A cell that forgot the rules
Your body is built from trillions of cells, and almost all of them follow strict rules about when to divide and when to stop. A skin cell divides to heal a cut, then settles down. A liver cell waits patiently for a signal. Cancer begins when a single cell stops obeying those rules — it keeps dividing when it should rest, refuses to die when it should, and slowly builds up into a mass of cells that crowd out and damage healthy tissue.
Where do those rules live? In the cell's DNA. Genes carry the instructions for the proteins that tell a cell to grow, to pause, to repair itself, or to die. When a mutation damages one of those instructions, the cell can lose a brake or get a stuck accelerator. That is why we call cancer a disease of the genome: it is driven by changes in DNA, even when those changes were never passed down from a parent.
“Genetic” does not always mean “inherited”
This is the single most confusing point in cancer genetics, so let's be careful. A mutation that happens in a body cell during your lifetime — from sunlight, smoke, a copying error, or just bad luck — is a somatic mutation. It affects only that cell and its descendants, and it cannot be passed to your children. The vast majority of cancers (often estimated around 90–95%) arise from such somatic changes and are called sporadic.
A mutation present in the egg or sperm — a germline mutation — is different. It sits in every cell of the body from birth and can be inherited. When such a mutation raises cancer risk, it can run in a family. We will explore that hereditary side in guide 4. For now, hold both ideas at once: cancer is always genetic at the level of the cell, but only sometimes hereditary at the level of the family.
It takes more than one change
A single mutation almost never causes cancer. Cells have many overlapping safeguards, so one broken brake is usually not enough — backup systems catch the problem or push the cell to self-destruct. Cancer typically requires several mutations to accumulate in the same cell lineage, knocking out different safeguards one after another. This is why most cancers take years to develop and why incidence rises steeply with age: it simply takes time to collect enough hits.
Over the next four guides we will name the genes behind these brakes and accelerators. The accelerators-stuck-on are oncogenes; the brakes-knocked-out are tumor-suppressor genes. Keep the picture simple for now: cancer is one cell's genome going wrong, step by step, until growth runs free.