They do not, however, develop polyps in the same number as in the FAP syndrome. They were relatively easy to discover because they exert strong effects. Cancer cells grow and spread in ways that normal cells do not. Cancer cells do this because their genes are different from the genes in normal cells. In other words, tumors have their own genetics.
Cancer starts in a normal cell when genes have become damaged.
Multiple genes must be damaged before a normal cell becomes a cancer cell. The diagram below shows one possible cascade of genes for colorectal cancer. Each gene has to be damaged to develop cancer. No one knows if this is all or some of the genes that must be damaged. Nor does anyone know if the damage must occur in a certain order.