TP53 and Pancytopenia: After IR exposure, the extents of HSC pool shrinking (Figure 6J), HSC apoptosis (Figure 6K), and pancytopenia (Figure 6L–N) were all less severe in the p53−/− mice comparing to those in their WT littermates, indicating that the p53−/− mice were more resistant to irradiation-induced myelosuppression.