Approximately half of human tumors bear p53 mutations (Hollestein et al., 1997). The most prevalent type consists of missense mutations that are frequently accompanied by loss of the remaining wild-type p53 (wt-p53) allele (Hainaut et al., 1997; Levine, 1997). The major site of the p53 mutations is the highly conserved DNA binding core domain (Hussain et al., 1998; Prives et al., 1999). Thus, mutant p53 (mt-p53) proteins are unable to specifically bind DNA and to activate specific wt-p53 target genes. Unlike wt-p53, whose half-life is short, mutant p53 proteins are quite stable and abundantly present in cancer cells. One certain outcome of p53 mutations is the loss of wild type activities such as growth arrest, apoptosis, and differentiation (Michalovitz et al., 1990; Yonish-Rouach et al., 1991; Soddu et al., 1996; Almog et al., 1997). However, at variance with other tumor suppressor genes, cells with p53 mutations maintain expression of the fulllength protein. This may suggest that, at least certain mutant forms of p53 can gain additional functions through which actively contribute to cancer progression (Prives et al., 1999; Sigal et al., 2000; Strano et al., 2001; Bullock et al., 2001). Such evidence is provided by several in vitro and in vivo studies (Haley et al., 1990; Dittmer et al., 1993; Gualberto et al., 1998; Frazier et al., 1998; Li et al., 1998; Blandino et al., 1999; Aas et al., 1996; Irwin et al., 2003; Strano et al., 2003).
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