p53, p63 and p73 are members of the p53 gene family involved in development, 
differentiation and response to cellular stress. p53 gene is a transcription 
factor essential for the prevention of cancer formation. The p53 pathway is 
ubiquitously lost in human cancer either by p53 gene mutation (60% of cancers) 
or by lost of cell signalling upstream and downstream of p53 in the remaining 
cancers expressing WTp53 gene. As p53 pathway inactivation is a common 
denominator to all cancers, the understanding of p53 tumour suppressor activity 
is likely to bring us closer to cancer therapy. However, despite all the 
experimental evidences showing the importance of p53 in preventing 
carcinogenesis, it is difficult in clinical studies to link p53 status to cancer 
treatment and clinical outcome. The recent discovery that p53 gene encodes for 
nine different p53 proteins (isoforms) may have a profound impact on our 
understanding of p53 tumour suppressor activity. Studies in several tumour types 
have shown that the nine different p53 isoforms are abnormally expressed in 
tumour tissues compared to normal cells. p53 protein isoforms modulate p53 
transcriptional activity and cell fate outcome in response to stress. Regulation 
of p53 function in normal and tumour tissues in man is likely to be more complex 
than has been hitherto appreciated. Therefore, the tumour p53 status needs to be 
determined more accurately by integrating p53 isoform expression, functional p53 
mutation analysis and a panel of antibodies specific of p53 and of its target 
genes.