Persistence of BCR/ABL⁺progenitors in multiple samples obtained from a patient in continued CCR on imatinib mesylate treatment. Results for (A) BCR/ABL-GAPDH mRNA levels measured by Q-PCR and (B) the percentage of BCR/ABL⁺ CD34⁺ cells detected by FISH are shown for BM samples obtained from one patient from 12 to 26 months after initiation of imatinib mesylate treatment. A percentage less than or equal to 6% of cells with a BCR/ABL fusion signal is considered within background limits for the FISH assay and is shown by the horizontal line.