Abstract： Objective To explore the effects of xemderma pigmentosum group D(XPD)/P44 subcomplex on the cell cycle of the hepatoma cells.Methods Human hematoma cells of the line SMMC7721 were cultured and transfected with human XPD gene by LipofectAMINE and 2 strains with stably transfected plasmid pEGFG-N2 and stably transfected recombinant plasmid pEGFG-N2/XPD were selected.After stably transfection,the antisense oligonucleotides of P44 were added to treat the stably transfected cells.The cells were divided into 6 groups:Group①(control group),Group②transfected with the blank plasmid pEGFP-N2,Group③transfected with the recombinant plasmid pEGFP-N2/XPD,Group④transfected with ASODN complementary to the translation initiation site of pEGFP-N2/XPD,Group⑤transfected with antisense oligodeoxynucleotides(ASODN)complementary to the translation terminal site of pEGFP-N2/XPD.and Group⑥transfected with ASODN complementary to the translation exon5 site of pEGFP-N2/XPD.The expression Ievels of wild-type P44,XPD,cdk7,cdk2,c-myc,and cdc25A were detected by RT-PCR and Westem blotting.The cell growth and the cell cycle were examined by MTT and flow cytometry(FCM)Results The P44 and XPD mRNA expression levels of Group③were significantly higher than those of Groups①and②(both P<0.01).Western blotting indicated that the changes of P44 and XPD protein expression levels were consistent with those of their mRNAs respectively:while the tuRNA and protein expression levels of cdk7,cdk2,c-myc,and cdc25A were all decreased.MTT method showed that the hepatoma cells grew slowly,FCM showed that the number of the cells arrested at the G1 stage of Group③were higher than those of Groups①and②.After the blockage of P44 gene expression,the expression levels of XPD mRNA and protein were decreased.The XPD mRNA and protein expression levels of Groups④,⑤,and⑥were significantly higher than those of Group③(all P<0.01).The mRNA and protein expression levels of cdk7,cdk2.c-myc,and cdc25A were upregulated.MTY method indicated that ceils grew fast.FCM showed that the nulnbers of the cells arrested at the G1 stage of Group④,⑤,and⑥were all lower than that of Group③.The expression levels of cell cycle regulatory genes including cdk7,cdk,c-myc,and cdc25A were markedly decreased,the hepatoma cells grew slowly;after the blockage of P44 gene expression the expression levels of XPD mRNA and protein were decreased.whereas the expression levels of the cell cycle regulatory genes mentioned above were enhanced.and the hepatoma cells grew faster.Conclusion XPD gene inhibits the proliferation and promotes the apoptosis of hepatoma ceils.The expression of XPD may be regulated by its molecular partner P44.XPD/P44 subcomplex iB involvod in the regulation of DNA damage checkpoint.