Medical Oncologist
CION Cancer Clinics
Oncogenes and tumour suppressor genes | CION Cancer Clinics
An oncogene is a gene that, when faulty, pushes a cell to grow when it should not. A tumour suppressor gene normally holds growth back or repairs damage, and a fault removes that protection. Cancer usually needs both kinds of fault working together in one cell. This page explains the difference and why it matters for how inherited risk works. At CION Cancer Clinics in Hyderabad, our oncologists review your family history with you and guide you to the right genetic counselling and testing.
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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 difference between an oncogene and a tumour suppressor gene?
- The accelerator and the brake, compared
- How a tumour suppressor fault plays out, using BRCA as an example
- The words you will meet, in plain language
- What each fault usually needs to cause trouble
- Why your counsellor cares which kind of gene is on your report
- Two mix-ups worth correcting
- Common questions about oncogenes and tumour suppressor genes
The short answer
What is the difference between an oncogene and a tumour suppressor gene?
An oncogene is a gene that, when faulty, pushes a cell to grow when it should not, like an accelerator stuck down. A tumour suppressor gene normally holds growth back or repairs damage, like a brake. Cancer usually needs the accelerator pushed and the brake released, not just one or the other.
Both start as ordinary, useful genes
Neither kind of gene exists to cause cancer. Both do necessary jobs in a healthy cell, controlling ordinary growth, healing and renewal. Cancer risk comes from a fault changing how they behave, not from carrying the gene itself.
The two faults behave in almost opposite ways
An oncogene usually needs only one copy switched on wrongly to cause trouble, because it is now actively pushing growth. A tumour suppressor usually needs both of your two copies knocked out before its protection is lost, because one working copy is normally enough to do the job.
Two kinds of gene
The accelerator and the brake, compared
Most inherited cancer genes are tumour suppressors, not oncogenes. This is one of the most useful distinctions on a genetic report.
Oncogene
Normally helps control ordinary cell growth. A fault switches it permanently on, pushing the cell to keep dividing.
Usually behaves like
- One faulty copy is often enough
- Acts like a stuck accelerator
- Rarely the gene tested for inherited risk
Tumour suppressor gene
Normally restrains growth or repairs damage. A fault removes that restraint once both copies are lost.
Usually behaves like
- Both copies usually need to fail
- Acts like a released brake
- Most inherited cancer genes are this kind
Why the second group matters more here
Being born with one faulty copy of a tumour suppressor gene means only one further fault, in the remaining copy, is needed in a cell for its protection to be lost entirely.
Neither works alone
Most cancers involve faults in several genes from both groups, working together in the same cell, rather than a single gene of either kind acting by itself.
Not sure whether this applies to you?
Ask an oncologistA worked example
How a tumour suppressor fault plays out, using BRCA as an example
The gene's normal job
BRCA1 and BRCA2 normally help repair a particular kind of DNA damage accurately, protecting the cell from picking up further faults.
One copy is inherited faulty
Someone carrying a faulty copy is born with one working copy instead of two, in every cell. The remaining copy still does the repair job.
The second copy is lost in one cell
Ordinary wear damages the remaining working copy in a single cell, somewhere in the body, at some point in life.
That cell can no longer repair itself well
Without either working copy, errors build up faster in that one cell and its descendants, raising the chance a further fault turns it cancerous.
On your report
The words you will meet, in plain language
- Oncogene
- A gene that, when faulty, actively pushes a cell to grow.
- Tumour suppressor gene
- A gene that normally restrains growth or repairs damage. Most inherited cancer genes are this kind.
- Loss of function
- A fault that stops a gene doing its normal job, typical of tumour suppressor faults.
- Gain of function
- A fault that switches a gene permanently on or makes it overactive, typical of oncogene faults.
- Second hit
- The further fault, damaging the remaining working copy, that a tumour suppressor gene usually needs before its protection is lost.
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Side by side
What each fault usually needs to cause trouble
Oncogene fault
Tumour suppressor fault
Usually one faulty copy is enough
Usually both copies must be lost
Rarely inherited; usually arises during life
Often the gene behind an inherited cancer
syndrome
Some are the direct target of specific cancer
drugs
Loss is harder to target directly with a
medicine
Why this distinction is useful to you
Why your counsellor cares which kind of gene is on your report
Knowing whether a gene acts as an accelerator or a brake changes how a family history is read. Most of the genes cancer genetics testing looks for, BRCA1, BRCA2, MLH1 and similar names, are tumour suppressors, which is why one inherited faulty copy is enough to raise risk sharply even though a second fault is still needed inside a cell.
It also shapes what treatment can target
Some oncogenes that are switched on inside a tumour can be targeted directly by specific medicines, because blocking an overactive signal is a clear target. Restoring a lost tumour suppressor is much harder, which is why treatment for these cancers more often relies on removing the affected cells rather than switching the gene back on.
Your own result still needs a specialist to read
Which category a specific gene falls into, and what a fault in it means for you, is a question for your genetic counsellor or oncologist. This page explains the general categories only.
Commonly believed
Two mix-ups worth correcting
"An oncogene is a gene nobody should have."
Everyone carries oncogenes, and in their normal form they do essential jobs controlling ordinary growth and healing. It is only a fault switching them permanently on that becomes a problem.
"If my tumour suppressor gene is faulty, cancer
is already certain."
Inheriting one faulty copy still leaves the second copy working in every cell. Cancer needs that remaining copy to fail too, in a specific cell, which is not certain and does not happen to everyone who carries the inherited fault.
"Only oncogenes can be targeted by modern cancer
drugs, so a tumour suppressor fault is untreatable."
Treatment does not have to switch a lost gene back on to work. Cells that have lost a tumour suppressor's protection are often more vulnerable in other ways, and several established treatments are built around exploiting exactly that weakness rather than repairing the gene itself.
Questions we are asked
Common questions about oncogenes and tumour suppressor genes
Is BRCA an oncogene or a tumour suppressor
gene?
BRCA1 and BRCA2 are tumour suppressor genes. They normally help repair a particular kind of DNA damage, and their protection is lost once both copies stop working in a cell.
Can testing tell me which kind of gene fault I
carry?
Yes. A genetic report names the specific gene involved, and your counsellor can explain whether it normally behaves as an accelerator or a brake, and what that means for how the fault is likely to act.
Why do most inherited cancer genes turn out to be
tumour suppressors?
An inherited fault affecting a tumour suppressor still leaves one working copy at birth, so a person can be born carrying it and stay well for years. A serious inherited oncogene fault would tend to cause problems too early and too often to be passed down as commonly.
Can a drug switch a tumour suppressor gene back
on?
This is an active area of research, but it remains much harder than blocking an overactive oncogene. Most current treatment for tumour suppressor faults focuses on removing the affected cells rather than restoring the gene's function directly.
Does having an oncogene mean I will get
cancer?
No. Everyone carries oncogenes in their normal, working form, and they cause no harm unless a fault switches them on wrongly. Carrying the ordinary gene is not the same as carrying a cancer-causing fault in it.
Is one type of fault more dangerous than the
other?
Neither is uniformly more dangerous; it depends on the specific gene, the specific fault and which other genes are also affected. Most cancers involve faults from both groups acting together rather than one type alone.
My report names a gene I have never heard of. What do I
do?
Do not search for isolated meanings online. Bring the report to the genetic counsellor or oncologist who ordered it, since the same gene name can carry very different implications depending on the exact fault found.
Where can I get my own result explained?
This page explains the general categories only. What your specific gene and fault mean for you is a question for a genetic counsellor or clinical geneticist. Call the CION helpline if you are not sure 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)
Medical Oncologist
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Sources
- National Cancer Institute — Oncogenes
- National Cancer Institute — Tumor Suppressor Gene
- Cancer Research UK — Genes and cancer
- MedlinePlus Genetics — What is a genetic predisposition to disease?
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
How a faulty gene leads to cancerThe general mechanism these two gene types fit intoThe two-hit hypothesis in plain EnglishWhy a tumour suppressor gene usually needs both copies lostDNA repair genes and why they matter mostA closer look at the tumour suppressor genes that repair damageThe BRCA1 geneThe worked example on this page, explained in full
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