Poly(ADP-ribose) polymerase-1 (PARP-1) is a nuclear DNA binding protein that participates in processes involving nicking and resealing DNA strands. A genomically unstable subpopulation of PARP-1(-/-) cells has recently been described, which disappears after stable transfection of the cells with complete PARP-1 cDNA. Here we investigate the role played by]PARP-1 in the maintenance of genomic stability, independently of its enzymatic activity. We used a PARP-1-deficient cell line to express a DNA construct encoding the PARP-1 DNA-binding domain (DBD) fragment and one encoding the mutant DBDbd-, defective in binding to DNA strand breaks. We found that, in the absence of DNA damage, expression of DBD or DBDbd- mutant induces increased genomic instability in the PARP1(-/-) cells. These results suggest that the DBD fragment of PARP-1, apart from its classical role of nick detection and DNA binding, is likely to participate in molecular complexes with proteins involved in genomic integrity.