Tumour protein p53 binding to Mdm2 protein. Mdm2 protein is shown as ribbons, while the binding site of p53 (which is much larger than the part seen here) is shown as a ball and stick model. p53 is a protein that plays an important part in the cell cycle of cell division. It checks the cell's DNA for errors and damage during the cycle, and either repairs the DNA or stops the cycle if excessive damage is found. An absence of p53 allows the replication of damaged DNA, which has been implicated in many cancers. Mdm2 is a protein that inactivates p53. Drugs that bind to and inactivate Mdm2 are therefore thought to have anticancer potential. In the p53 fragment, atoms are colour-coded: carbon is grey, oxygen red, nitrogen blue and sulphur yellow.