High sensitivity of Parp-2^−/− mice to sublethal γ-radiation dose caused by BM failure. (A) Kaplan-Meier survival curves, after 5 Gy of total body γ-irradiation (TBI), of 5-week-old Parp-2^−/− (n = 13), Parp-1^−/− (n = 8), wild-type (WT) (n = 15), and Parp-2^−/− mice injected with 2 × 10⁶ WT BM cells (Parp-2^−/− [WT BM]; n = 5). Survival was monitored for 35 days. (B) Kaplan-Meier survival curves of reconstituted Parp-2^−/− or WT mice after γ-irradiation. Parp-2^−/− recipient mice were given a lethal dose of radiation (9.5 Gy) and reconstituted with 10 × 10⁶ total BM cells from WT (6 recipient mice) or Parp-2^−/− mice (6 recipient mice). Similarly, lethally irradiated WT mice were reconstituted with WT or Parp-2^−/− BM cells. After 6 weeks, these mice were given a single dose of γ-irradiation (5 Gy) and survival was monitored for 35 days. (C) Red blood cells (RBC) and white blood cells (WBC) in WT and Parp-2^−/− mice at different time points after TBI (5 Gy). (D) Total number of BM cells (two femurs per mouse) in WT and Parp-2^−/− mice at different time points after TBI (5 Gy). Values represent the mean ± SEM of at least 6 mice of each genotype. *Statistically significant difference (P < .05). (E) Hematoxylin and eosin staining of fixed femur sections from WT and Parp-2^−/− mice before and at days 6 and 12 post irradiation (5 Gy) (original magnification ×100).