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They are mutated. But the biggest thing that is different and what makes them so dangerous is that their "checkpoints" don't work. Basically, cells produce new cells. And in this cycle, there are several 'checkpoints', where the process could be aborted if things aren't right. With cancer, the checkpoints don't work, so the cells just keep duplicating and the checkpoints don't stop them. An analogy would be being in a car with the gas pedal being stuck to the floor. Compounding this, the new cancer cells can change the normal cells into cancer cells, which then do the same thing...reproduce and convert more normal cells. These are a few of the reasons why cancer is difficult to "cure".