Poly(ADP-ribose) polymerase (PARP) is implicated in the maintenance of genomic integrity, given that inhibition or depletion of this enzyme increases genomic instability in cells exposed to genotoxic agents. We previously showed that immortalized fibroblasts derived from PARP(-/-) mice exhibit an unstable tetraploid population, and partial chromosomal gains and losses in PARP(-/-) mice and immortalized fibroblasts are accompanied by changes in the expression of p53, Rb, and c-Jun, as well as other proteins. A tetraploid population has also now been detected in primary fibroblasts derived from PARP(-/-) mice. Oligonucleotide microarray analysis was applied to characterize more comprehensively the differences in gene expression between asynchronously dividing primary fibroblasts derived from PARP(-/-) mice and their wild-type littermates. Of the 11,000 genes monitored, 91 differentially expressed genes were identified. The loss of PARP results in down-regulation of the expression of several genes involved in regulation of cell cycle progression or mitosis, DNA replication, or chromosomal processing or assembly. PARP deficiency also up-regulates genes that encode extracellular matrix or cytoskeletal proteins that are implicated in cancer initiation or progression or in normal or premature aging. These results provide insight into the mechanism by which PARP deficiency impairs mitotic function, thereby resulting in the genomic alterations and chromosomal abnormalities as well as in altered expression of genes that may contribute to genomic instability, cancer, and aging.