Binding of soluble FN to human CD34⁺ cells. Purified CD34⁺ cells were incubated with biotin-labeled soluble FN in the presence (right panel, shaded histogram) or absence (left panel, shaded histogram) of a 20-fold excess of unlabeled FN, and then stained with FITC-conjugated avidin. The stained cells were analyzed by a single-color flow cytometry. The level of background staining with unlabeled FN and FITC-conjugated avidin (open histogram) is shown in each panel.