CDK6 and its kinase activity are required for stem cell/multipotent progenitor development. (A) Staining for hematopoietic stem cells (LSK fraction). The relative percentage of selected cell populations in BM is shown. (B) Total BM cellularity from 2- to 3-month-old mice. The cell numbers are mean ± SE (N is indicated). *P < .05, significantly different from the control levels. (C) The percentage of LSK in the total cell population was calculated based on the percentage of Lin⁻c-Kit⁺ and c-Kit⁺ Sca-1⁺ populations and is the mean ± SE. *P < .05, significantly different from the WT/WT-Δ control, which was arbitrarily defined as 1 unit (100%). (D) The absolute cell numbers for the LSK were calculated based on the total number of cells (B) and the percentage of LSK (C) in the total cell population from BM, and are the mean ± SE (N is indicated). *P < .05, significantly different from the WT/WT-Δ controls, which was arbitrarily defined as 1 unit (100%). (E) Proliferative fraction of LSK. BM suspensions were surface-stained before permeabilization and then stained with 7-amino-actinomycin D (7-AAD). The percentage of (S-G₂-M) from 4 separate experiments is the mean ± SE. *P < .05, significantly different from the WT/WT-Δ controls, which was arbitrarily defined as 1 unit (100%). (F) Apoptotic fraction from LSK cells using annexin V staining. Data are mean ± SE (N is indicated below each mutant mouse). *P < .05, significantly different from the WT/WT-Δ control, which was arbitrarily defined as 1 unit (100%).