Correlation of KREC quantification with B-cell subset reconstitution post allo-HSCT. (A-C) Flow cytometrically determined cell counts of patients’ peripheral blood B-cell subsets were correlated with RT-PCR quantified KREC copy numbers at indicated days after allo-HSCT (D14, n = 16; D28, n = 34; D60, n = 24; D90, n = 26; D180, n = 11). Shown are mean values of KREC copy number per microliter of blood (gray line) and number of (A) transitional, (B) naïve, and (C) CD27⁺ memory B cells per microliter of blood (black line) ± SEM. (D-E) Graphs illustrate numbers of transitional B cells per µl blood ± SEM for patients with an early (D) or late (E) onset of B-cell reconstitution and KREC copy numbers per microliter of blood ± SEM for the respective patient group at indicated time points (early/late: D14, n = 9/4; D28, n = 13/17; D60, n = 14/10; D90, n = 18/8; D180, n = 6/5). Statistically significant correlations between kinetics of KREC copy and B-cell subset number are displayed as regression coefficient β (β*) and P values.