Comparison between the ability of Tpo and Epo to promote viability of CD34⁺CD38⁻ progenitor cells. CD34⁺CD38⁻ BM cells were cultured at a density of 1 cell per well in 10 μL serum-depleted medium and predetermined optimal concentrations of the indicated cytokines. After 116 hours of preincubation, 10 μL medium containing IL-1, IL-3, IL-6, G-CSF, GM-CSF, CSF-1, FL, KL, Epo, and Tpo was added to yield predetermined optimal concentrations. Clones (≥5 cells) were scored after an additional 14 days of incubation at 37°C and 5% CO₂ in air (▪). A total of 180 CD34⁺CD38⁻ cells were also cultured at a density of 1 cell per well in 20 μL serum-depleted medium alone or supplemented with Tpo or Epo and scored for clonal growth (≥5 cells) after 14 days in culture at 37°C and 5% CO₂ in air (□). The results represent the means (+SEM) of total number of clones per 180 cells of three individual experiments, with 180 wells scored per group in each experiment. *No clones were formed by cells incubated for 14 days in medium alone.