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CION Cancer Clinics
What the SMARCA4 and SMARCB1 genes do, and why it matters | CION Cancer Clinics
SMARCB1 and SMARCA4 help build a machine inside every cell that opens and closes packed DNA, so the right genes are switched on at the right time. When both copies of either gene stop working in one cell, a fast-growing rhabdoid tumour can form, usually in a baby or young child. This page explains what the genes do, and how an inherited fault differs from one found only in a tumour. 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 do the SMARCA4 and SMARCB1 genes actually do?
- Which conditions are linked to these two genes?
- How does a fault in one gene turn into a tumour?
- What do the words on the report mean?
- A tumour test or a blood test: what does each one tell you?
- What this page cannot tell you
- Four things parents tell us, and what is actually true
- Common questions about SMARCA4 and SMARCB1
The short answer
What do the SMARCA4 and SMARCB1 genes actually do?
Both genes carry the instructions for parts of one machine inside the cell. That machine loosens and tightens packed DNA, which decides which genes can be read and which stay silent. It is one of the main ways a cell knows when to keep growing and when to settle into its adult job.
Why losing them matters so much
When a cell loses both working copies of SMARCB1 or SMARCA4, the machine stops working properly. Genes that should switch off during early development stay switched on. The cell keeps dividing like a very young cell and never grows up. That is what a rhabdoid tumour is: a tumour made of cells that were never told to stop.
Why these tumours appear so young
Most cancers need many faults that build up over decades. Rhabdoid tumours usually need very little beyond the loss of this one gene. That is why they tend to appear in babies and toddlers rather than adults, and why they can grow quickly once they start.
The gene fault explains why the tumour formed. It says nothing about anything the parents did or did not do.One gene name, several meanings
Which conditions are linked to these two genes?
The same gene name can appear on very different reports. What matters is which gene, what kind of change, and whether it was found in the blood or only in the tumour.
SMARCB1 and rhabdoid tumours
SMARCB1 is the gene most often lost in rhabdoid tumours. These can form in the brain, where they are called atypical teratoid rhabdoid tumours, in the kidney, or in soft tissue elsewhere in the body.
Usually looks like
- A tumour in a baby or young child
- Loss of the INI1 protein on the biopsy report
- Occasionally, more than one tumour at once
SMARCA4 and rhabdoid tumours
SMARCA4 does a similar job and is lost in a smaller group of rhabdoid tumours. An inherited SMARCA4 fault is rarer than an inherited SMARCB1 fault. The biopsy report may mention loss of the BRG1 protein.
SMARCA4 and a rare ovarian cancer
SMARCA4 is also behind small cell carcinoma of the ovary, hypercalcaemic type. This is a rare cancer of girls and young women, sometimes with a raised calcium level in the blood. Some of these women carry the fault from birth.
Other changes in the same genes
Milder SMARCB1 changes are linked to schwannomatosis, a condition of benign nerve tumours in adults. Other changes in both genes cause developmental conditions such as Coffin-Siris syndrome. These are managed very differently, and a counsellor will tell you which one your report points to.
Not sure whether this applies to you?
Ask an oncologistHow a fault becomes a tumour
How does a fault in one gene turn into a tumour?
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Every cell carries two copies
One copy came from the mother and one from the father. A single working copy is enough to keep the machine running and the cell behaving normally.
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Most rhabdoid tumours start in one cell only
In most children, both copies are lost in a single cell during early development. Nothing was inherited, and nothing can be passed on. This is the commonest pattern.
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Some children are born with one faulty copy
A smaller group carry a germline fault, meaning it is present in every cell from birth and can be passed on. Often it arose new in that child and came from neither parent.
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Losing the second copy starts the tumour
In a child who already carries one fault, a single further loss in one cell is enough. That is why these children can be very young at diagnosis, or have more than one tumour.
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Not every carrier develops a tumour
Some parents are found to carry the fault and were never ill. Doctors call this incomplete penetrance, meaning the fault does not always lead to a tumour.
On your report
What do the words on the report mean?
- SWI/SNF complex
- The machine these genes help to build. It unpacks and repacks DNA so the right genes can be read at the right time.
- INI1
- The protein made by SMARCB1. A report saying INI1 is lost means the tumour cells have stopped making it.
- BRG1
- The protein made by SMARCA4. Its loss on a tumour report points to SMARCA4.
- Rhabdoid tumour
- A fast-growing tumour of early childhood, named after the way its cells look under the microscope.
- Somatic
- Found only in the tumour. A somatic fault is not in the rest of the body and cannot be passed on.
- Germline mosaicism
- When a parent carries the fault in some egg or sperm cells but not in their blood. It explains how a second child can be affected.
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Side by side
A tumour test or a blood test: what does each one tell you?
Test on the tumour
Test on the blood
Shows whether the tumour cells lost the gene
Shows whether the child was born with the fault
Helps confirm the diagnosis
Helps plan checks and test the family
Almost always abnormal in a rhabdoid tumour
Abnormal in some children, not most
Read by the pathologist
Explained by a genetic counsellor
Being straight with you
What this page cannot tell you
It cannot tell you whether your child's fault was inherited, or what a particular variant on a report means. The same gene can carry changes with very different effects. What your specific variant means is a question for the counsellor who ordered the test.
It cannot guide treatment
Treatment of a rhabdoid tumour is planned by a paediatric oncology team who can see the scans and the biopsy. Testing a tumour to choose medicines is a separate question, covered under targeted therapy.
Who this does not apply to
Most children with cancer have no SMARCB1 or SMARCA4 fault at all. Adults who see SMARCA4 on a lung tumour report usually have a change found only in that tumour. It was not inherited and does not need a family test. Studies of inherited SMARCA4 faults are still small, so some answers are simply not known yet.
If a report names either gene and you are unsure whether it came from the tumour or the blood, ask. The answer changes everything that follows.Commonly believed
Four things parents tell us, and what is actually true
"One of us must have passed it on."
Many children with an inherited fault are the first in the family to have it. It arose new in the egg, the sperm or very early in the pregnancy. Testing both parents is still advised, because the answer matters for future children.
"If we both test negative, our other children are safe."
The chance is usually low, but it is not nil. A parent can carry the fault in some egg or sperm cells without it showing in the blood. That is why brothers and sisters are often offered a test anyway.
"A SMARCB1 result means the same thing in every family."
Different changes in this gene cause different conditions, from rhabdoid tumours in babies to benign nerve tumours in adults. The exact change and the family story decide which one applies.
"Something in the pregnancy caused this."
Nothing a mother ate, did or took during pregnancy is known to cause these faults. They arise by chance when cells copy their DNA. Guilt is common and understandable, but it is not deserved.
Questions we are asked
Common questions about SMARCA4 and SMARCB1
Is a rhabdoid tumour always caused by an inherited fault?
No. The tumour cells almost always lose SMARCB1 or SMARCA4, but in most children that loss happened only in the tumour. A blood test is what tells you whether the child was born with the fault. Guidance advises offering that blood test to every child with a rhabdoid tumour.
What does INI1 loss on the biopsy report mean?
It means the tumour cells have stopped making the protein that SMARCB1 codes for. It supports the diagnosis of a rhabdoid tumour. On its own it does not tell you whether the fault is inherited. Only a blood test answers that question.
Can adults carry these faults?
Yes. Some parents learn they carry the fault only after their child is diagnosed, and they have never been ill. Women who carry a SMARCA4 fault have a raised risk of a rare ovarian cancer, and should talk through their options with a counsellor.
Why do these tumours appear so early in life?
Because very little else needs to go wrong. Once a cell loses both copies, it can keep behaving like a very young, dividing cell. Most other cancers need many faults to build up, and that usually takes decades.
Can the gene fault be repaired?
No. An inherited fault cannot be corrected or reversed. What helps is finding any tumour early and treating it well, which is why carrier children are checked closely in the first years of life. The checks are planned by a paediatric team.
Are there medicines aimed at these genes?
Some drugs aimed at the pathways switched on when these genes are lost are being studied in clinical trials. They are treatments for the tumour, not a way to repair an inherited fault. Your child's oncologist will say whether any trial is relevant.
Should brothers and sisters be tested?
If a child carries an inherited fault, siblings are usually offered a test, especially young ones. Checks for carrier children start in infancy, so the answer matters early. The counsellor will explain the order and the timing for your family.
Can this be explained to us in Telugu?
Ask for counselling in Telugu if you prefer it, and bring a family member to sit in. Call the CION helpline and someone will point you to the right counsellor and paediatric team for your child.
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) — Rhabdoid Tumor Predisposition Syndrome
- MedlinePlus Genetics — SMARCB1 gene
- MedlinePlus Genetics — SMARCA4 gene
- National Cancer Institute — Childhood Central Nervous System Atypical Teratoid/Rhabdoid Tumor 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
Rhabdoid tumour predisposition syndromeThe inherited condition itself, explained in fullSMARCA4 and SMARCB1 positive: what happens nextThe first steps once a fault is found in the bloodTesting the family for SMARCA4 and SMARCB1Why siblings are tested even when parents are negativeSMARCA4 and small cell carcinoma of the ovaryThe rare ovarian cancer linked to SMARCA4
Talk to us
Has a report named SMARCB1 or SMARCA4?
Tell us what the report says and who was tested. We will help you reach a genetic counsellor and a paediatric oncology team who can explain it properly. One helpline serves every CION centre.