Proportions of CD20⁺ blasts in paired samples from diagnosis and follow-up. Comparisons of PB samples from day 0 versus day 8 (left plot), as well as of BM samples from diagnosis versus day 15 (center) and day 33 (right), are shown. Each data point represents one BCP-ALL case. Thresholds for determining CD20 expression on leukemic cells were set by using the background fluorescence of residual non-B lymphocytes within the same acquisition. Follow-up samples only with more than or equal to 0.1% residual leukemic cells were analyzed. Dashed lines mark the cutoff of 20% used to describe a sample as positive or negative. The numbers of samples located within each of the 4 quadrants, which are built by the cutoff lines, are shown. Identical results in paired samples would fit closely to the dotted diagonal line. In all comparative plots, it can be seen that follow-up samples have higher proportions of CD20⁺ leukemic cells than the paired samples at diagnosis.