(A) Two-color flow cytometric analysis of human CD34⁺ cells mobilized into the peripheral blood. CD34⁺ progenitor cells were isolated from peripheral blood following in vivo administration of cyclophosphamide and G-CSF. A total of 5 × 10⁵ cells was fixed, stained, and analyzed by two-color flow cytometry for DNA and protein content as described in Materials and Methods. The quiescent cells form a homogeneous population (marked C) with 2n DNA and low protein content. (B) Two-color flow cytometric analysis of stimulated peripheral blood CD34⁺ cells. CD34⁺ progenitor cells as above were cultured in vitro with SCF, IL-3, and IL-6 for 2 days. Cells were then fixed, stained, and analyzed by two-color flow cytometry for DNA and protein content as described in Materials and Methods. The stimulated cells show two populations (marked C and D) with 2n DNA, but D has an increased protein content. This is consistent with the cells having moved from G0 (C) into the G1 phase of the cell cycle (D). Cells to the right of box D (marked E) are in S and G2/ M phases.