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CHEK2 1100delC: what this variant means | CION Cancer Clinics
1100delC is a single missing letter in the CHEK2 gene that stops that copy from working. It is the most studied CHEK2 change and raises breast cancer risk moderately, less than a BRCA fault. This page explains what the name on your report means, how it compares with other CHEK2 changes, and what usually happens once it is found. 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 is the CHEK2 1100delC variant?
- What does each part of "c.1100delC" mean?
- How does 1100delC compare with other CHEK2 changes?
- What happens once 1100delC is found?
- How is 1100delC different from a BRCA fault?
- What people read about 1100delC, and what is true
- What this page cannot tell you
- Common questions about CHEK2 1100delC
The short answer
What is the CHEK2 1100delC variant?
1100delC is one specific fault in the CHEK2 gene, where a single letter of the DNA code is missing. That missing letter breaks the instruction, so the gene's copy from that parent does not work. It is the most studied CHEK2 change in the world, and it raises the risk of breast cancer moderately.
Why this one change is so well known
1100delC is common in some parts of northern and eastern Europe, where it has been passed down through many generations. That made it possible to study thousands of carriers. Much of what doctors know about CHEK2 risk comes from those studies, which is why the name turns up so often in articles and on reports.
How often it is found in India
Studies in Indian families have found 1100delC rarely. Indian families who carry a CHEK2 fault more often have a different change in the same gene. The studies so far are small, so this may change as more Indian families are tested. If your report names 1100delC, it is a real and well understood finding.
Reading about 1100delC online will mostly describe European families. Your counsellor will explain what it means in yours.Reading the name
What does each part of "c.1100delC" mean?
- c.
- Short for coding DNA. The numbers that follow count letters along the part of the gene that holds the instruction.
- 1100
- The position of the change, counted letter by letter from the start of the instruction.
- del
- Short for deletion. A letter that should be there is missing.
- C
- The missing letter, cytosine. Newer reports often drop it and write c.1100del. It is the same change.
- Frameshift
- What a missing letter does. The code is read in groups of three, so every group after the gap is misread and the protein is cut short.
- Pathogenic
- The laboratory's classification. It means the change is known to break the gene and raise cancer risk.
Not sure whether this applies to you?
Ask an oncologistNot all CHEK2 changes are equal
How does 1100delC compare with other CHEK2 changes?
A report that says CHEK2 is not the whole story. The exact change matters, and so does how it is classified.
1100delC and other truncating changes
These changes cut the CHEK2 protein short so that it does not work. They carry the clearest evidence of a raised breast cancer risk, and management guidelines are largely built on them.
Missense changes such as I157T
Here one building block of the protein is swapped for another. The protein still works, only less well. Studies suggest the risk is lower than with 1100delC, and screening is often guided more by the family history.
Variants of uncertain significance
The laboratory has found a change it cannot yet classify. It should not change screening or treatment on its own. It may be reclassified as evidence grows.
Two copies of 1100delC
Very rarely a person inherits the change from both parents. This is more likely where parents are related by blood. Small studies suggest a higher breast cancer risk than one copy.
Tell your counsellor if your parents are related. It changes who in the family should be tested.Step by step
What happens once 1100delC is found?
A results appointment
A genetic counsellor explains the result, what 1100delC is and what it is not. This is where your own questions about risk are answered, in Telugu, Hindi or English.
The family history is looked at again
With CHEK2, the family history shapes the plan as much as the gene does. A carrier with several close relatives with breast cancer may get a different plan from one with none.
A screening plan is agreed
For women, this usually means breast screening that starts earlier and may add MRI. Men discuss prostate checks. Bowel screening depends on the family history.
Treatment is reviewed if you have cancer
If you are already being treated for breast cancer, your oncologist will say whether the result changes the plan. Often it does not change the treatment itself.
Relatives are offered testing
A family letter names 1100delC so that parents, brothers, sisters and adult children can be tested for that one change.
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Side by side
How is 1100delC different from a BRCA fault?
CHEK2 1100delC
BRCA1 or BRCA2 fault
Moderately raised breast cancer risk
A much higher breast cancer risk
No clear link to ovarian cancer
A clearly raised ovarian cancer risk
Family history strongly shapes the plan
The gene alone largely sets the plan
Preventive surgery not routinely advised
Preventive surgery is often discussed
Commonly believed
What people read about 1100delC, and what is true
"1100delC is a European gene, so my result must be wrong."
It is more common in Europe, but it is found in other populations too. Accredited laboratories confirm what they report. If in doubt, ask the counsellor how the result was checked.
"It is as serious as BRCA."
It is not. The breast cancer risk is moderate, lower than with BRCA1 or BRCA2. Most women who carry it are managed with closer screening rather than surgery.
"Since it is only moderate, I can ignore it."
Moderate still means higher than most people. Earlier and closer screening finds cancers at a stage when treatment is simpler. That is the point of knowing.
"My tumour report found it, so my family must have it."
A change found only in a tumour may not be inherited. A blood or saliva test is needed to know whether it was present from birth and can be passed on.
Being straight with you
What this page cannot tell you
It cannot give you your own risk. That depends on 1100delC, on your family history, your age and other factors a counsellor weighs together. What your specific variant means is a question for the counsellor who ordered the test.
It cannot replace the report itself
Reports differ in how they write the change and how they classify it. Bring the whole report, not a photo of one line, so the counsellor can check the gene, the change and the classification together. Searching the variant name online will mostly bring up research on European families. Those numbers were measured in other populations and may not describe yours.
It cannot say how the evidence will change
What is known about 1100delC today comes largely from large European studies. As more Indian families are tested, the picture here may sharpen. Ask your counsellor how you will be told if the advice for carriers changes, and keep your report somewhere you can find it.
Who this does not apply to
If your report names a different CHEK2 change, this page is only partly about you. If a change was found only in a tumour, that belongs with targeted therapy, not inherited testing. Most people never need a CHEK2 test at all.
Questions we are asked
Common questions about CHEK2 1100delC
My report says c.1100del, not 1100delC. Is it the same?
Yes. Current naming rules leave out the missing letter, so newer reports write c.1100del. Older reports and most research papers use 1100delC. Both describe the same missing letter at the same place in CHEK2.
Does 1100delC mean I will get breast cancer?
No. It raises the risk above that of the general population, but most carriers never develop breast cancer. The purpose of knowing is to screen earlier and more closely, so that any cancer is found early.
Does it raise the risk of other cancers?
Some studies link CHEK2 faults with a modestly raised risk of prostate and bowel cancer, and male breast cancer. The evidence is less settled than for breast cancer. Your family history decides whether any extra screening is worthwhile.
Should I have preventive surgery?
It is not routinely advised on a CHEK2 result alone. It may be discussed where the family history is very strong. Risk-reducing surgery is one option among several and is covered on its own pages.
Can men carry and pass on 1100delC?
Yes. A man can carry it without ever being unwell and pass it to a son or daughter. Each child has a one in two chance. Leaving men out of family testing is a common reason the pattern is missed.
Does it change my breast cancer treatment?
Often not the treatment itself. It may affect decisions about surgery on the other breast and about follow-up screening. Your oncologist will say whether the result changes anything in your case.
My parents are related. Does that matter?
It can. Both parents may carry the same change, so a child could inherit two copies. That is rare, and small studies suggest a higher risk than one copy. Tell your counsellor, because it changes who in the family should be tested.
Where can I get the result explained?
With a genetic counsellor or clinical geneticist. Bring the full report and a list of who in the family had cancer, and at what age. Call the CION helpline and someone will arrange an 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)
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Sources
- MedlinePlus Genetics — CHEK2 gene
- National Cancer Institute — Genetics of Breast and Gynecologic Cancers (PDQ)–Health Professional Version
- NCCN — Genetic/Familial High-Risk Assessment: Breast, Ovarian, Pancreatic, and Prostate
- HGVS — HGVS Nomenclature: sequence variant nomenclature
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
The CHEK2 gene: what it doesWhat the gene does before the variant breaks itLow-penetrance variants and why they confuse peopleWhy moderate-risk changes are harder to act onCHEK2 mutation: which cancers and what riskThe linked cancers, one by oneTesting the family for CHEK2Who to test once 1100delC is found
Talk to us
Does your report name CHEK2 1100delC?
Bring the full report and your family history to a genetic counsellor who can explain what it means for you. We can arrange that appointment and help plan testing for relatives. One helpline serves every CION centre.