If you are unsure where to begin, call the CION helpline and someone will point you to the right clinic.
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
- GeneReviews (NCBI) — Lynch Syndrome
- MedlinePlus Genetics — Constitutional mismatch repair deficiency syndrome
- National Cancer Institute — Genetics of Colorectal Cancer (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
Lynch syndrome: the complete pictureWhat each parent's own Lynch diagnosis means for themLiving with Lynch syndrome in a large familyOrganising testing once two sides of the family are involvedConsanguinity and cancer riskWhy related parents raise the chance of a shared faultWhich syndromes require childhood testingWhy CMMRD is one of the exceptions to waiting until adulthood
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Worried your child's history might fit?
Tell us about your child's diagnosis and the cancers on both sides of the family. We will help you reach the right paediatric oncology and genetics team, with counselling in Telugu if you prefer. One helpline serves every CION centre.