Objective   To study if the key factor PARP1 for base excision repair (BER) is involved in repair of DNA double strand breaks (DSB) when Ku70 is silenced.    Methods    A shRNA expression vector against human Ku70 was constructed and then were transfected to produce silent Ku70 cell line. DSB were induced with calicheamycin

a high efficient inductor of DNA DSB. Effect of DSB on cell survival rate was detected when Ku70 was silent or PARP1 was inhibited with crystal violate staining. Phosphorylation of H2AX was detected by Western blotting. Cell fractions were extracted according to the difference in interactions of chromatin. Distribution of PARP1 in cell fractions was assayed by Western blotting.    Results   The protein level in Ku70 was significantly down-regulated by Ku70 shRNA. Silent Ku70 and PARP inhibitor significantly decreased the survival rate of cells (P0.01). PARP1 was mainly distributed in fractions of cells closely bound to chromatin.    Conclusion   PARP1 can bind to chromatin in the absence of Ku70 and decrease the survival time of cells

indicating that PARP1 may participate in repair of DNA DSB independent of Ku70.