A representative experiment depicting sequential cytokine stimulation of G₀CD34⁺ cells in a two-dimensional tracking experiment. Sorted G₀CD34⁺ cells were stained with PKH2 on day 0 and cultured for 7 days (day 0-7, on the left) in the presence of one primary cytokine. On day 7, cells were harvested, washed, stained with PKH26, and cultured for an additional 7 days (day 7-14) in the presence of the secondary cytokine/cytokine combinations indicated to the right. Definition of phase III and IV cells, depicted here as red and blue events, respectively, was accomplished according to criteria described in Fig 6. A total of nine such experiments were performed with similar results obtained in all. However, because of the relatively small number of G₀CD34⁺ cells obtained from a normal donor, FL was used in four experiments only, and IL-6 was used in only three experiments. SCF and IL-3 were used in all nine experiments.