The p53 tumor suppressor protein plays a critical role in the cellular response to DNA damage leading to cell cycle arrest or apoptosis depending on cell type, culture conditions, and the extent of DNA damage. Loss of the p53-dependent DNA damage response can lead to genomic instability and the survival of cells carrying mutations and carcinogenic lesions thereby contributing to malignancy (1). DNA strand breaks produced by ionizing radiation (IR) 1 or by DNA repair intermediates following treatment with UV radiation or chemotherapeutic agents result in the accumulation of p53 protein and in the activation of its transcriptional activity (reviewed in Refs. 2 and 3). Elevated levels of p53 protein are believed to be important to initiate the events that lead to G 1 arrest or apoptosis after DNA damage. There is compelling evidence that post-translational modification of p53 is required for its stabilization, as well as for activation of its latent sequence-specific DNA-binding and transactivation functions. Once p53 becomes activated it binds as a tetramer to p53 responsive elements on double stranded DNA consisting of two half-sites (5Ј-PuPuPuC(A/T)(T/A)GPyPyPy-3Ј) separated by a spacer consisting of 0 -13 nucleotides (4). The site-specific DNA-binding activity of p53 leads to transcriptional activation of p53 target genes. Covalent modification of p53 has also been shown to regulate its subcellular localization, tetramerization, interaction with other proteins, and degradation. p53 protein is modified in vivo through phosphorylation, acetylation, poly(ADP-ribosyl)ation, ubiquitination, and sumoylation reactions (reviewed in Refs. 2 and 5).The events upstream of p53 activation are complex and not well understood. Several DNA damage sensory molecules are believed to relay the DNA damage signal to p53; each may be involved in the response to one or more types of DNA damage. For example, ataxia-telangiectasia-mutated kinase protein is involved in the activation of p53 in response to IR (6, 7), and ataxia telangiectasia-related kinase protein is involved in the activation of p53 in response to UV irradiation (8).Poly(ADP-ribose) polymerase (PARP-1) is an abundant nuclear enzyme that binds to, and is activated by, DNA single and double strand breaks (reviewed in Refs. 9 -11). Its activation represents one of the earliest responses to DNA damage in the cell. PARP-1 catalyzes the sequential transfer of ADP-ribose monomers onto nuclear protein acceptors using NAD ϩ as substrate. During the process of poly(ADP-ribosyl)ation, NAD ϩ is hydrolyzed and released as free nicotinamide. More than 30 nuclear proteins have been identified as poly(ADP-ribose) (pADPr) acceptors, with PARP-1 itself being the major target, via its automodification domain. pADPr acceptor proteins may be modified through covalent as well as through non-covalent association with pADPr, either free or bound to PARP-1. Poly-(ADP-ribose) glycohydrolase is the major enzyme responsible for the hydrolysis of pADPr. There is general agreement, based on genetic ...