P53 is one of the most important tumour suppressor proteins. In resting cells, p53 is in a latent state. In the presence of DNA damage or upon other conditions with oncogenic potential, p53 becomes rapidly activated and slows down proliferation and/or eliminates cells with damaged DNA from the population. The importance of these activities becomes particularly clear in patients with Li-Fraumeni Syndrome. Carriers of this disease have inactivating mutations of p53 in the germ line which leads to tumourigenesis at a particular early age.