Mitotic quiescence of the primitive hematopoietic cells in the leukemic hosts and expression of several cell-cycle regulators. As illustrated in Figure 1A, 10⁷ BMNCs from B6.SJL mice were transplanted with or without 10⁶Notch1-induced leukemia cells (CD45.2⁺GFP⁺) into lethally irradiated C57BL/6J recipients. At the 2-week time point, LKS cells from CD45.1⁺ BMNCS were sorted for staining with PI to assess the general cell-cycle profile (G₀/G₁ vs S/G₂ + M) or staining with PY in conjunction with Hoechst 33342 (HO) to specifically determine the portion of cells in G₀ vs G₁ with flow cytometry. An aliquot of the cells was also used to examine the expression of several cell-cycle regulators with real time RT-PCR. (A) PI staining. A representative figure is shown from 1 of 2 experiments with similar results. (B) PY staining. Cells residing in G₀ appear at the bottom of the G₀/G₁ peak as shown in the representative plot. Values are mean ± SD. **P = .017 (t test). (C) Expression of cell-cycle regulators. The CD45.1⁺ LKS cells were sorted directly into lysis buffer for real-time RT-PCR analysis. Data shown are the ratios between leukemic and control groups from 1 of 2 independent experiments with similar results. Values are mean ± SD. *P < .05 (t test). **P < .01 (t test).