Persistence ofBCR/ABL⁺cells with CFC and LTCIC capacity in CML patients in CCR with imatinib mesylate treatment. CD34⁺ cells were plated in CFC (committed progenitor, n = 15) and LTCIC (primitive progenitor, n = 13) assays, and the resulting colonies were harvested and analyzed for BCR/ABL⁺ cells by FISH as shown in the bottom half of the figure. Results from the first sample studied are shown and represent the percentage of BCR/ABL⁺ cells in colonies resulting from CFC and LTCIC assays. The percentage of CD34⁺ cells (n = 15) that were BCR/ABL⁺ is shown for comparison. A percentage less than or equal to 6% of cells with a BCR/ABL fusion signal is considered within background limits and is shown by the horizontal line. CD34⁺ cells from 4 patients, CFCs from 4 patients, and LTCICs from 3 patients were BCR/ABL^– by FISH.