P53 mutants are found in more than half of human cancers and ar e considered as the most important human cancer related gene. p53 is detected at 53 kDa position by electrophoresis and is composed of 393 amino acids. In the unstressed normal cells the p53 level is low and it is inactive. However, with stress, especially with DNA damage, it is activated to promote arrest of cell cycle and repair of DNA damage, or induction of apoptosis. The functions of p53 are regulated by the phosphorylation of serine and threonine, and th e acetylation of lysine at various sites in the molecule (Ref 1). p53 at Ser20 is phosphorylated by Chk1 and Chk2 kinases and this enhances tetramerization, stability and activity of p53 (Ref 2,3)Synonyms: Cellular tumor antigen p53, NY-CO-13, Phosphoprotein p53, Tumor suppressor p53