oncogenes and tumour-suppressor genes
/ TOO-mer sup-PRESS-or /
Think of a car. To drive safely you need a working accelerator and a working brake. A cell deciding whether to divide has the same two systems: genes that say "grow now" and genes that say "stop, wait, repair." Cancer is a crash, and it happens when the accelerator gets stuck down or the brakes fail — or both.
The accelerators are called oncogenes. In a healthy cell their normal form (a proto-oncogene) tells the cell to grow only when it should; a mutation can jam this gene permanently on, so the cell hears "grow" forever. The crucial point is that this is a gain-of-function change, and one over-active copy is enough — like a gas pedal stuck to the floor, you only need one. The brakes are the tumour-suppressor genes. Their job is to slow division, repair DNA, or trigger a damaged cell to kill itself. They cause trouble when they are lost, and because there are usually two copies, you typically have to knock out both before the brake fails — a "two-hit" pattern. The famous tumour suppressor TP53 is nicknamed "the guardian of the genome" because it halts division and orders self-destruction when DNA damage is severe; it is mutated in about half of all human cancers.
This accelerator-and-brake picture is the core logic of cancer genetics and the reason cancer needs multiple mutations to take hold — you must press the gas and cut the brakes. It also explains a clinical asymmetry: drugs can often block an over-active oncogene protein (you can plug a stuck accelerator), but it is far harder to restore a missing brake, so for decades tumour suppressors were called "undruggable." That is finally starting to change, but it remains one of cancer therapy's hardest problems.
Children born with one already-faulty copy of the tumour-suppressor RB1 develop the eye cancer retinoblastoma when a chance mutation knocks out the second copy in a single retinal cell — the classic "two-hit" case where inheriting the first hit makes the cancer far more likely.
Oncogene = one stuck-on copy is enough; tumour suppressor = usually both must be lost.
A common confusion: oncogenes act dominantly (one over-active copy causes trouble) while tumour suppressors usually need both copies knocked out — that asymmetry is why suppressors are so much harder to drug.