Primitive hematopoietic cells from Vav-Cre Rb KO animals are impaired in cell-cycle exit upon replicative stress. BM cells were pulse-labeled with BrdU in vivo, and the frequency of HPCs (L⁻S⁻K⁺ cells) and HSCs (L⁻S⁺K⁺) in the G₀/G₁, S, and G₂/M phases of the cell division cycle was analyzed by flow cytometry after 5-FU treatment (150 mg/kg) at (A) 16 hours (n = 3 for both control and Vav-Cre Rb KO) and (B) 4 days (n = 9 for both control and Vav-Cre Rb KO). (C) Frequency of HPCs (L⁻S⁻K⁺) and HSCs (L⁻S⁺K⁺) undergoing apoptosis in control and Vav-Cre Rb KO animals either 16 hours or 4 days after 5-FU treatment, determined by staining for annexin V and 7AAD by flow cytometry (n = at least 3 for each data point). Shown are mean values plus or minus 1 SEM; *P < .05.