Poly(ADP-ribose) polymerization is a unique post-translation protein modification that utilizes an ADP-ribose moiety from NAD+ to form long and branched polymers attached via glutamic acid residues to nuclear acceptor proteins. The corresponding enzyme, poly(ADPribose) polymerase (PARP-1), is a zinc finger-containing protein, which allows PARP-1 binding to either double- or single-strand DNA breaks. The catalytic activity of PARP-1 is strictly dependent on the presence of strand breaks in DNA, and is modulated by the level of automodification. PARP-1 is regarded as an intracellular sensor for DNA strand breaks, and its function has been implicated in cellular processes that require DNA cleavage and rejoining reactions, such as DNA replication, recombination and repair. Recent studies have also implicated PARP-1 in the regulation of gene expression through modification of transcription factors by poly(ADP-ribosyl)ation or its direct binding to gene-regulating DNA sequences. The latter is attributable to PARPs ability to recognize and bind to various structural discontinuities in the DNA duplex in the absence of DNA strand breaks, such as three- or four-way junctions, bent DNA, and base unpaired regions. Cumulatively, these findings indicate that PARP-1 plays a pivotal role in the maintenance of the genome integrity during the normal functioning of eukaryotic cells as well as in the cellular responses to DNA damage, and that PARP-DNA interactions are indispensable for PARP function. This review summarizes the data on DNA-binding properties of PARP- 1 and relates them to the development of strategies for sensitizing tumor cells to genotoxic treatments.