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Mitosis is closely controlled by the genes inside every cell. Sometimes this control can go wrong.
If that happens in just a single cell, it can replicate itself to make new cells that are also out of control. These are cells.
They continue to replicate rapidly without the control systems that normal cells have.
Cancer cells will form lumps, or tumours, that damage the surrounding tissues. Sometimes, cancer cells break off from the original tumour and spread in the blood to other parts of the body.
When a tumour spreads to another part of the body it is said to have metastasised.
They continue to replicate and make more tumours. These are called secondary tumours.