Necdin-null hematopoietic cells are highly sensitive to irradiation. (A) Survival curves for mice reconstituted with wild-type (WT) or necdin-null (Ndn null) fetal liver cells, given total body irradiation (6.5 Gy), and monitored regularly for survival (P = .0017, n = 10). (B) Survival curves for mice reconstituted with wild-type or necdin-null fetal liver cells given TBI (8 Gy) and monitored regularly for survival (P = .0091, n = 8). (C) HSCs (Lin⁻Sca-1⁺c-Kit⁺CD48⁻CD150⁺ cells) from the BM of the mice repopulated with wild-type or necdin-null fetal liver cells were assessed for apoptosis 12 hours after a dose of total-body irradiation (6.5 Gy) using DAPI and annexin V staining (left panels). Data shown are the mean percentage (± SD) of the annexin V⁺/DAPI⁻ population in Lin⁻Sca-1⁺c-Kit⁺CD48⁻CD150⁺ cells (P = .0098, n = 5, right panel). (D) Cell-cycle analysis of HSCs after G-CSF treatment (200 μg/kg daily). Lin⁻Sca-1⁺c-Kit⁺CD48⁻CD150⁺ cells isolated from mice reconstituted with wild-type or necdin-null fetal liver cells after 5 days of G-CSF treatment were analyzed by staining with Hoechst 33342 and Ki67. Data shown are the mean values of G₀ cells (± SD; P = .75, n = 5). (E) Apoptosis of HSCs after G-CSF treatment (200 μg/kg daily). HSCs from the BM of the 5-day G-CSF–treated mice repopulated with wild-type or necdin-null fetal liver cells were assessed for apoptosis 12 hours after a dose of total body irradiation (6.5 Gy) using DAPI and annexin V. Data shown are the mean percentage (± SD) of the annexin V⁺/DAPI⁻ population in Lin⁻Sca-1⁺c-Kit⁺CD48⁻CD150⁺ cells (P < .005, n = 4). (F) HSCs (Lin⁻Sca-1⁺c-Kit⁺CD48⁻CD150⁺) from the BM of mice repopulated with wild-type, necdin-null, p53-null, or necdin/p53 double-null fetal liver cells were assessed for apoptosis 12 hours after a dose of total body irradiation (6.5 Gy) using DAPI and annexin V staining. Data shown are the mean percentage ± SD of the annexin V⁺/DAPI⁻ population within HSCs (P < .0001 by 1-way ANOVA, n = 4). Significant differences were observed between wild-type and necdin-null, wild-type and p53-null, necdin-null and p53-null, necdin-null and necdin/p53 double-null, and p53-null and necdin/p53 double-null recipients.