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Testing the family for the xeroderma pigmentosum genes | CION Cancer Clinics
After a child is diagnosed with xeroderma pigmentosum, brothers and sisters should be tested first, even if they look well, because protection started early changes the most. Adult relatives can then be offered carrier testing when they plan a family. This page explains who to test, in what order, what each result means, and why marriages within the family matter here. 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
- Who in the family should be tested after an XP diagnosis?
- What does testing mean for each relative?
- How does family testing work, step by step?
- The testing terms, in plain language
- What a brother's or sister's result means
- What this page cannot tell you
- Four things families tell us, and what is actually true
- Common questions about testing the family for XP
The short answer
Who in the family should be tested after an XP diagnosis?
Brothers and sisters of the affected child come first, even if they look perfectly well. After that, testing is offered to adult relatives who are planning to have children, especially where marriages within the family are common.
Why brothers and sisters come first
XP can be mild in early childhood, and some forms show little more than freckling for a while. A brother or sister may have the condition without anyone noticing. Finding them now means protection starts years earlier than it otherwise would, which is the single biggest thing that changes how XP unfolds.
Why testing wider relatives is about the next generation
XP only appears when a child inherits the same faulty gene from both parents. Aunts, uncles and cousins are not at risk of XP themselves, but some will be carriers. That matters when they choose a partner, and matters most when the partner is also a relative who may carry the same fault.
Testing relatives works best once the affected child's two faults are known exactly. Start there.Who, and why
What does testing mean for each relative?
The same test answers a different question for each person, so the reason for testing shapes the timing.
Brothers and sisters
When both parents are carriers, each child has a one-in-four chance of XP, a one-in-two chance of being a healthy carrier, and a one-in-four chance of neither. Only testing can tell which.
Test soon if a sibling
- Freckles early on sun-exposed skin
- Burns or blisters easily
- Finds light hurts their eyes
Parents
Both parents are almost always carriers. Testing them confirms which fault came from which side, which tells the family where else to look. It rarely changes anything for the parents' own health.
Adult relatives
Aunts, uncles and cousins on each parent's side may be carriers. Carrier testing is offered when they are planning a family, and it matters most if they plan to marry within the family.
Partners of carriers
A carrier's children can only have XP if the partner carries a fault in the same gene. Testing the partner answers that. If the partner is clear, XP in their children is extremely unlikely.
Not sure whether this applies to you?
Ask an oncologistOne circle at a time
How does family testing work, step by step?
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The affected child's two faults are confirmed
The laboratory names the gene and the exact change on each copy. Everything after this step tests relatives for those same changes, not for every XP gene again.
-
The family receives a letter
The genetics team writes a family letter naming the gene and the faults. Relatives take it to their own doctor or laboratory, anywhere in India or abroad.
-
Brothers and sisters are tested
A blood or saliva sample is checked for the two known faults. This is quicker and usually cheaper than the original test, and the answer is clear.
-
Parents are tested to confirm each side
Knowing which parent carries which fault shows which side of the family to offer carrier testing to next.
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Wider relatives are offered carrier testing
Adults on the carrier side decide for themselves whether to be tested, usually when marriage or children are being planned.
Words you will hear
The testing terms, in plain language
- Cascade testing
- Testing relatives one circle at a time, starting with the closest, for a fault already found in the family.
- Targeted test
- A test that looks only for the family's known faults rather than reading whole genes. It is faster and gives a clearer answer.
- Carrier
- Someone with one faulty copy and one working copy. A carrier is healthy and does not have XP.
- Obligate carrier
- A relative who must be a carrier because of how the family is connected, such as both parents of a child with XP.
- Consanguinity
- A marriage between two people related by blood, such as cousins or an uncle and niece. It raises the chance both carry the same fault.
- Prenatal diagnosis
- Testing a pregnancy for the family's known faults, offered to couples who are both carriers.
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Side by side
What a brother's or sister's result means
The result
What it means for them
Both family faults found
They have XP, and protection and checks start now
One family fault found
A healthy carrier, relevant only for their own children later
Neither fault found
Not affected and not a carrier of this fault
Result unclear or sample failed
Repeat the test, and protect them as if affected meanwhile
Being straight with you
What this page cannot tell you
It cannot tell you which of your relatives carry the fault, or how to handle each conversation. Every family is different, and a counsellor who has drawn your family tree can plan who to approach and in what order. What your specific variant means is a question for the counsellor who ordered the test.
It cannot settle the question of testing young children
Testing a brother or sister for XP itself is usually urgent. Testing a young child only to find out whether they are a carrier usually waits until they are old enough to decide. A sibling test for XP often reveals carrier status too, so talk through with your counsellor what you want to be told.
Who this does not apply to
Relatives of someone with an ordinary skin cancer in later life do not need XP testing. Neither do relatives of a carrier who has no affected child in the family, unless they are planning children with a partner who may also carry it. Most people in the wider family will never need this test.
Relatives can refuse testing. That is their right, and it does not stop anyone else in the family from being tested.Commonly believed
Four things families tell us, and what is actually true
"Our other child looks healthy, so there is no need to test."
Some forms of XP show very little in early childhood. A sibling can look well for years while sun damage builds up. A simple test settles it, and an early answer changes the most.
"One child has XP, so the next one will be fine."
Chance has no memory. Every pregnancy between two carriers carries the same one-in-four chance, however many children came before.
"It comes from the mother's side."
XP needs a faulty copy from both parents. Neither side is to blame, and both sides of the family may include carriers.
"A carrier should not marry."
Carriers are healthy and can marry freely. The only question is whether their partner carries a fault in the same gene. If not, their children cannot be expected to have XP.
Questions we are asked
Common questions about testing the family for XP
How is a brother or sister tested?
With a blood or saliva sample, checked only for the faults already found in the affected child. The laboratory needs the family letter or the original report to do this. The result is usually simpler to read than the first one, because the laboratory knows exactly what it is looking for.
Should a new baby in the family be tested?
If both parents are carriers, yes. Many families protect a newborn from daylight as if affected until the result is back. Testing during the pregnancy is also possible once the family's faults are known. A counsellor can explain each option.
Should cousins be tested?
Cousins are not at risk of XP themselves but may be carriers. Carrier testing is worth offering when they plan to marry, and especially if they plan to marry another relative. Adults decide for themselves, and the family letter lets them test anywhere.
Does a carrier need to disclose it before marriage?
There is no legal duty to disclose. Many families choose to share it, because a partner's test answers the only question that matters. How and when to raise it is a fair topic to talk through with a counsellor.
Is testing a relative expensive?
A targeted test for known family faults is usually cheaper than the original panel. Prices vary between laboratories, and some offer reduced rates for relatives. Ask for a written estimate before the sample is taken.
What if a relative does not want to be tested?
That is their decision. You can give them the family letter and let them decide in their own time. Testing is never forced, and a refusal does not affect anyone else's right to be tested.
Can relatives abroad or in another state be tested?
Yes. The family letter names the gene and the exact faults, so any accredited laboratory can run the targeted test. They can also arrange genetic counselling locally or online.
Where do we start?
Ask the team that diagnosed your child for the family letter and a referral for sibling testing. Write down who is in the family on both sides before the appointment. Call the CION helpline if you are unsure who to approach.
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) — Xeroderma Pigmentosum
- MedlinePlus Genetics — Xeroderma pigmentosum
- MedlinePlus Genetics — What are the different ways a genetic condition can be inherited?
- National Cancer Institute — Genetic Testing for Inherited Cancer Susceptibility Syndromes
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
XP positive: what happens nextThe care plan for a sibling who tests positiveConsanguineous families and cascade testingWhy marriages within the family change the carrier pictureHow cascade testing worksTesting relatives one circle at a timeWhich syndromes do require childhood testingWhy XP is tested in children when most genes wait
Talk to us
Not sure which relatives should be tested?
Tell us who in the family has been diagnosed and what the report says. We will help you reach a genetics team who can plan testing for the rest of the family. One helpline serves every CION centre.