Cancer is a result of the breakdown of the controls that regulate cells. The causes of the breakdown always include changes in important genes. These changes are often the result of mutations, changes in the DNA sequence of chromosomes. Mutations can be very small changes, affecting only a few nucleotides or they can be very large, leading to major changes in the structure of chromosomes.

Both small and large mutations can affect the behavior of cells. Combinations of mutations in important genes can lead to the development of cancer. The material covered in these sections describes the relationship between mutation and cancer, the different kinds of mutations and what causes them.