Tumour suppressor p53 degraded by proteasome. Computer illustration showing the structure of a molecule of tumour protein p53 (blue), tagged with ubiquitin (yellow), being degraded into small peptides by a proteasome (violet). p53 prevents cancer formation by acting as a tumour suppressor. It binds to the DNA (deoxyribonucleic acid) of cancerous cells and triggers self-destruction of that cell. Mutations in the p53 gene prevent it from carrying out this roll and therefore contribute to about half of the cases of human cancer. Proteasomes are protein complexes that degrade unneeded or damaged proteins.