Membrane phenotype of Step IIIP HPCs grown in FCS^- liquid suspension culture in the presence of 100 ng/mL Tpo. Cells were harvested at various stages of maturation and analyzed by single (top) and double (bottom) fluorescence labeling using anti-CD4, -CD61, -CD34 MoAbs (mean ± standard error of mean [SEM] values from three independent experiments). (▪) CD34⁺; (▵) CD61⁺; (▴) CD4⁺; (○) CD34⁺/4⁺; (•) CD34⁺61⁺; (□) CD4⁺/61⁺.