Thymic regeneration failure after CBT. The frequency of recent thymic emigrants in peripheral blood was quantitated using a real-time PCR assay measuring T-cell receptor (TCR) δ excision circles produced during TCR-α rearrangement. (A) Longitudinal assessment of thymic function in CB transplant recipients (n = 26, median age = 33) demonstrates reduced thymic function in baseline and post-CBT thymopoietic recovery that was measurable in only 2 subjects and significant and sustained in only 1 recipient. The horizontal line depicts the number of TRECs present in a group of healthy control subjects who were age-matched (n = 18, median age = 34). (B) Thymic regenerative failure in CB transplant recipients relative to autologous (auto) and allogeneic (allo) SCT recipients. Median values of TRECs in CBT were dramatically reduced relative to other prospectively analyzed SCT recipients, despite a significantly reduced median age of the CBT population (median age 33 vs 43 for Colorado allogeneic SCT recipients and 50 for Colorado autologous SCT recipients). Values for CB transplant recipients were significantly lower than other groups at all time points (P < .01).