Importantly, when pMSV-T24-H-ras/MSV-c-myc was converted to linear molecules, this plasmid was found to be about thirty-fold more active, with 800 pg now inducing tumors in newborn CD3 epsilon mice. The availability of a sensitive in vivo system should make feasible the GS-5734 AbMole analysis of cellular and viral oncogenes following direct inoculation of DNA without the usual approach of expressing these oncogenes in cells in vitro followed by analyzing the phenotypes of these transformed cells in vivo. Such an approach might reveal hitherto unrecognized activities and functions of these oncogenes, which might also reveal novel mechanisms of oncogenesis. In addition, the identification of a highly susceptible mouse strain for the detection of the oncogenic activity of DNA prompted us to test the oncogenicity of DNA from four human tumors: HeLa, A549, HT1080, and CEM. While tumors were induced in all animals inoculated with cellular DNA when the control pMSV-T24-Hras/MSV-c-myc plasmid was co-injected, demonstrating that none of the cellular DNAs had inhibitory activity, no tumors were induced in mice that were injected with the tumor-cell DNA alone, which suggests that detecting activated oncogenes in cellular DNA might be problematic even with sensitive animal models such as the newborn CD3 epsilon mouse. The results described in this paper demonstrate that the CD3 epsilon mouse is sensitive to tumor induction by activated H-ras plus c-myc DNA. The results also confirm our earlier finding that the newborn mouse is more sensitive to this oncogenic insult than is the corresponding adult animal, which appears to be the case regardless of strain. Whether the sensitivity of the CD3 epsilon mouse is due to its defects in T-cell function and/or NK-cell function, or whether it is due to the particular genetic background, is unknown. With respect to the first question, we are using several other mouse strains with various immunological deficiencies to try to assess what components of the immune system might be contributing to the efficiency of tumor induction by the ras/myc DNA. With respect to the second issue, viz., the contribution of mouse strain background, this is more difficult to address, since the CD3 epsilon mouse is a hybrid between a CBA mouse and a C57BL/6 mouse and therefore there is no parental strain with which to investigate this issue. In earlier studies, we had evaluated adults and newborns of both the C57BL/6 mouse and the CBA mouse, and neither of these mouse strains was as sensitive to the ras/myc DNA-induced tumor formation as the CD3 epsilon mouse. Therefore, at present, the reason for the high sensitivity of the CD3 epsilon mouse is not known.