All cancer is genetic, caused by changes in genes, but in most people, these changes have occurred sporadically by chance or with age. However, some people have developed cancer, or are at high-risk of certain cancers, because of a genetic susceptibility they have inherited.
Genes are present in every cell in our body, contained in structures called chromosomes, which are found in the nucleus of a cell. We have 46 chromosomes that come in 23 pairs. One chromosome in each pair comes from our father, and the other comes from our mother. Each chromosome, in turn, is made up of thousands of genes. Genes are coded segments of DNA that, like chromosomes, come in pairs. DNA contains instructions for making the substances that help our bodies to function properly. A change in the sequence of the DNA that makes up the gene can cause the gene to stop functioning. These genetic changes are called mutations.