In normal adult tissues, the rate at which new cells are made (cell growth) and old cells die is in balance. Genes inside each cell maintain this balance by controlling how the cell behaves.
There are many different types of genes that control cell behaviour. Cancer is the result of faults (mutations) in genes that control cell growth and death and that repair mistakes in our DNA. Cells with these mutations may begin to grow in an uncontrolled way, forming tumours. Usually several gene mutations must happen in a group of cells before cancer develops. This can take many years, and is why cancer is more common in older people.