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Why NUDT15 matters especially for South Asian patients | CION Cancer Clinics
Faulty versions of the NUDT15 gene are much more common in South and East Asian people than in Europeans. NUDT15 controls how safely the body handles thiopurine tablets such as mercaptopurine and azathioprine. Older safety testing looked only at TPMT, so Indian patients with a normal TPMT result can still react badly. This page explains why, who it affects, and why both genes should be tested. At CION Cancer Clinics, our team helps carriers and their families plan checks, next steps and support after a genetic result.
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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
- Why does NUDT15 matter more for Indian patients?
- Who in India is affected by a NUDT15 result?
- Why was NUDT15 missed for so long?
- The NUDT15 words you may see, in plain language
- What changes when NUDT15 is added to the test?
- What this page cannot tell you
- Four things families assume about NUDT15
- Common questions about NUDT15 in South Asian patients
The short answer
Why does NUDT15 matter more for Indian patients?
Faulty versions of the NUDT15 gene are far more common in people of South Asian and East Asian ancestry than in people of European ancestry. Early safety testing for thiopurine tablets was built around a different gene, TPMT, because that was the gene that mattered most in the patients first studied. So a patient in Hyderabad can have a perfectly normal TPMT result and still be at real risk.
How the gap opened up
For decades, thiopurine safety meant checking TPMT. Most of that research came from Europe and North America, where faulty TPMT explains a good share of severe reactions. Doctors in Asia kept seeing patients with normal TPMT whose blood counts still collapsed on a standard dose. The missing piece turned out to be NUDT15, a gene that breaks down the active form of the drug before it harms the bone marrow.
What that means at the bedside
In Indian patients, NUDT15 often explains more of the problem than TPMT does. Testing TPMT alone can give a family false comfort. Testing both genes together gives the oncologist the fuller picture before the first tablet is swallowed, rather than after the first dangerous drop in counts.
Your ancestry does not decide your result. It only decides how likely it is that the test finds something.Who it touches
Who in India is affected by a NUDT15 result?
Only people who take a thiopurine. That group is larger than most families expect, and it is not limited to cancer.
Children with leukaemia
Mercaptopurine is the backbone of maintenance treatment for acute lymphoblastic leukaemia, the most common childhood cancer. It is taken daily at home for a long stretch, so the dose matters every single day.
Adults with leukaemia
Adults with the same leukaemia take the same drugs, often on protocols adapted from children's treatment. The gene result applies in exactly the same way.
People on azathioprine
Azathioprine is a thiopurine used widely outside cancer care. A NUDT15 result is just as relevant here.
Commonly prescribed for
- Crohn's disease and ulcerative colitis
- Lupus and some kidney diseases
- Autoimmune liver and skin conditions
- Preventing rejection after a transplant
Families who marry within a community
Where parents are related, or come from the same small community, a child is more likely to inherit a faulty copy from both sides. That is the pattern that produces the most severe reactions.
Not sure whether this applies to you?
Ask an oncologistHow the gene was found
Why was NUDT15 missed for so long?
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Thiopurine safety was built on TPMT
TPMT was linked to severe drug reactions decades ago. Dose advice, laboratory tests and textbooks all grew up around that one gene.
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Asian doctors saw reactions TPMT could not explain
Faulty TPMT is uncommon in Asian populations, yet severe drops in blood counts on standard doses were not. Many of these patients had entirely normal TPMT results.
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Gene-wide studies pointed at NUDT15
A little over a decade ago, a study of Korean patients with bowel disease linked one NUDT15 variant to low white cell counts on thiopurines. Studies of children with leukaemia soon confirmed the same effect across Asian and Hispanic children.
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Guidelines added NUDT15 to the dose advice
International pharmacogenomic guidelines now give dose advice for TPMT and NUDT15 side by side. The American drug regulator's biomarker list names both genes for thiopurines.
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Practice is still catching up
Some laboratories and order forms still offer TPMT alone. That is why it is worth asking, by name, whether NUDT15 was included.
On your report
The NUDT15 words you may see, in plain language
- NUDT15
- A gene that makes an enzyme which clears the active form of thiopurine drugs. When it works poorly, the drug builds up in the bone marrow.
- NUDT15 *3
- The most common faulty version worldwide. Some reports describe it by its change instead, written as p.R139C or c.415C>T.
- Heterozygous
- One faulty copy and one working copy. This usually means an intermediate metaboliser and a reduced starting dose.
- Homozygous
- Two faulty copies, one from each parent. This usually means a poor metaboliser and a far smaller dose.
- Ancestry
- Where your forebears came from. It shifts the odds of carrying a variant but never tells you whether you actually do.
- Consanguinity
- Marriage between blood relatives, such as cousins. It raises the chance that a child inherits the same variant from both parents.
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Side by side
What changes when NUDT15 is added to the test?
TPMT tested alone
TPMT and NUDT15 together
Can miss many at-risk South Asian patients
Checks both known genetic causes
A NUDT15 problem shows up as falling counts
The dose is planned before the first tablet
Unexplained intolerance leads to guesswork
Intolerance has a likely explanation
Relatives learn nothing useful
Relatives know to mention it if prescribed a thiopurine
Being straight with you
What this page cannot tell you
It cannot tell you your own result. Most people of South Asian ancestry have two working copies of NUDT15, and only a test shows which group you are in. What your specific variant means is a question for the oncologist or genetic counsellor who ordered the test.
The Indian evidence is still thin
Most NUDT15 research comes from East Asian, American and European centres. Indian studies so far are small, and the gene has not been mapped properly across India's many communities and regions. Rarer variants may not be on every laboratory panel, so a normal result means the common variants were not found.
Who this does not apply to
Most people never take a thiopurine, and for them this result has no use. It is not a cancer risk test and says nothing about whether cancer runs in your family. It only matters when mercaptopurine, thioguanine or azathioprine is planned or already being taken.
A result is a starting point
Even with both genes tested, the dose is still steered by regular blood counts. Infection, the illness itself and other medicines can all lower counts too.
Commonly believed
Four things families assume about NUDT15
"NUDT15 is a problem for Chinese and Japanese patients, not Indians."
Faulty versions are found across South Asia too, at levels high enough that international guidelines apply to Indian patients. It is a pan-Asian finding, and it also appears in people of Hispanic ancestry.
"We are Indian, so my child must be a poor metaboliser."
Most Indian children have a normal NUDT15 result. Ancestry raises the odds of finding a variant. It does not predict your child's result, which is why the test exists.
"The hospital tested TPMT, so the genes have been checked."
A TPMT result says nothing about NUDT15. They are separate genes on separate lines of the report. If only one line is there, only one gene was tested.
"If one child has it, every child in the family has it."
Each child inherits their own mix from their parents. A brother or sister may share the result or may not. It only matters for them if they are ever prescribed a thiopurine.
Questions we are asked
Common questions about NUDT15 in South Asian patients
Is NUDT15 testing available in Hyderabad?
Yes, several laboratories in the city offer it, often as a combined thiopurine panel with TPMT. The key is the order form. Make sure NUDT15 is named on it, because a request that only says TPMT will be tested as TPMT alone.
Our marriage is between cousins. Does that change anything?
It raises the chance that a child inherits the same faulty copy from both parents, which is the pattern that causes the most severe reactions. It does not mean your child has one. It is a good reason to make sure both genes are tested before a thiopurine starts.
My child's TPMT was normal but counts crashed. Could it be NUDT15?
It is one of the possibilities worth checking, especially in a South Asian child. Infection, the leukaemia itself and other medicines can also cause it. Ask the oncologist whether a NUDT15 test was done, and if not, whether it should be sent now.
I take azathioprine for Crohn's disease. Does this apply to me?
Yes. Azathioprine is handled by the same two genes, and the same results apply. Many gastroenterologists now test both before starting. If you started without a test, ask your doctor whether one would help, especially if your counts have been low.
Does NUDT15 affect other cancer drugs?
Its known clinical importance is with the thiopurine family: mercaptopurine, thioguanine and azathioprine. Other cancer drugs have their own gene tests, such as DPYD before capecitabine or fluorouracil. A NUDT15 result does not answer those questions.
Can adults be tested, or is it only for children?
Adults can be tested in exactly the same way, from a blood sample. The result does not change with age, so a test done once applies for life. Keep the report and show it whenever a thiopurine is prescribed again.
Should the whole family get tested?
Not routinely. The result carries no cancer risk, so healthy relatives gain nothing from testing now. What helps is telling blood relatives about the result, so they can mention it if a doctor ever prescribes them a thiopurine.
Is the test covered by Aarogyasri or Ayushman Bharat?
It depends on the treatment package and the hospital. Some leukaemia packages include thiopurine gene testing and some do not. Ask the billing desk before the sample is sent so that the cost is clear in advance.
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
- Clinical Pharmacogenetics Implementation Consortium (CPIC) — CPIC Guideline for Thiopurines and TPMT and NUDT15
- NCBI Medical Genetics Summaries — Mercaptopurine Therapy and TPMT and NUDT15 Genotype
- US Food and Drug Administration — Table of Pharmacogenomic Biomarkers in Drug Labeling
- National Cancer Institute — Childhood Acute Lymphoblastic Leukemia Treatment (PDQ) - Patient 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
TPMT and NUDT15 before thiopurinesWhat both tests check and how results are labelledThiopurine dosing in childhood leukaemia maintenanceHow the dose is managed through maintenanceWhat a poor metaboliser result meansIf the report says two faulty copiesWhy this testing is not routine yet in IndiaThe gap between guidelines and everyday practice
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