How Do Genetic Changes Turn a Normal Cell into Cancer?
Cancer begins with damage to a cell's genetic instructions, not with a single broken gene. This article explains the different kinds of genetic and gene-regulation changes involved in cancer, why a cell usually needs several of these changes working together, and how they build up over a person's lifetime.
By Chris Morais, MSc, MPhil, PhD
Cancer develops when changes build up in three main types of genes: proto-oncogenes that get stuck switched on, tumor suppressor genes that stop working as brakes, and DNA repair genes that can no longer catch copying mistakes. A single change rarely causes cancer on its own; it usually takes several of these changes acting together over time.
- What Genetic Changes Are Involved in Cancer?
- What Is the Difference Between a Driver Mutation and a Passenger Mutation?
- How Do Changes in Proto-Oncogenes Promote Cancer?
- How Do Tumor Suppressor Genes Normally Prevent Cancer?
- How Can Defects in DNA Repair Genes Promote Cancer?
- Can Changes in Gene Regulation Cause Cancer Without Changing the DNA Sequence?
- Why Are Several Genetic Changes Usually Needed Before Cancer Develops?
- How Do Cancer-Promoting Changes Build Up Over Time?
- Are Cancer-Causing Genetic Changes Always Inherited?
- Do All Cells With Cancer-Related Mutations Become Cancer Cells?
- Frequently Asked Questions
What Genetic Changes Are Involved in Cancer?
The genetic changes involved in cancer generally fall into three categories of genes: proto-oncogenes, tumor suppressor genes, and DNA repair genes[1]. Each of these gene types normally plays a distinct role in keeping cell growth under control, and damage to any of them removes a different safety check.
| Gene Type | Normal Job | Simple Analogy | Effect When Damaged |
|---|---|---|---|
| Proto-oncogene | Helps start and support normal cell growth | A gas pedal used only when needed | Can become an oncogene that is stuck in the "on" position |
| Tumor suppressor gene | Slows or stops cell division, repairs damage, or removes badly damaged cells | A brake pedal and an inspection station combined | The brake fails, allowing the cell to divide without limit |
| DNA repair gene | Fixes copying mistakes in DNA before they are passed on | A proofreader checking a document before it is filed | Mistakes elsewhere in the DNA build up faster and go uncorrected |
What Is the Difference Between a Driver Mutation and a Passenger Mutation?
A driver mutation directly helps a cell grow and divide abnormally, giving it an advantage that allows it to outcompete normal cells nearby[3]. A passenger mutation, by contrast, rides along in the same cell without giving it any growth advantage on its own[4]. Most tumors contain only a small number of driver mutations, but they can carry many more passenger mutations that came along for the ride during repeated cell division[4]. Doctors focus on driver mutations because these are usually the changes responsible for the cancer and the ones that current treatments are designed to target.
How Do Changes in Proto-Oncogenes Promote Cancer?
Changes in proto-oncogenes promote cancer by turning a gene that normally supports healthy growth into one that pushes growth forward without the usual limits[1]. A proto-oncogene works like a gas pedal that a normal cell presses only when a growth signal calls for it. Once damaged, the same gene can behave as though the pedal is pressed down at all times.
How Does a Proto-Oncogene Become an Oncogene?
A proto-oncogene becomes an oncogene when a genetic change makes the gene more active than normal or makes it impossible for the cell to switch off[1]. This can happen in a few different ways, including a change in the gene's own sequence, extra copies of the gene being made, or the gene being moved next to a different part of the DNA that keeps it constantly switched on. Once this happens, the resulting oncogene keeps signaling the cell to grow even when no real growth signal is present.
How Do Tumor Suppressor Genes Normally Prevent Cancer?
Tumor suppressor genes normally prevent cancer by slowing or stopping cell division, helping repair damaged DNA, or triggering the removal of a cell that is too damaged to fix[1]. These genes work like a combined brake pedal and inspection station, holding growth back until everything checks out.
What Happens When Tumor Suppressor Genes Stop Working?
A cell usually needs both working copies of a tumor suppressor gene to be damaged or lost before that particular brake fails completely[1], [2]. A person is normally born with two copies of most genes, one from each parent. If only one copy is damaged, the other working copy can often still do the job. Cancer risk rises sharply once both copies stop working, since nothing is left to hold that particular safety check in place.
How Can Defects in DNA Repair Genes Promote Cancer?
Defects in DNA repair genes promote cancer indirectly, by allowing mistakes elsewhere in the DNA to build up faster than they normally would[1], [2]. A healthy cell relies on its repair genes to catch and correct copying errors before they are passed on to new cells. When these repair genes are themselves damaged, that proofreading step weakens, and other genetic changes, including changes in proto-oncogenes and tumor suppressor genes, are able to accumulate more quickly than usual.
Can Changes in Gene Regulation Cause Cancer Without Changing the DNA Sequence?
Yes, changes in gene regulation, called epigenetic changes, can promote cancer without changing the underlying DNA sequence at all[4]. A useful comparison is a light switch and its wiring. The DNA sequence is the wiring itself, while epigenetic changes work more like a hand that flips certain switches on or off, changing which lights are lit without touching a single wire.
How Can Epigenetic Changes Alter Cancer-Related Genes?
Chemical tags, most often through a process called DNA methylation, can attach to a gene and silence it, or can be removed from a gene to switch it on, without changing the gene's DNA sequence[4]. In cancer, this process can silence a tumor suppressor gene as effectively as a direct mutation would, or activate an oncogene that would otherwise stay quiet. Because these changes do not alter the DNA sequence itself, they are, in principle, reversible, which is part of why researchers are interested in medicines that target this process.
Why Are Several Genetic Changes Usually Needed Before Cancer Develops?
Several genetic changes are usually needed because a single change rarely disables enough of the cell's safety checks on its own[2], [4]. A cell generally needs to lose the function of at least one tumor suppressor gene, gain an active oncogene, and often pick up further changes affecting DNA repair or gene regulation before it behaves as a true cancer cell. This is similar to a vehicle with several separate safety systems. Losing the brakes alone might not cause a crash if the steering and other controls still work normally; a crash becomes far more likely only once several systems fail together.
How Do Cancer-Promoting Changes Build Up Over Time?
Cancer-promoting changes build up over time through a process similar to natural selection, in which a cell that gains a helpful change divides faster than its neighbors and leaves behind more descendants carrying that same change[3], [4]. Over months or years, one of those descendant cells may pick up a second helpful change, and its own descendants then outcompete the rest. This repeats gradually, generation of cells after generation of cells, until a lineage has accumulated enough combined changes to behave as cancer.
Are Cancer-Causing Genetic Changes Always Inherited?
No, most cancer-causing genetic changes are not inherited. The large majority develop during a person's lifetime rather than being passed down from a parent[1]. In a smaller number of cases, a person is born with one damaged copy of a cancer-related gene already inherited from a parent, which raises their lifetime risk but does not guarantee that cancer will occur.
What Is the Difference Between Germline and Somatic Changes?
Germline changes are present in the egg or sperm cell and are therefore copied into every cell of a person's body from birth, which means they can be passed on to children[1]. Somatic changes, by contrast, occur later in a specific cell or tissue during a person's lifetime and are only copied into that cell's own descendants, so they are not passed on to children. Most of the genetic changes involved in cancer are somatic rather than germline.
Do All Cells With Cancer-Related Mutations Become Cancer Cells?
No, a single cell carrying even a genuine driver mutation does not automatically become a cancer cell[2], [4]. The body constantly produces cells with small genetic changes, and most of these cells are repaired, removed by programmed cell death, or simply outcompeted by their normal neighbors before they cause any harm. Only in rare cases does a single lineage of cells manage to build up enough combined changes, avoid the body's normal safeguards, and go on to develop into cancer.
Frequently Asked Questions
Can a Person Inherit a Cancer-Related Gene and Never Develop Cancer? Yes, inheriting a damaged copy of a cancer-related gene raises a person's lifetime risk but does not guarantee that cancer will develop. Many people who carry an inherited change in a gene such as a tumor suppressor gene never go on to develop cancer, because the remaining healthy copy of the gene, along with other protective factors, may continue to function well enough throughout that person's lifetime.
Is There a Single Gene That Causes All Cancers? No, there is no single gene responsible for all cancers. Different cancers, and even different cases of the same type of cancer, commonly involve different combinations of genetic changes. Some genes, such as certain tumor suppressor genes, are altered relatively often across many cancer types, but no single gene accounts for the disease on its own.
Can Diet or Lifestyle Change the Genes Involved in Cancer? Diet and lifestyle do not typically rewrite a person's DNA sequence, but some habits, including smoking and long-term sun exposure, are established causes of the DNA damage that can lead to cancer-related mutations over time. Research also suggests that some lifestyle factors may influence epigenetic changes, which affect how genes are switched on or off without altering the DNA sequence itself, although this area of research is still developing and specific dietary claims should be treated with caution.
Do Doctors Test Tumors for These Genetic Changes? Yes, testing a tumor's genetic changes, often called biomarker testing or molecular testing, is now a routine part of care for many types of cancer. Identifying which specific driver mutations are present in a person's tumor can help doctors choose treatments that are designed to target those particular changes, rather than relying on a one-size-fits-all approach.
Can Epigenetic Changes Be Reversed? Epigenetic changes are, in principle, reversible, since they do not alter the underlying DNA sequence. This is different from a direct DNA mutation, which cannot simply be switched back. Researchers are actively studying medicines designed to reverse harmful epigenetic changes in cancer cells, though this remains an active area of research rather than a routine treatment for every type of cancer.
Morais C. How Do Genetic Changes Turn a Normal Cell into Cancer? [Internet]. BiologyWithin.com; 2026 Sep 7. Available from: https://www.biologywithin.com/2026/09/how-genetic-changes-turn-normal-cell-into-cancer.html
- National Cancer Institute. What is cancer? Cancer.gov. 2026.
- Koya AI, Ibrahim SA. Carcinogenesis. StatPearls [Internet]. 2024.
- National Cancer Institute. Driver mutation. NCI Dictionary of Cancer Terms.
- Shlyakhtina Y, Moran KL, Portal MM. Genetic and non-genetic mechanisms underlying cancer evolution. Cancers (Basel). 2021;13(6):1380.
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Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about personal health concerns.
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