But in contrast to retrovirally delivered DDp53, we also observed modest selection for p53−/− cells in non-irradiated CD4+ and CD8+ cells, which could either reflect the pre-existence of partially transformed p53−/− cells in the T-cell lineage (consistent with eventual T lymphoma development) or skewed selection for p53−/− T progenitors in the abnormal thymic environment of irradiated recipient mice. The gene discussed is CD4; the disease is lymphoma.