Tumour suppressor protein and DNA. Computer artwork showing a molecule of the tumour suppressor protein p53 (blue and pink) bound to a molecule of DNA (deoxyribonucleic acid, yellow and orange). p53 is crucial in multicellular organisms, where it regulates the cell cycle (the means by which cells replicate) and prevents the proliferation of cells with damaged DNA. It is produced in response to radiation and chemicals that damage the structure of DNA, when it binds to specific sequences in the DNA and halts the cell cycle. 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