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.
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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 changed into oncogenes, they push growth too hard. Tumor suppressor genes normally slow growth or remove damaged cells; 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 — from normal aging, from things like tobacco smoke or ultraviolet light, or simply 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.
Words to know
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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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