Mutation of the p53 protein may represent the commonest genetic event in human malignancy. Mutation results in the overexpression and thus detectability of the protein by immunohistochemistry. p53 appears to have both a function as an oncoprotein, perhaps by complexing in its mutant form with wild type p53, and as a tumour suppressor gene in its normal state. The gene for p53 is located on chromosome 17p, a frequent site of allele loss in many tumours including lung colon, breast and brain. The p53 oncoprotein is expressed at a level detectable by immunohistochemistry in carcinomas from various sites including breast, colon, bladder and lung and the role of the protein in malignant disease is currently being investigated in many centres. Expression has been shown to correlate with poor prognosis in breast cancer. In colonic tumours, p53 was expressed in 47 per cent of cancers and in 9 per cent of adenomas. Expression was not detected in normal mucosa.