Methylcellulose assay illustrating the inhibitory effect of imatinib on normal lineage-committed progenitor cells in vitro. To evaluate imatinib-dependent CFU growth, 500 cells each were cultured in methylcellulose medium supplemented with different concentrations of imatinib ranging from 0 to 10 μM. The mean ± SE of the plating efficiency (in percent) of CFUs was determined 14 to 16 days after incubation (n = 3 individual donors; except for 0.625 μM, where results from only 2 donors were available). The asterisk (*) indicates lowest imatinib concentration at which significant differences compared to untreated cells were observed for total colonies (P < .002), BFU-Es (P < .002), and CFU-GMs (P < .001).