The ability of Tpo and other putative viability factors to promote viability of CD34⁺CD38⁻ and CD34⁺CD38⁺ progenitor cells. CD34⁺CD38⁻ (A) or CD34⁺CD38⁺ (B) 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 of medium containing IL-1, IL-3, IL-6, G-CSF, GM-CSF, CSF-1, FL, KL, Epo, and Tpo was added to each well 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⁻ (A) or CD34⁺CD38⁺ (B) cells were also cultured at a density of 1 cell per well in 20 μL serum-depleted medium and incubated with each of the putative viability factors alone and scored for clonal growth (≥5 cells) after 14 days of culture at 37°C and 5% CO₂ in air (□). The results represent the means (+SEM) of total number of clones per 180 cells of five and three individual experiments (A and B, respectively), with 180 wells scored per group in each experiment. *No clones were formed by cells incubated for 14 days in medium alone.