Medical Oncologist
CION Cancer Clinics
How a faulty gene leads to cancer | CION Cancer Clinics
A faulty gene means one of a cell's instructions for growing, repairing itself or self-destructing has stopped working. On its own that is rarely enough. Cancer begins once several of these safeguards fail together inside a single cell. This page explains that process plainly, and why an inherited fault changes the starting point rather than guaranteeing the outcome. 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 does a faulty gene actually do to cause cancer?
- Four kinds of instruction a faulty gene can break
- How faults accumulate in a single cell over time
- The words you will meet, in plain language
- An inherited fault compared with one that builds up in life
- Why one inherited fault can be enough to raise risk sharply
- Three things families assume, and what is actually true
- Common questions about how a gene fault leads to cancer
The short answer
What does a faulty gene actually do to cause cancer?
Every cell in your body follows a set of instructions telling it when to grow, when to stop, when to repair itself and when to self-destruct if it cannot be repaired. A faulty gene means one of those instructions has stopped working correctly. Cancer begins when enough of these instructions fail inside one single cell.
It almost never takes just one fault
A single faulty gene rarely causes cancer by itself. Most cells have backup systems, so it usually takes several faults building up in the same cell, in the right combination, before that cell breaks free of normal control.
Where the fault came from changes the starting point, not the ending point
Some people are born with one fault already present in every cell, inherited from a parent. Everyone else starts from cells with no faults at all. Either way, further faults still have to build up in one cell before cancer can begin.
This is a cell problem before it is a body problem
Cancer starts as one cell that has broken free of the rules and then divides into a growing family of identical cells. Everything that follows, the lump, the scan finding, the diagnosis, traces back to faults in that one original cell.
What can break
Four kinds of instruction a faulty gene can break
Genes linked to cancer usually fall into one of these four jobs.
Telling a cell to grow
Some genes act like an accelerator, pushing a cell to divide. A fault that jams this accelerator down is one route to uncontrolled growth.
Telling a cell to stop
Other genes act like a brake, holding growth back until conditions are right. A fault that disables this brake removes a safeguard the cell relied on.
Repairing damage
A separate set of genes checks new copies of DNA for mistakes and fixes them. When these fail, errors elsewhere in the cell build up far faster than normal.
Ordering self-destruction
A badly damaged cell is normally instructed to destroy itself rather than carry on. A fault in this instruction lets a damaged cell survive when it should not.
Not sure whether this applies to you?
Ask an oncologistHow it builds up
How faults accumulate in a single cell over time
-
A normal cell, following every instruction
Growth, repair and self-destruction are all working as they should. This describes almost every cell in your body, almost all of the time.
-
One instruction fails
Ordinary wear, a copying error during cell division, or an inherited fault present from birth damages one gene in one cell.
-
The cell still has other safeguards
Losing one instruction is rarely enough on its own. Other control genes are still working, so the cell usually behaves normally or is cleared away.
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A second, unrelated fault lands in the same cell
Over months or years, further damage can strike the same cell again, this time affecting a different safeguard.
-
Enough safeguards are gone
Once growth, repair and self-destruction have all been compromised in one cell, it can begin dividing without the normal checks, and a tumour can form from its descendants.
On your report
The words you will meet, in plain language
- Mutation
- A change in the spelling of a gene. Most are harmless; some stop the instruction working properly.
- Oncogene
- A gene that, when faulty, pushes a cell to grow when it should not.
- Tumour suppressor gene
- A gene that normally restrains growth or repairs damage. Cancer often needs this kind of gene to fail, not just an oncogene to switch on.
- Driver mutation
- A fault that actively pushes a cell towards cancer, as opposed to a change that has no real effect.
- Somatic
- A fault that arose in one cell during a person's life. It is not in the rest of the body and cannot be inherited by a child.
- Germline
- A fault present from birth in every cell, including egg or sperm cells, which is how it can pass to the next generation.
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Two different starting points
An inherited fault compared with one that builds up in life
Inherited (germline) fault
Fault acquired during life (somatic)
Present from birth, in every cell of the body
Arises later, only inside the affected organ
Can be passed to children
Cannot be passed to children
Gives a cell a head start, one safeguard
already down
All safeguards start intact and have to fail one
by one
Relatives can be tested for the exact same
fault
Relatives are not affected and are not tested
for it
Why an inherited fault matters so much
Why one inherited fault can be enough to raise risk sharply
A cell needs several safeguards to fail before it turns cancerous. Someone born with one fault already present has, in effect, a head start in every single cell of that type in their body. The remaining safeguards still have to fail through ordinary wear, but that is a smaller gap to close than starting from a fully intact cell.
This is why the same gene can behave differently
Two people can inherit the exact same faulty gene and have very different outcomes, because the remaining steps still depend on chance, on other genes acting alongside it, and on exposures over a lifetime. An inherited fault changes the odds. It does not fix the outcome.
Repair genes deserve their own explanation
The genes that repair damage matter more than almost any other group, because losing them speeds up every other kind of fault. That is covered on its own page, since it is central to how several of the most common inherited cancer syndromes work.
Commonly believed
Three things families assume, and what is actually true
"One faulty gene is all it takes."
Usually not. Most cancers need several separate faults to build up inside a single cell before it breaks free of normal control. One faulty gene, inherited or acquired, is a step along that road rather than the whole journey.
"If I live perfectly, I can stop faults building
up entirely."
Some damage comes from ordinary copying errors every time a cell divides, which happens regardless of lifestyle. A healthy lifestyle lowers the odds; it cannot bring them to nothing.
"A faulty gene works the same way as a
virus."
A gene fault cannot spread from person to person the way an infection can. It can only pass from parent to child, at conception, and only if it was already present in the egg or sperm cell.
Questions we are asked
Common questions about how a gene fault leads to cancer
Does having a faulty gene mean I already have
cancer?
No. A faulty gene, inherited or acquired, is one step in a longer process. Most people carrying an inherited fault never develop the safeguard failures needed for cancer to actually begin, though their risk is raised compared with someone starting from an intact cell.
How many faults actually have to happen in one
cell?
It varies by cancer type and by which genes are involved, but it is essentially always more than one. This is part of why cancer risk rises with age: more time allows more faults to accumulate in the same cell line.
Can stress or diet directly cause a gene
fault?
Ordinary stress does not damage genes in a way linked to cancer. Certain exposures, tobacco smoke being the clearest example, can damage DNA directly and add to the accumulation of faults. Diet's role is much smaller and less direct than either of these.
If I inherited a fault, will my children definitely
have cancer?
No. Inheriting the fault only means a child starts with the same head start you did. Whether the remaining safeguards later fail in any particular cell is still not certain, and many carriers of an inherited fault never develop cancer at all.
What is the difference between a driver mutation and
an ordinary one?
Most spelling changes in DNA have no real effect on how a cell behaves. A driver mutation is one of the smaller number that genuinely pushes a cell towards uncontrolled growth, and it is what treatment and testing are usually focused on finding.
Can a fault in one gene be repaired by the body
later?
Repair genes fix many small errors as they happen, which is exactly why they matter so much. Once a fault has become fixed in a gene's permanent spelling, it stays that way in that cell and every cell descended from it.
Is this the same explanation for every type of
cancer?
The broad idea, several safeguards failing in one cell, applies widely, but the specific genes involved and how many faults are needed differ by cancer type. A specialist looking at your particular diagnosis can explain what is known for that cancer.
Where can I ask what this means for my own
situation?
This page explains the general mechanism only. What it means for you specifically, including whether a gene test is worth having, is a question for a genetic counsellor or your oncologist. 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
Medical Oncologist
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Sources
- National Cancer Institute — The Genetics of Cancer
- Cancer Research UK — How cancer starts
- MedlinePlus Genetics — What is a gene mutation and how do mutations occur?
- NHS — Causes of cancer
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
Oncogenes and tumour suppressor genesA closer look at the two kinds of gene involvedThe two-hit hypothesis in plain EnglishWhy one inherited fault alone is usually not enoughDNA repair genes and why they matter mostThe safeguard that, when lost, speeds up every other faultCancer genetics explainedThe bigger picture this mechanism sits inside
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