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CION Cancer Clinics
De novo mutations: a gene fault with no family history | CION Cancer Clinics
A de novo fault is a gene change that starts with one person, not passed down from either parent. It happens by chance during the formation of an egg or sperm cell, or very early in development, and it is nobody's fault. This page explains how it is confirmed and what it means for the rest of the family. At CION Cancer Clinics in Hyderabad, our oncologists review your family history with you and guide you to the right genetic counselling and testing.
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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 a de novo mutation, and where did it come from?
- The moments when a de novo fault can arise
- What happens once a de novo fault is suspected
- The words a de novo report uses, in plain language
- What a de novo finding usually means for the family
- What this page cannot tell you
- Four things families assume about de novo faults, and what is true
- Common questions about de novo mutations
The short answer
What is a de novo mutation, and where did it come from?
A de novo fault is a gene change that appears for the first time in one person, not inherited from either parent. It happens in the egg, the sperm, or very early after conception, and it is nobody's fault. Neither parent carries it, and neither parent did anything to cause it.
Why "no family history" does not mean "not inherited"
Many families assume a genetic result must be wrong if nobody else has been affected. A de novo fault is exactly the situation where that assumption fails. The fault is real, present from birth, and can still be passed on to that person's own children.
Why this comes as such a shock to parents
A parent who is told their child's cancer gene fault is de novo often first hears it as blame, or wonders what they missed. Neither reaction fits the biology. The change happened by chance in the reproductive cells or very early cell divisions, well before anyone could have known or prevented it.
A de novo result usually means parents do not need to be tested themselves, though a counsellor will usually confirm this directly.Where the change actually happens
The moments when a de novo fault can arise
All of these are ordinary, unavoidable parts of how cells copy their genetic material.
In an egg or sperm cell
A copying error occurs while the egg or sperm cell is forming, before conception. The resulting embryo carries the fault in every cell.
Just after conception
A very early cell division after the egg and sperm join can introduce an error that then gets copied into most or all of the developing embryo's cells.
Confirmed by testing both parents
A de novo finding is only confirmed once both biological parents are tested for that exact change and neither carries it. Without that step it remains a possibility, not a certainty.
Why some cases stay unconfirmed
Sometimes one parent is unavailable or unwilling to be tested. In that situation the fault is treated cautiously, as possibly rather than confirmed de novo.
Not sure whether this applies to you?
Ask an oncologistAfter the finding
What happens once a de novo fault is suspected
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The lab flags the fault in the child or patient
Testing finds the change in the person who has, or is at risk of, the cancer in question.
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Both parents are offered testing for that exact change
This is a targeted test for one specific change, far simpler and cheaper than the original broad test.
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Neither parent carries it
This confirms the fault most likely arose new, either in a reproductive cell or very early in development.
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Wider relatives are usually reassured, not tested
Because the fault began with this one person, siblings, aunts and uncles are not at raised risk from it, unless there is a separate, unrelated reason to test them.
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The person's own future children remain a separate question
The fault is now permanently part of that person's genetic makeup and can be passed on by them in the ordinary way, even though it began with them.
On your report
The words a de novo report uses, in plain language
- De novo
- Latin for "from new". A gene fault that arose in this person, not inherited from either parent.
- Germline
- Present in every cell from birth, and therefore able to be passed on. A de novo fault is still germline, despite being new.
- Trio testing
- Testing the patient and both biological parents together, the usual way a de novo finding is confirmed.
- Parental confirmation
- The specific, targeted test of each parent for the exact change found in the patient.
- Gonadal mosaicism
- A rare situation where a parent carries the fault only in some reproductive cells and tests negative in blood, which can affect risk to a future sibling.
- Recurrence risk
- The chance a future child of the same parents would also be affected, which is usually low but not zero after a de novo finding.
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Side by side
What a de novo finding usually means for the family
Usually true
Usually not true
Parents are not carriers and do not need routine surveillance for it
A parent secretly carries the fault and was never tested
Siblings are not at raised risk from this fault
Siblings should be tested as a precaution regardless
The affected person can pass it to their own children
The fault disappears because it was not inherited
The fault is confirmed only after both parents test negative
A lab can call it de novo without testing the parents
Being straight with you
What this page cannot tell you
It cannot tell you whether your family's specific finding is truly de novo without both parents actually being tested. Assuming it is de novo before that testing is complete can miss a rare situation, such as one parent carrying the fault only in some cells.
It cannot settle guilt that has no basis
Nothing a parent ate, did, or was exposed to caused a de novo fault in the vast majority of cases. If guilt is sitting heavily with a family, a genetic counsellor can go through the biology in detail, which usually helps more than reassurance alone.
Who this does not apply to
Most inherited cancer gene faults are not de novo. If your family has a pattern of cancer across generations, a de novo explanation is unlikely and testing usually points instead to a fault passed down the ordinary way.
If you have been told a result is "probably" de novo without both parents being tested, ask whether that testing can still be arranged.Commonly believed
Four things families assume about de novo faults, and what is true
"If it's de novo, one of us must have hidden something from the doctor."
A de novo fault means neither parent carries the change at all, in any cell that was tested. There is nothing to have hidden, because there was nothing there to find beforehand.
"My other children must have it too, since it's already in the family now."
A de novo fault began with one person. It is not automatically present in siblings, who can still be reassured, usually without needing to be tested themselves.
"There's no point testing us as parents if there's no family history."
Parental testing is exactly what confirms whether a fault is truly de novo. Skipping it leaves open rarer possibilities, including a parent carrying the fault only in reproductive cells.
"A de novo fault is somehow less serious than an inherited one."
The fault itself behaves the same way in the body whether it was inherited or arose new. What differs is only where it came from, not how it affects risk once it is present.
Questions we are asked
Common questions about de novo mutations
Did we do something to cause our child's de novo fault?
No. A de novo fault arises from an ordinary copying error during the formation of an egg or sperm cell, or very early in development. It is not linked to anything a parent did, ate or was exposed to.
Do our other children need to be tested?
Usually not, once a fault is confirmed de novo in one child. Their risk is not raised by that fault, though your counsellor may still discuss testing in certain family situations.
Could I still pass this fault to a future child?
The recurrence risk to a future child of the same parents is usually low but not zero, because of a rare possibility called gonadal mosaicism. Your counsellor can explain whether this applies in your case.
Will my child pass this fault to their own children one day?
Yes, in the ordinary way, once the fault exists in the person's own body. Each child of theirs would have the usual chance of inheriting it, exactly as with any inherited fault.
How is a de novo finding actually confirmed?
Both biological parents are tested for the exact change found in the patient. If neither carries it, the fault is confirmed as most likely de novo.
What if one parent refuses to be tested?
The fault is then treated as possibly, rather than confirmed, de novo. This mainly affects how confidently other relatives can be reassured, and a counsellor can discuss the practical impact.
Is a de novo fault rarer than an inherited one?
For most cancer genes, yes, inherited faults are more common than de novo ones. Some specific genes and conditions have a higher share of de novo cases, which a counsellor can explain for the gene involved in your family.
Who can explain our specific result properly?
A genetic counsellor or clinical geneticist can walk through the testing already done and what, if anything, is still needed. Call the CION helpline if you are not sure who to approach, 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
- MedlinePlus Genetics — What does it mean if a disorder seems to run in my family?
- National Cancer Institute — Genetic Testing for Inherited Cancer Susceptibility Syndromes
- GeneReviews (NCBI) — GeneReviews: An overview of hereditary cancer syndromes
- Cancer Research UK — Inherited cancer genes and increased cancer risk
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
Mosaicism explainedA related situation where a fault is not present in every cellCan a cancer gene skip a generation?Another pattern families mistake for a de novo faultDominant and recessive inheritance explainedHow a fault behaves once it does exist in a familyFamily planning with a cancer geneThe next question once a fault is confirmed, de novo or not
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Told your result has no family history behind it?
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