A CML sample that showed minimal response to imatinib mesylate was found to have a very high percentage of

BCR/AB-negative cells. (A) The frequency of total (▪) and BCR/ABL-positive (░) LTCIC and CFC progenitors is shown relative to results in cells not exposed to imatinib mesylate. (B) CFSE analysis of primitive progenitors after 96 hours in the absence of imatinib mesylate (left panel) revealed a mixture of proliferative cells (33%) and quiescent cells (67%). Exposure to 1 μM imatinib mesylate increased the percentage of quiescent cells (90%; right panel). The PI is shown below each plot.