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.

In addition, p53 is increasingly recognized as a DNA repair factor and it also has some, although as yet poorly characterized, functions during embryonic development. While maintaining genomic integrity is important for differentiated cells, it is absolutely fundamental for stem cells since this cell type provides the proliferative pool of a whole organism. Stem cells therefore should respond very efficiently to lesions in the DNA.

The altered DNA damage response and the poorly characterized function of p53 in stem cells raises several questions. We are particularly focusing on the following issues:

• How do stem cells respond to DNA damage and what is the role of p53?
• Is p53 important for stem cell development and if yes, what is its function?