Tumour suppressor protein. Molecular model of the tumour suppressor protein p53 (left and right) bound to a molecule of DNA (deoxyribonucleic acid, down centre) at the p53 response element. P53 prevents the proliferation of cells with damaged DNA. Its production occurs in response to radiation and chemicals that damage the structure of DNA. The protein binds to specific sequences in the DNA and halts the cell cycle (the means by which cells replicate). If there is only minor damage p53 activates genes involved in DNA repair, but if the damage cannot be repaired it initiates cell death (apoptosis). P53 plays a very important role in preventing the replication of cancerous cells, so much so that p53 is found to be inactive in 50% of all cancers.