Frequency of AML1/ETO+ cells in B cells and monocytes estimated by limit dilution analyses. The percent of samples negative by RT-PCR for AML1/ETO transcripts is plotted on the y axis versus the number of cells per sample tested. According to the Poisson statistics, the frequency of AML1/ETO+ cells can be estimated as numbers of cells in samples that show 37% of detection failures (arrows). The frequency of AML1/ETO+ cells was estimated to be 1 in 41,000 (case 5) and 38,000 (case 6) mature B cells in remission marrow (○), and 1 in 5,100 (case 5) and 6,200 (case 6) mature B cells in leukemic marrow (●), indicating that leukemic BM contains t(8;21)+ B cells at ≈10-fold higher frequency, compared with remission BM. AML1/ETO+ cell was estimated to be in 1 in 3,800 (case 5) and 4,700 (case 6) monocytes in remission marrow (□).