Increased proliferative capacity of Δ122p53. (A) Photomicrographs illustrate more cells in S phase of spleens from Δ122p53 mice compared with p53^+/+ and p53^−/− mice. Mice were pulse-labeled with BrdU for 100 minutes to label proliferating cells. Organs were harvested, and BrdU⁺ cells were detected with an horseradish peroxidase-labeled antibody and light microscopy. Images were taken with a Leica DM 2000 microscope, Leica DFC 295 camera, and Leica Version 3.5.0 Application Suite (Software). Lens aperture was 0.3 with a 10× objective. Images were cropped using Photoshop Version CS2 (Adobe). (B) Quantitation of BrdU⁺ cells in different tissues. BrdU was administered for 100 minutes (thymus) or 24 hours (other organs). The proportion of BrdU⁺ cells per 2000 to 5000 total cells was determined. (C) Colony-forming assays show that Δ122p53 mice have more granulocyte and macrophage colonies compared with p53^+/+ and p53^−/− mice. Bone marrow was cultured in methylcellulose-based media, and colonies were counted at day 10. *P < .05. **P < .01. ***P < .001. All experiments were performed using at least 3 mice per genotype. Results are mean plus or minus SD. (D) The p53 null 10–1 fibroblasts transduced with the Δ122p53 retrovirus (red triangles) or vector control (black diamonds) were seeded at a density of 0.6 × 10⁵ cells/35-mm dish, and viable cell numbers were determined over time by trypan blue counting. (Inset) Expression of Δ122p53 protein by Western blotting (PAb 240 antibody) in Δ122p53-transduced cells but not in the vector control. Lower (pink) bands illustrate equal loading by Ponceau S staining.