Several cases of T-cell leukemia caused by gammaretroviral insertional mutagenesis in children treated for x-linked severe combined immunodeficiency (SCID) by transplantation of autologous gene-modified stem cells were reported. In a comparative analysis, we recently showed that mature T cells, on the contrary, are highly resistant to transformation by gammaretroviral gene transfer. In the present study, we observed immortalization of a single T-cell clone in vitro after gammaretroviral transduction of the T-cell protooncogene LMO2. This clone was CD4/CD8 double-negative, but expressed a single rearranged T-cell receptor. The clone was able to overgrow nonmanipulated competitor T-cell populations in vitro, but no tumor formation was observed after transplantation into Rag-1 deficient recipients. The retroviral integration site (RIS) was found to be near the IL2RA and IL15RA genes. As a consequence, both receptors were constitutively upregulated on the RNA and protein level and the immortalized cell clone was highly IL-2 dependent. Ectopic expression of both, the IL2RA chain and LMO2, induced long-term growth in cultured primary T cells. This study demonstrates that insertional mutagenesis can contribute to immortalization of mature T cells, although this is a rare event. Furthermore, the results show that signaling of the IL-2 receptor and the protooncogene LMO2 can act synergistically in maligniant transformation of mature T lymphocytes.