TP53 and cancer: P53 mutations occur in almost half cancer types ever known (Hollstein et al., 1991), and loss of p53 function is linked to drug resistance in several tumor entities, including carcinoma of the breast, ovary, and colorectum as well as melanoma, acute lymphoblastic leukemia, neuroblastoma and osteosarcoma (Li et al., 1998; Asada et al., 1999; Lam et al., 1999; Righetti et al., 1999; Berns et al., 2000).