[en] A major challenge in radiation carcinogenesis is to identify the somatic (gene)s involved in this process. To study this question, the author examined the transforming activity of the DNA isolated from x-ray (600 rad) transformed 10T 1/2 cells using both NIH 3T3 and C3H10T 1/2 cells as recipients in the transfection assay. The resulting frequencies of transfection were 0.02-0.1 foci/μg genomic DNA with NIH 3T3 cells, and 0 - 0.04foci/μg of genomic DNA with 10T 1/2 cells as recipients. The authors also examined oncogene expression during the process of x-ray induced morphological transformation of 10T 1/2 cells. DNA obtained from cells during different stages of transformation (1,2,3,4,5, and 6 weeks) after irradiation and from Type III transformed foci) were transfected into NIH 3T3 cells. Preliminary results indicate that the activation of certain oncogens may occur 2-3 weeks after irradiation, well before the appearance of morphologically transformed foci. The results of these and of more detailed studies currently in progress are discussed in the light of previous observations on the kinetics of the development of malignant transformation in irradiated 10T 1/2 cells