The human body is made up of a vast collection of cells and every cell contains 23 pairs of chromosomes. Winding through each pair is a double spiral of DNA molecules, the genetic blueprint for life. Each of these molecules contains numerous numbers of genes.

Almost all our cells need to be replaced regularly. Some cells divide every few hours and are shed after living for only a few days. Others live for years. The process of all divisions and growth are controlled by genes that start and stop the growth process. Some of these growth-controlling genes may undergo changes (mutation) that cause them to malfunction and cell growth is then uncontrolled........