A gene found in every cell in your body. It tells the cell's machinery how to make a protein, also called p53. This protein has several roles in a normal cell, the most important of which is to stop cells dividing if they become damaged - it is a tumour suppressor gene. If the p53 gene in one of your cells becomes damaged and starts to make a protein that does not function properly, that cell can become cancerous. Damaged ( mutated) p53 genes are found in nine out of ten cancers.