Illustration of the DNA-binding domain (red) of a p53 tumour suppressor protein bound to a molecule of DNA (deoxyribonucleic acid), with a nucleosome (green) in the background. Nucleosomes are the fundamental repeating unit used to package DNA inside a cell. They consist of DNA wrapped around histone proteins, a material known as chromatin. The p53 DNA-binding domain targets a short DNA sequence near the entry/exit region of the nucleosome. Binding of p53 changes the chromatin structure for gene activation. P53 prevents the proliferation of cells with damaged DNA. When it binds to DNA it 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).