Objective:To investigate the expression of tumor suppressor gene PTEN in the radiation-induced mouse thymoma cells,and to observe the inhibitory effect of exogenous PTEN transfection on the in vitro proliferation and in vivo tumor forming ability of radiation-induced thymoma.Methods: Immunohistochemistry SP and Western blotting assay were used to examine the expression of PTEN,γ-H2AX,and Rad51 protein in radiation-induced mice thymoma and normal thymus tissues.RT-PCR assay was conducted to examine the PTEN gene loss.Exogenous PTEN gene was transferred into mouse thymoma cells and its inhibitory effects on cell proliferation and tumor-forming ability were observed.Results: The positive rate of PTEN protein expression was 22.73% (5/22) in radiation-induced thymoma tissue,significantly lower than that in the normal thymus tissue (P<0.01).Western blotting assay showed that the expression of PTEN protein in thymoma was markedly lower than that in the normal thymus tissue (P<0.01).RT-PCR found that in tumor tissue there was high-frequency of PTEN gene loss.Exogenous PTEN expression in thymoma significantly inhibited the cell proliferation and the tumor-forming ability (P<0.01).The expression of γ-H2AX protein in the thymoma tissue was significantly higher than that in the normal thymus tissue; the expression of Rad51 protein was significantly lower than that in the normal tissue. Conclusion: Loss of PTEN gene may contribute to radiation-induced thymoma via influencing the Rad51-mediated DNA repair pathway.Exogenous PTEN gene transfer can inhibit the in vitro proliferation of thymoma cells,which may contribute to the treatment and prevention of radiation-induced tumor.