Oligoclonal CML in mice treated with PD166326 or imatinib mesylate. Genomic DNA was prepared from the spleens of mice with the CML-like illness after 3 to 4 weeks of treatment with the indicated drug or placebo and analyzed for the number of unique BCR/ABL proviral integration sites by Southern blot as described in “Materials and methods.” The number of leukemic clones for each sample is indicated at the top. Genomic DNA from a clonal P210 and parental Ba/F3 cell line are shown at left as controls. DNA size markers are indicated at left.