Telomerase activity and telomere length of flow-sorted populations. (A) Telomerase activity was assayed in B cells from 60 patients with CLL using the TRAP assay and percentage of CD38⁺ CLL cells plotted versus TPG (total product generated) units in the same patients. CD19⁺CD5⁺ cells from 20 patients with CLL were sorted into CD38⁺ and CD38⁻ subpopulations and processed for quantification of telomere length and telomerase activity. (B) Lines connect data points that indicate telomerase activity in the cell subsets of each individual patient. (C) Lines connect data points for mean telomere lengths of flow-sorted CD38⁺ and CD38⁻ CLL cells. (D) Significant positive correlation (r = .465; P < .045) exists between estimated telomerase activity and observed telomerase activity (TPG units) in CD38⁺ cells from 20 patients with CLL shown in panels B and C; this correlation does not exist in the CD38⁻ cells. Diagonal lines in panels A and D indicate best fits based on linear regression of data.