multi-hit model
/ MUL-tee-hit MOD-el /
Think of a bank vault protected by several independent locks. A thief who picks just one lock still cannot open it; only when every lock has been defeated does the vault swing open. Your cells are guarded the same way — several different safeguards must all fail before a cell turns fully cancerous. The multi-hit model is the idea that cancer is not caused by a single mutation but by an accumulation of several separate genetic 'hits' in the same cell line.
More precisely, the model says a cell becomes cancerous step by step: it might first gain a mutation that switches on an oncogene (a stuck accelerator), then lose one copy of a tumor suppressor, then lose the second copy, then break a DNA-repair gene, and so on. Each hit gives that cell line a small growth advantage, so its descendants multiply and become a larger target for the next mutation. The famous special case is Alfred Knudson's 'two-hit hypothesis' for tumor suppressors, where both copies of a gene like RB1 must be knocked out before its protection is gone.
This model explains several real-world facts at once. It explains why most cancers take many years to develop and why their incidence rises steeply with age — there is more time to collect hits. It explains why inherited cancers tend to appear earlier: a person born with one hit already in place needs fewer additional ones. And it offers hope, because interrupting the chain at any step — by repairing DNA, removing carcinogens, or early detection — can prevent the full sequence from completing.
Colon cancer is a textbook example: tumors often progress through a known sequence of hits — losing the APC tumor suppressor, then activating the KRAS oncogene, then losing p53 — each step matching a more advanced, more dangerous growth. Researchers can almost read the tumor's history in the order of its mutations.
Cancer usually needs several mutations to accumulate, not just one.
A single mutation rarely causes cancer; this is also why cancer risk climbs with age, as there is more time to accumulate hits.