Effect of cytokines on colony formation by CD34⁺CD41⁺ MK progenitors. CD34⁺CD41⁺ BM cells were incubated for 7 days in serum-free plasma clot culture in 96-well plates (100 cells in 100 μL of media) with different combinations of cytokines. After 7 days, the wells were fixed, stained as described, and examined microscopically. Clusters of 3 or more cells were scored as CFU-MK colonies. The results shown are the mean ± SD of four experiments.