Comparison of the effect of different concentrations of IM on colony formation by CFCs from IM responders and nonresponders. CD34⁺ cells from 11 patients classified clinically as imatinib mesylate (IM) responders and 14 as nonresponders were assayed in duplicate 1-mL methylcellulose cultures containing 0, 1, 5, or 10μM IM. Two weeks later, the colonies were enumerated to determine the numbers derived from BFU-E, CFU-GEMM, or CFU-GM. The total number of colonies obtained in the IM-supplemented cultures was then calculated as a percentage of those obtained in the absence of IM. (♦) Results obtained from the 3 patients who rapidly developed BC; and cross bars, mean plus or minus SEM of data for each patient group. Differences between the results for the IM responders and nonresponders at all doses of IM tested are significant (P < .001).