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Objective: Recent studies have demonstrated that cells exposed to ionizing radiation or alkylating agents can develop prolonged genetic instability. But its mechanism is still unknown. A cDNA fragment (fragment 9) has been isolated in MNNG-exposed vero cell by mRNA differential display in this lab. After antisense blocking the expression of its relevant gene (fragment 9 related gene, FNR gene), we found that nontargeted mutation frequency induced by MNNG was enhanced significantly. which implicated that the product of the blocked gene may be involved in the inhibition of nontargeted mutation. In order to elucidate the functional mechanism of the FNR gene, we try to separate the proteins from the established cell line expressing antisense fragment 9 to find out the FNR gene-coded protein. Method: The total cellular proteins of MNNG-exposed vero cell transfected with antisense RNA expression plasmid (vero-pM-amp--9-) and those with vector DNA (vero-pM-amp-) were separated by two-dimensional gel electrophoresis, and the resulting maps were analyzed with 2-D analysis software packages to find out the differentially expressed protein spots. Then the related 2-D PAGE database (http://biobase.dk/cgi-bin/celis/) was searched according to the protein spots information obtained from 2-DE including the position in the gel, isoelectric point (pl) and molecular weight (Mr). Result: Twelve proteins were specifically expressed only in vero-pM-amp-, and 2 proteins in vero-pM-amp--9-. In addition, there were 24 proteins expressed in higher level in vero-pM-amp--9- as compared with vero-pM-amp- (P<0.05), among them the expression of 7 proteins were enhanced by greater than 5 folds. On the other hand, no sequence similarity was found by homology analysis in GenBank through comparing the fragment 9 with the cDNA sequences of those proteins found in this study. Conclusion: Gene expression alterations bave occurred after antisense blocking of the FNR gene expression as demonstrated by proteomic analysis, and the FNR gene may code a gene expression regulatory factor.