cancer as a disease of the genome

Every cell in your body follows a set of rules: grow when told, stop when told, repair your DNA, and if you are too damaged, kill yourself for the good of the whole. Cancer is what happens when a single cell tears up that rulebook — and the rulebook is written in DNA. So at its root, cancer is a disease of the genome: it begins when the genes that enforce the rules get damaged.

It almost never takes one mutation. Cancer is a multistep process: a cell picks up a damaging mutation, divides, its descendants pick up more, and over years a lineage accumulates the handful of changes needed to break free. The mutations cluster in two kinds of genes — accelerators (oncogenes) jammed permanently on, and brakes (tumour-suppressor genes) snapped off — plus damage to the DNA-repair genes that should have caught the errors, which then lets mutations pile up even faster. This is Darwinian evolution running inside your body on a timescale of years: cells with mutations that help them grow outcompete their neighbors, so a tumour is a population that has been selected for selfishness. Crucially, almost all of these are somatic mutations, arising in body cells during life, not inherited from a parent.

Reading cancer this way changed everything. Sequencing a tumour's genome reveals exactly which genes are broken, which lets doctors match a drug to the specific fault rather than to the organ where the cancer sits — the heart of precision oncology. The honest limits are sobering: tumours are genetically diverse within a single patient and keep evolving, so they develop resistance to drugs the way bacteria develop resistance to antibiotics; and most cancer mutations are not inherited, so a clean family history does not mean you are safe.