Number of clones derived from single-cell expansion cultures. Distribution of clones derived from single-cell expansion cultures of CD34⁺38^– cells from human umbilical CB (n = 2) in SFM supplemented with 6 growth factors and either with or without imatinib (concentration, 5 μM). Note that whereas 15% of the single CD34⁺38^– cells were expandable to more than 10⁵ cells in vitro, none of the imatinib-treated clones reached that margin. The asterisk (*) indicates significant difference between imatinib-treated and control cells at day 18 (P < .001).