Aneuploidy and extensive chromosomal rearrangements are common in human tumors. The role of DNA damage response proteins p53 and p21(CIP1/WAF1) in aneugenesis and clastogenesis was investigated in telomerase immortalized diploid human fibroblasts using siRNA suppression of p53 and p21(CIP1/WAF1). Cells were exposed to the environmental carcinogen sodium arsenite (15 and 20 microM), and the induction of micronuclei (MN) was evaluated in binucleated cells using the cytokinesis-block assay. To determine whether MN resulted from missegregation of chromosomes or from chromosomal fragments, we used a fluorescent in situ hybridization with a centromeric DNA probe. Micronuclei were predominantly of clastogenic origin in control cells regardless of p53 or p21(CIP1/WAF1) expression. MN with centromere signals in cells transfected with NSC siRNA or Mock increased 30% after arsenite exposure, indicating that arsenite induced aneuploidy in the tGM24 cells. Although suppression of p53 increased the fraction of arsenite-treated cells with MN, it caused a decrease in the fraction with centromeric DNA. Suppression of p21(CIP1/WAF1) like p53 suppression decreased the fraction of MN with centromeric DNA. Our results suggest that cells lacking normal p53 function cannot become aneuploid because they die by mitotic arrest-associated apoptosis, whereas cells with normal p53 function that are able to exit from mitotic arrest can become aneuploid. Furthermore, our current results support this role for p21(CIP1/WAF1) since suppression of p21(CIP1/WAF1) caused a decrease in aneuploidy induced by arsenite, suggesting that p21(CIP1/WAF1) plays a role in mitotic exit.