Not all mutations that lead to cancerous cells result in the cells reproducing
at a faster, more uncontrolled rate. For example, a mutation may simply cause a
cell to keep from self-destructing. All normal cells have surveillance
mechanisms that look for damage or for problems with their own control systems.
If such problems are found, the cell destroys itself.
Over time and after many cell divisions, a third mutation may arise. If the
mutation gives the cell some further advantage, that cell will grow more
vigorously than its predecessors and thus speed up the growth of the