Lung tissue (Figure 4A, second panel from top) and thymus tissue sections (data not shown) taken from the p53−/− mice that showed lymphoma invasion in lung and thymus did not show a significant increase in proliferation above control levels, suggesting that the cells contributing to lymphomagenesis in the double mutant mice had a proliferative advantage that could contribute to the decreased latency in their lymphoma progression. This evidence concerns the gene TP53 and lymphoma.