Scl gene dosage and LT-HSC cell cycle in response to cytokine stimulation. (A) Scl^+/+ and Scl^+/− lineage-depleted bone marrow cells harvested from adult mice were stimulated with Steel Factor, IL-11, and IL-6 for 24 hours before cell-cycle analysis. The percentages shown in each quadrant are the mean ± SEM of 3 experiments with representative contour plots. (B-C) Stacked columns represent the relative proportion of cells in G₀, G₁, and S-G₂M within LT-HSCs (B) or progenitors (C) from Scl^+/+ and Scl^+/− mice (mean ± SEM of 3 experiments). The left panels represent cells at steady state before culture; the right panels represent growth factor (GF) stimulated cells after 24 hours in culture (*P = .02-.03; **P = .005). (D) Cell-cycle analysis of LT-HSCs from 3-week-old mice after 24 hours of cytokine stimulation. The percentages shown in each quadrant are the mean ± SEM of 3 experiments with representative contour plots.