ku80 gene of radiosensitive cell line SX-9 was cloned by RT-PCR.Compared cDNA of the radiosensitive cell line with its parental cell line, it was found that there were two mutation sites in ku80 gene which could interact with DSB DNA and Ku70.It was confirmed by EMSA and Southwestern blot that the DSB end binding activity of Ku80 in the nucleus extract of the radiosensitive cell decreased greatly，which infered that the radiosensitivity of the cell line is caused by dysfunction of its Ku80 protein.