p53 was discovered in 1979 as a cellular protein associating with the transforming protein of SV40 tumor virus. Since then, many different biochemical functions have been attributed to the 53 kD phosphoprotein. Experimental evidence has suggested that p53 acts as a negative regulator of cell growth in normal cells (Finlay, 1989). Thus, the inactivation or mutation of p53 may be an essential step in the development of malignancy (Lane and Benchmol, 1990). Wild-type p53 levels in normal cells and tissues were found to be very low. Mutant p53 polypeptide, however, is often found to be present at high concentrations in mammalian tumors and tumor cell lines. For example, in an immuno-histochemistry study 40% of human breast cancer showed elevated levels of mutant p53 in the cell nucleus. Mutations of the p53 protein have some characteristic features: a) Most of them are missense point mutations giving rise to an altered protein function. b) Many -but not all- mutant p53 proteins