To identify a set of genes involved in the development of radiation-induced tumorigenesis, we used DNA microarrays consisting of 1176 mouse genes and compared expression profiles of radioresistant cells, designated NIH3T3-R1 and NIH3T3-R4. These cells were tumorigenic in a nude mouse grafting system, as compared with the parental NIH3T3 cells. Expression of MDM2, CDK6 and CDC25B was found to increase more than 3-fold. Entactin protein levels were down-regulated in NIH3T3-R1 and NIH3T3-R4 cells. Changes in gene expression were confirmed by reverse transcription-PCR or western blotting. When these genes were transfected into NIH3T3 cells, CDC25B and MDM2 overexpressing NIH3T3 cells showed radio-resistance, while CDK6 overexpressing cells did not. In the case of entactin, overexpressing NIH3T3-R1 and NIH3T3-R4 cells were still radioresistant. Furthermore, CDC25B and MDM2 overexpressing cells grafted into nude mice were tumorigenic. NIH3T3-R1 and NIH3T3-R4 cells showed increased radiation-induced apoptosis accompanied by a faster growth rate, rather than an earlier radiation-induced G2/M phase arrest, suggesting that the radioresistance of NIH3T3-R1 and NIH3T3-R4 cells was due to a faster growth rate rather than induction of apoptosis. In the case of MDM2 and CDC25B overexpressing cells, similar phenomena, such as increased apoptosis and a faster growth rate, were shown. The above results, therefore, demonstrate involvement of CDC25B and MDM2 overexpression in radiation-induced tumorigenesis and provide novel targets for detection of radiation-induced carcinogenesis.