The tumor suppressor gene p53, is a transcription factor involved in cell cycle arrest, apoptosis and cell senescence. In normal cells, DNA damage activates a signal cascade that results in p53 undergoing a number of posttranslational modifications, one of these being acetylation. p53 is acetylated by HATs (Histone Acetyltransferases) p300 and CBP in the C-terminal regulatory domain at lysine residues. Published data have shown that acetylation by these HATs increases its stability being less susceptible to MDM2 mediated degradation, increases DNA binding in-vitro and increases transcription activation of promoters of downstream target genes, such as the cdk inhibitor p21. Coincidently, the mentioned lysine residues have been shown to be ubiquitinated by MDM2 ubiquitin ligase, tagging p53 for nuclear export and proteosomal degradation. This possibility of acetylation versus ubiquitation at the same lysines has led to a model where there is a competition between the HATs and MDM2, helping to govern the level of p53 in the cell. To further study the role of these lysine residues in p53 regulation and function, p53 mutants have been generated. Lysine residues were substituted with glutamine residues (which is to structurally mimic acetylated lysine) or arginine residues (which is to mimic unmodified lysine).