The tumor suppressor p53 is a multifunctional protein whose main duty is to preserve the integrity of the genome. This function of wild-type p53 as "guardian of the genome" is achieved at different levels, as a cell cycle checkpoint protein, halting the cell cycle upon DNA damage, and via a direct involvement in processes of DNA repair. Alternatively, p53 can induce apoptosis. Mutations in the p53 gene occur in about 50% of all human tumors and eliminate the tumor suppressor functions of p53. However, many mutant p53 proteins have not simply lost tumor suppressor functions but have gained oncogenic properties which contribute to the progression of tumor cells to a more malignant phenotype. The molecular basis for this gain of function of mutant p53 is still unknown. However, mutant (mut) p53 specifically binds to nuclear matrix attachment region (MAR) DNA elements. MAR elements constitute important higher order regulatory elements of chromatin structure and function. By binding to these elements, mut p53 could modulate important cellular processes, like gene expression, replication, and recombination, resulting in phenotypic alterations of the tumor cells. Mut p53 thus could be the first representative of a new class of oncogenes, which exert their functions via long-range alterations or perturbation of chromatin structure and function.