A generation or two ago the word cancer was avoided in polite conversation. The diseases collectively known as cancer seemed at that time somehow too horrible and too fatal to discuss. Each molecule of DNA contains thousands of individual genes, the units of heredity.
Genes have the power to direct the production of specific proteins.
These proteins then carry out the development and operation of the cell. In nearly all cancers more than one gene must be damaged for the cells to reproduce and spread. Because such a combination is needed, cancers develop in stages. In the earlier stage, some of the DNA is defective, but by itself the damaged genes cannot greatly affect cell growth. Later another gene is damaged by something in the environment or perhaps just in the course of normal cell division.