Persistence of BCR/ABL⁺CD34⁺cells in CML patients in CCR with imatinib mesylate treatment. CD34⁺ cells were selected from MNCs of CML patients in CCR (CP, n = 12; and AP, n = 3) by immunomagnetic column separation and analyzed for the presence of BCR/ABL⁺ cells by FISH. The percentage of BCR/ABL⁺ cells in the CD34⁺ cell fraction from the first sample studied is shown. A percentage less than or equal to 6% of cells with a BCR/ABL fusion signal is considered within background limits based on laboratory validation studies and is shown by the horizontal line. CD34⁺ cells from 4 patients in CP were BCR/ABL^– by FISH.