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DNA and tumour suppressor complex. Molecular model showing a molecule of the p53 tumour suppressor protein (green, centre) bound to a 'damaged' DNA (deoxyribonucleic acid) strand. p53 is an important checkpoint in the cell cycle, ensuring the integrity of DNA replication and thus supressing tumours and preventing cancers. Defects in p53 proteins, caused by defects in the TP53 gene that encodes them, are implicated in 50% of all cancers. Here such as defect has caused the DNA strand to be replicated poorly. (KEYSTONE/SCIENCE PHOTO LIBRARY/LAGUNA DESIGN)