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CION Cancer Clinics
A de novo TP53 result when nobody else had cancer | CION Cancer Clinics
Finding an inherited TP53 fault in a family with no history of cancer is more common than people expect. Sometimes the fault arose new in you. Sometimes it came from a parent whose risk never showed, and sometimes the change is not inherited at all. This page explains the possibilities, how the genetics team tells them apart, and what each one means for your children and relatives. 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 a de novo TP53 result mean?
- Why a TP53 fault can appear with no family history
- How does the team tell these explanations apart?
- The words you will meet, in plain language
- What each explanation means for your relatives
- What this page cannot tell you
- Four things families tell us, and what is actually true
- Common questions about a de novo TP53 result
The short answer
What does a de novo TP53 result mean?
De novo means new. A de novo TP53 fault arose for the first time in you, in the egg or sperm you came from or very early in your development. Neither parent carries it. You can still pass it on, so it matters for your children even though it did not come from the family before you.
How often does this happen?
More often than people expect. Among people with an inherited TP53 fault, a meaningful minority have no parent who carries it. That is more common for TP53 than for many other cancer genes. A clear family history is therefore not a reason to doubt the result straight away.
Why the family history looked clear
Sometimes it really is new. Sometimes it came from a parent who stayed well, or from a side of the family that is small, scattered or poorly recorded. Before calling a result de novo, the genetics team has to rule those possibilities out. That takes some tests and some time.
What to tell relatives while you wait
It is fine to tell close family that a result has come back and that more tests are needed before anyone else acts on it. Brothers, sisters and cousins should not rush to private testing on their own. Once the source is known, the counsellor can advise exactly who should be tested, and for which fault.
A result is only called de novo once both parents have been tested and neither carries the fault.Four possibilities
Why a TP53 fault can appear with no family history
Each explanation leads to different advice for you and your relatives, which is why the team works to find out which one fits.
It is new in you
The fault arose in the egg or sperm before you were conceived. It is in every cell of your body. Your parents and siblings are very unlikely to carry it, but each of your children has a one in two chance of inheriting it.
A parent carries it and stayed well
Not every carrier develops cancer, and some variants carry a lower risk than others. A parent may carry the fault without knowing it. Relatives who died young of other causes can also hide a pattern.
It is in only some of your cells
If the change arose a little after conception, only some cells carry it. This is called mosaicism. The laboratory often suspects it when the change shows more weakly than expected.
It can affect
- How high your own risk is
- Whether it reaches your children
It is in your blood cells only
In older adults, and after chemotherapy, a TP53 change can build up in blood cells alone. It is not inherited and not in the rest of the body. Doctors call this clonal haematopoiesis.
Not sure whether this applies to you?
Ask an oncologistWorking it out
How does the team tell these explanations apart?
Looking at how strongly the change shows
An inherited fault usually shows in about half of the gene copies read. A much weaker signal hints at mosaicism or a blood-only change.
Testing a second tissue
A small skin sample, or sometimes hair roots, is tested. If the fault is there too, it is not confined to the blood.
Testing both parents
Each parent gives a blood sample, tested only for your exact fault. If neither carries it, the result is confirmed as de novo.
Revisiting the family history
The counsellor asks again about relatives who died young, lived abroad or had an illness nobody named. Details often surface on the second telling.
On your report
The words you will meet, in plain language
- De novo
- New in this person. Not inherited from either parent, but it can be passed to children.
- Germline
- Present in every cell from birth, including eggs or sperm, and therefore inheritable.
- Mosaicism
- A change present in some cells of the body but not others, because it arose after conception.
- Variant allele fraction
- The share of the gene copies read by the laboratory that carry the change. It helps separate an inherited fault from a mosaic one.
- Clonal haematopoiesis
- A change that builds up in blood-forming cells with age or after treatment. It is not inherited.
- Gonadal mosaicism
- A parent who carries the change only in their eggs or sperm. Their own blood test is normal, yet more than one child can inherit it.
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Side by side
What each explanation means for your relatives
If the result turns out to be
What it usually means for the family
New in you, in every cell
Each child has a one in two chance; parents are clear
Inherited from a parent who stayed well
That parent's side of the family is offered testing
Mosaic in you
Children's chance is lower and harder to predict
In blood cells only
Not inherited; relatives need no TP53 test for it
Confirmed de novo, but a sibling is worried
A small chance remains; siblings are often offered a test
Being straight with you
What this page cannot tell you
It cannot tell you which explanation fits your result. That depends on the laboratory data, a second tissue test and your parents' results. What your specific variant means is a question for the counsellor who ordered the test.
Parental testing can raise private questions
Testing both parents occasionally reveals something unexpected about family relationships. Counsellors discuss this before any sample is taken, and results are shared with care and privacy. If a parent has died or cannot be tested, the team works with what is available.
Who this does not apply to
A TP53 change found only inside a tumour is very common and is not an inherited result at all. That belongs with your oncologist and the tumour testing pages. If a relative has already been confirmed to carry the same fault, your result is inherited, and this page does not apply.
Commonly believed
Four things families tell us, and what is actually true
"Nobody in our family had cancer, so the lab made a mistake."
Laboratory errors are rare, and de novo TP53 faults are well known. The right response is further testing to understand the result.
"It is new in me, so my children are safe."
A new fault behaves like any inherited one from you onwards. If it is in every cell, each child has a one in two chance of carrying it.
"We need to find out which parent is to blame."
Nobody chooses a gene fault, and in de novo cases neither parent passed it on. Parents are tested to guide the family's care, never to assign fault.
"De novo means a lower cancer risk for me."
A fault in every cell carries the same risk whether it is new or inherited. Only a mosaic change may carry a lower one, and your screening plan is set with that in mind.
Questions we are asked
Common questions about a de novo TP53 result
Do both my parents need to be tested?
Yes, if they are able and willing. Testing only one parent leaves the question open. Each parent is tested for your exact fault, which is simpler than your original test and usually quicker.
What if one of my parents has died?
A stored tissue sample from an old surgery or biopsy can sometimes be tested. If nothing is available, the team uses the other parent's result and the family history. The result may then stay uncertain, and relatives are advised accordingly.
Could my brother or sister carry it too?
If the fault is truly new in you, it is unlikely. A parent can rarely carry it only in their eggs or sperm, so a small chance remains. Many teams offer siblings a test for peace of mind.
Does a de novo result change my screening?
Usually not. If the fault is in every cell, you are offered the same lifelong checks as any carrier, including whole-body MRI. A confirmed mosaic result may lead the team to adjust the plan.
Will my children inherit it?
Each child has a one in two chance if the fault is in every cell. If it is mosaic, the chance is lower and harder to predict. Your counsellor can discuss testing children and options for future pregnancies.
Can clonal haematopoiesis be mistaken for an inherited fault?
Yes, especially in older adults or after chemotherapy. That is why a weak TP53 signal on a blood test is checked with a second tissue before anyone is told they carry an inherited fault.
How long does it take to get a clear answer?
It depends on how quickly parents can give samples and whether a skin test is needed. Screening does not wait for the answer. Your team will start the checks you need while the family testing is sorted out.
Where do I start?
Ask the counsellor who gave you the result about parental testing and whether a second tissue test is needed. If you have no counsellor yet, the CION helpline can arrange a genetic counselling appointment.
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) — Li-Fraumeni Syndrome
- MedlinePlus Genetics — TP53 gene
- MedlinePlus Genetics — What is mosaicism?
- NCCN — Genetic/Familial High-Risk Assessment: Breast, Ovarian, Pancreatic, and Prostate
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
Testing the family for TP53How parents, siblings and children are offered testingGermline TP53 vs a TP53 change in the tumourMake sure your result is the inherited kindTP53 and family planningOptions for future pregnancies once a fault is foundChompret criteria and when TP53 testing is offeredWhy some carriers are found outside the usual checklist
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Puzzled by a TP53 result nobody expected?
We can arrange genetic counselling to work out where the result came from and what it means for each relative. One helpline serves every CION centre.