The short answer
Cancer begins when changes in a cell's genes let it grow out of control. These changes can switch on growth genes, break the genes that normally stop growth, or disable repair.
Cancer is a genetic disease — it is caused by changes in genes that control how cells grow.
These gene changes can be inherited, but most happen during a person's life.
Oncogenes are changed genes that tell cells to grow too much.
Tumor suppressor genes normally slow growth; cancer can switch them off.
Watch: What actually causes cancer?
1 min · Captioned · Cancer starts with gene changes that build up over years — here's the mechanism.
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The full explanation.
Cancer is a disease of genes
Every cell follows instructions stored in its genes. Cancer begins when some of those instructions get changed and a cell starts to grow and divide when it should not. This is why cancer is called a genetic disease — though that does not mean it is always inherited.
Three kinds of genes that matter
Scientists focus on three groups of genes. Proto-oncogenes help cells grow normally. When they are changed into oncogenes, they push growth too hard. Tumor suppressor genes normally slow growth or remove damaged cells, and cancer can switch them off. DNA repair genes fix copying mistakes. When they fail, other changes pile up faster.
It usually takes several of these changes in the same cell, over time, for cancer to form.
Where the changes come from
Some gene changes are inherited from a parent. But most happen during a person's life. They come from normal aging, from things like tobacco smoke or ultraviolet light, or from random errors when cells copy their DNA. This build-up over years is part of why cancer becomes more common with age.
Why this matters for treatment
Knowing which genes are changed in a tumor can guide treatment. Some newer drugs are designed to target a specific gene change, which is why doctors sometimes order genetic tests on the tumor itself.
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Common questions
Is cancer caused by one change?
Usually not. Cancer typically develops after several gene changes build up in the same cell over time, which is part of why cancer risk rises with age.
What is an oncogene?
An oncogene is a gene that has been changed so that it drives too much cell growth. A normal version of the gene (a proto-oncogene) has a healthy job in the cell.
What is a tumor suppressor gene?
A tumor suppressor gene normally helps control cell growth or triggers damaged cells to die. When these genes are turned off, cells can grow unchecked.
Are all gene changes inherited?
No. Most cancer-related gene changes happen during life from things like aging, tobacco, UV light, and chance copying errors. Only some are inherited from a parent.
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Written by: Cancer ExplainedSources last checked: 2026-07-14 what this meansLast updated: 2026-08-11Next planned review: 2028-07-14
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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